IXPE, the first observatory dedicated to imaging x-ray polarimetry, was launched on Dec 9, 2021 and is operating successfully. A partnership between NASA and the Italian Space Agencey (ASI) IXPE features three x-ray telescopes each comprised of a mirror module assembly with a polarization sensitive detector at its focus. An extending boom was deployed on orbit to provide the necessary 4 m focal length. A three-axis-stabilized spacecraft provides power, attitude determination and control, and commanding. After one year of observation IXPE has measured statistically significant polarization from almost all the classes of celestial sources that emit X-rays. In the following we describe the IXPE mission, reporting on its performance after 1.5 year of operations. We show the main astrophysical results which are outstanding for a SMEX mission.
The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX) is a NASA-funded sounding rocket instrument that can make the first measurement of the linear X-ray polarization of an extragalactic source in the 0.2-0.4 keV band. We employ multilayer-coated mirrors as Bragg reflectors at the Brewster angle. By matching the dispersion of a dispersive spectrometer using critical angle transmission gratings to three laterally graded multilayer mirrors (LGMLs), we achieve polarization modulation factors over 90%. We will describe new prototyping work as well as extensions of the design for an orbital version.
This work is supported in part by NASA grant 80NSSC23K0644.
The Chandra X-ray Observatory (CXO) was launched over 23 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imaging Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of −120°C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60°C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. In this paper, we evaluate the performance of the current contamination model using the most recent calibration observations conducted in 2021 and 2022. The contamination model has required several revisions over the course of the mission as the properties of the contamination layer have changed and our understanding of the layer has improved. We show that the current calibration model underestimates the additional absorption of the contamination layer by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC), spectral data from the cluster of galaxies known as Abell 1795, and high resolution x-ray spectra of Mrk 421 and other active galaxies. The current model of the contamination layer under-estimates the optical depth by ∼17% at 0.66 keV and ∼10% at 1.49 keV. We suggest that the model may be improved with a change to the temporal component of the model only. This revised model is expected to be included in a future release of the CXO calibration database.
Supermassive black holes (SMBH) interact with gas in the interstellar and intergalactic media (ISM/IGM) in a process termed “feedback” that is key to the formation and evolution of galaxies and clusters. Characterizing the origins and physical mechanisms governing this feedback requires tracing the propagation of outflowing mass, energy and momentum from the vicinity of the SMBH out to megaparsec scales. Our ability to understand the interplay between feedback and structure evolution across multiple scales, as well as a wide range of other important astrophysical phenomena, depends on diagnostics only available in soft x-ray spectra (10-50 Å). Arcus combines high-resolution, efficient, lightweight x-ray gratings with silicon pore optics to provide R~2500 with an average effective area of ~200 cm2, an order of magnitude larger than the Chandra gratings. Flight-proven CCDs and instrument electronics are strong heritage components, while spacecraft and mission operations also reuse highly successful designs.
Launched on 2021 December 9, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer Mission in collaboration with the Italian Space Agency (ASI). The mission will open a new window of investigation—imaging x-ray polarimetry. The observatory features three identical telescopes, each consisting of a mirror module assembly with a polarization-sensitive imaging x-ray detector at the focus. A coilable boom, deployed on orbit, provides the necessary 4-m focal length. The observatory utilizes a three-axis-stabilized spacecraft, which provides services such as power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of x-ray sources, with follow-on observations of selected targets.
Scheduled to launch in late 2021 the Imaging X-ray Polarimetry Explorer (IXPE) is a Small Explorer Mission designed to open up a new window of investigation -- X-ray polarimetry. The IXPE observatory features 3 identical telescope each consisting of a mirror module assembly with a polarization-sensitive imaging x-ray detector at its focus. An extending beam, deployed on orbit provides the necessary 4 m focal length. The payload sits atop a 3-axis stabilized spacecraft which among other things provides power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of x-ray sources, with follow-on observations of selected targets. IXPE is a partnership between NASA and the Italian Space Agency (ASI).
We describe an implementation of a broad-band soft X-ray polarimeter, substantially based on previous designs. The Globe-Orbiting Soft X-ray Polarimeter (GOSoX) is a SmallSat. As in a related mission concept the PiSoX Polarimeter, the grating arrangement is designed optimally for the purpose of polarimetry matching the dispersion of a spectrometer to a laterally graded multilayer (LGML). For GOSoX, the optics are lightweight Si mirrors in a one-bounce parabolic configuration. The instrument covers the wavelength range from 31 A to 75 A (165 - 400 eV). Upon satellite rotation, the intensities of the dispersed spectra, after reflection and polarizing by the LGMLs, give the three Stokes parameters needed to determine a source's linear polarization fraction and orientation. The design can be extended to higher energies as LGMLs are developed further. We describe the potential scientific return and the proposed mission concept following the results of a JPL Team X concept study.
The soft x-ray band covers the characteristic lines of the highly ionized low-atomic-number elements, providing diagnostics of the warm and hot plasmas in star atmospheres, interstellar dust, galaxy halos and clusters, and the cosmic web. High-resolution spectroscopy in this band is best performed with grating spectrometers. Soft x-ray grating spectroscopy with R = λ / Δ λ = > 104 has been demonstrated with critical-angle transmission (CAT) gratings. CAT gratings combine the relaxed alignment and temperature tolerances and the low mass of transmission gratings with high diffraction efficiency blazed in high orders. They are an enabling technology for the proposed Arcus grating explorer and were selected for the Lynx Design Reference Mission grating spectrometer instrument. Both Arcus and Lynx require the manufacture of hundreds to perhaps ~2000 large-area CAT gratings. We are moving toward CAT grating volume manufacturing using 200 mm silicon-on-insulator wafers, 4X optical projection lithography tools, deep reactive-ion etching, and KOH polishing. We have, for the first time, produced high-throughput 200 nm-period CAT gratings ~50% deeper than previously fabricated. X-ray diffraction efficiency is significantly improved in the ~1:25 - 1.75 nm wavelength range, peaking above 40% (sum of blazed orders). A new grating-to-grating alignment technique utilizing cross-support diffraction of visible light is presented, as well as the results of CAT grating emissivity measurements.λ
IXPE, the Imaging X-ray Polarimetry Explorer, is a NASA SMEX mission with an important contribution of ASI that will be launched with a Falcon 9 in 2021 and will reopen the window of X-ray polarimetry after more than 40 years. The payload features three identical telescopes each one hosting one light-weight X-ray mirror fabricated by MSFC and one detector unit with its in-orbit calibration system and the Gas Pixel Detector sensitive to imaging X-ray polarization fabricated by INAF/IAPS, INFN and OHB Italy. The focal length after boom deployment from ATK-Orbital is 4 m, while the spacecraft is being fabricated by Ball Aerospace. The sensitivity will be better than 5.5% in 300 ks for a 1E-11 erg/s/cm2 (half mCrab) in the energy band of 2-8 keV allowing for sensitive polarimetry of extended and point-like X-ray sources. The focal plane instrument is completed, calibrated and it is going to be delivered at MSFC. We will present the status of the mission at about one year from the launch.
X-ray polarimetry is still largely uncharted territory. With the upcoming launch of IXPE, we will learn a lot more about X-ray polarization at energies above 2 keV, but so far no current or accepted mission provides observational capabilities below 2 keV. We present ray-tracing results for a small orbital mission that could be launched within NASA’s Pioneer or SmallSat cost-cap to provide X-ray polarimetry below 2 keV. The design is based on the use of laterally-graded multi-layer (ML) mirrors, a concept that we have developed theoretically for the REDSoX Polarimeter,1 for which most components have been verified in the laboratory. In this contribution, we describe a single channel orbital mission based on the same idea, but modified to the unique cost and space requirements. All results scale up easily to two or more polarimetry channels. Scaling up would simply increase the effective area and reduce the need to rotate the instrument to measure the different polarization directions. In particular, we use the ray-traces to define the maximum size of the dispersion gratings and to determine an alignment budget.
The Chandra X-ray Observatory (CXO) was launched over 21 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imager Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of -120°C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60°C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. All three have exhibited significant changes with time. In this paper, we present a revision to the time dependence of the accumulation rate. In a previous paper, we described a model in which the accumulation rate started to decrease in 2017. We present new data which show that the accumulation rate has been roughly constant and linear from 2017 until the present. We show that the current calibration file underestimates the additional absorption of the contamination layer by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC) and spectral data from the the cluster of galaxies known as Abell 1795. We present a revised model that produces consistent line normalizations and fluxes in 0.5–1.0 and 1.0-2.0 keV bands for these two sources over the course of the mission. This revised model is expected to be released in December 2020 in the next release of the CXO The Chandra X-ray Observatory (CXO) was launched over 21 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imager Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of -120 C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60 C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. All three have exhibited significant changes with time. In this paper, we present a revision to the time dependence of the accumulation rate. In a previous paper, we described a model in which the accumulation rate started to decrease in 2017. We present new data which show that the accumulation rate has been roughly constant and linear from 2017 until the present. We show that the current calibration file underestimates the additional absorption of the contamination layer by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC) and spectral data from the the cluster of galaxies known as Abell 1795. We present a revised model that produces consistent line normalizations and fluxes in 0.5–1.0 and 1.0-2.0 keV bands for these two sources over the course of the mission. This revised model is expected to be released in December 2020 in the next release of the CXO Calibration Databasealibration Database
We present an update on our work measuring the performance and alignment of the critical-angle transmission (CAT) gratings for the proposed sounding Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX) mission, as well as a possible orbital version. We built and verified a grating alignment system that could be used for REDSoX Polarimeter fabrication. The performances of the gratings were measured using the MIT polarimetry beamline. The beamline is a monochromator and has been used to measure the absolute efficiencies of not only the REDSoX prototype gratings but also the Arcus Phase A gratings. It is also capable of producing and measuring polarized soft X-rays to aid in the development and testing of future missions. Lastly, we present an update on our effort applying twisted crystals to X-ray polarimetry. Support for this work was provided in part by the NASA grant NNX15AL14G and a grant from the MIT Kavli Institute Research Investment Fund.
We describe a new implementation of a broad-band soft X-ray polarimeter, substantially based on a previous design. This implementation, the Pioneer Soft X-ray Polarimeter (PiSoX) is a SmallSat, designed for NASA’s call for Astrophysics Pioneers, small missions that could be CubeSats, balloon experiments, or SmallSats. As in REDSoX, the grating arrangement is designed optimally for the purpose of polarimetry with broad-band focussing optics by matching the dispersion of the spectrometer channels to laterally graded multilayers (LGMLs). The system can achieve polarization modulation factors over 90%. For PiSoX, the optics are lightweight Si mirrors in a one-bounce parabolic configuration. High efficiency, blazed gratings from opposite sectors are oriented to disperse to a LGML forming a channel covering the wavelength range from 35 Å to 75 Å (165 - 350 eV). Upon satellite rotation, the intensities of the dispersed spectra, after reflection and polarizing by the LGMLs, give the three Stokes parameters needed to determine a source’s linear polarization fraction and orientation. The design can be extended to higher energies as LGMLs are developed further. We describe examples of the potential scientific return from instruments based on this design.
We present a novel nested-shell Wolter x-ray telescope design that can achieve diffraction-limited performance over the entire telescope aperture. The new design features a compact single-spacecraft mirror assembly, similar in form to that proposed for Lynx, and a separate detector spacecraft. A wide range of design parameter space was considered. We present a specific example which features 10 micro-arcsec resolution, a wide band (0.1-10 keV) with > 2 m2 effective area out to 10 keV, a large flat field (>1010 pixels), achieving diffraction-limited performance (Strehl ratio > 0.8). We will also briefly discuss a potential science case for this telescope.
The Imaging X-ray Polarimetry Explorer (IXPE) will add polarization to the properties (time, energy, and position) observed in x-ray astronomy. A NASA Astrophysics Small Explorer (SMEX) in partnership with the Italian Space Agency (ASI), IXPE will measure the 2–8-keV polarization of a few dozen sources during the first 2 years following its 2021 launch. The IXPE Observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazingincidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU), separated by a deployable optical bench. The Observatory’s Spacecraft provides typical subsystems (mechanical, structural, thermal, power, electrical, telecommunications, etc.), an attitude determination and control subsystem for 3-axis stabilized pointing, and a command and data handling subsystem communicating with the science instrument and the Spacecraft subsystems.
We present the performance and recent results of the MIT polarimetry beamline. Originally designed for testing Chandra HETG gratings, the beamline has been adapted to test components for soft x-ray polarimetry applications. Since then, its monochromator capabilities have also been used to test gratings. We present results on the measured absolute efficiencies of the Arcus Phase A gratings using the B-K, O-K, and C-K emission lines. The beamline has also been used to develop tools and techniques to measure the linear polarization of soft X-rays (0.2-0.8 keV), which form the basis for a sounding rocket mission REDSoX (Rocket Experiment Demonstration of a Soft X-ray Polarimeter) and a possible orbital mission. We present our tests to align the REDSoX gratings, as well as our idea to use thin twisted crystals as a possible alternative to laterally-graded multilayer mirrors. Support for this work was provided in part by the NASA grant NNX15AL14G and a grant from the MIT Kavli Institute.
Arcus provides high-resolution soft X-ray spectroscopy in the 12-50 Å bandpass with unprecedented sensitivity, including spectral resolution < 2500 and effective area < 250 cm2. The three top science goals for Arcus are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback, and (3) to explore how stars form and evolve. Arcus uses the same 12 m focal length grazing-incidence Silicon Pore X-ray Optics (SPOs) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. Combined with the high-heritage NGIS LEOStar-2 spacecraft and launched into 4:1 lunar resonant orbit, Arcus provides high sensitivity and high efficiency observing of a wide range of astrophysical sources.
The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX Polarimeter) is a sounding rocket instrument that can make the first measurement of the linear X-ray polarization of an extragalactic source in the 0.2-0.5 keV band as low as 10%. We employ multilayer-coated mirrors as Bragg reflectors at the Brewster angle. By matching the dispersion of a dispersive spectrometer using critical angle transmission gratings to three laterally graded multilayer mirrors (LGMLs), we achieve polarization modulation factors over 90%. We will describe new prototyping work as well as extensions of the design for an orbital version.
This work was supported in part by NASA grants NNX15AL14G and NNX17AE11G to develop the design for a soft X-ray polarimeter.
The Imaging X-ray Polarimetry Explorer (IXPE) will expand the information space for study of cosmic sources, by adding polarization to the properties (time, energy, and position) observed in x-ray astronomy. Selected in 2017 January as a NASA Astrophysics Small Explorer (SMEX) mission, IXPE will be launched into an equatorial orbit in 2021. The IXPE observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazing-incidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU). The optical bench separating the MMAs from the DUs is a deployable boom with a tip/tilt/rotation stage for DU-to-MMA (gang) alignment, similar to the configuration used for the NuSTAR observatory. The IXPE mission will provide scientifically meaningful measurements of the x-ray polarization of a few dozen sources in the 2-8 keV band, over the first two years of the mission. For several bright, extended x-ray sources (pulsar wind nebulae, supernova remnants, and an active-galaxy jet), IXPE observations will produce polarization maps indicating the magnetic structure of the synchrotron emitting regions. For many bright pulsating x-ray sources (isolated pulsars, accreting x-ray pulsars, and magnetars), IXPE observations will produce phase-resolved profiles of the polarization degree and position angle.
We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be "standard candles" in the classic sense, in that the only constraint placed on the source flux is that it is the same for all instruments. We apply a technique developed by Chen et al. (submitted to J. Amer. Stat. Association) that involves a popular statistical method called shrinkage estimation, which effectively reduces the noise in disparate measurements by combining information across common observations. We can then determine effective area correction factors for each instrument that brings all observatories into the best agreement, consistent with prior knowledge of their effective areas. We have preliminary values that characterize systematic uncertainties in effective areas for almost all operational (and some past) X-ray astronomy instruments in bands covering factors of two in photon energy from 0.15 keV to 300 keV. We demonstrate the method with several data sets from Chandra and XMM-Newton.
Arcus is a high-resolution soft x-ray spectroscopy mid-size Explorer mission selected for a NASA Phase A concept study. It is designed to explore structure formation through measurements of hot baryon distributions, feedback from black holes, and the formation and evolution of stars, disks, and exoplanet atmospheres. The design provides unprecedented sensitivity in the 1.2-5 nm wavelength band with effective area up to 350 cm2 and spectral resolving power R > 2500. The Arcus technology is based on a highly modular design that features 12 m-focal length silicon pore optics (SPO) developed for the European Athena mission, and critical-angle transmission (CAT) x-ray diffraction gratings and x-ray CCDs developed at MIT. CAT gratings are blazed transmission gratings that have been under technology development for over ten years. We describe technology demonstrations of increasing complexity, including mounting of gratings to frames, alignment, environmental testing, integration into arrays, and performance under x-ray illumination with SPOs, using methods proposed for the manufacture of the Arcus spectrometers. CAT gratings have demonstrated efficiency > 30%. Measurements of the 14th order Mg-Kα1,2 doublet from a co-aligned array of four CAT gratings illuminated by two co-aligned SPOs matches ray trace predictions and exceeds Arcus resolving power requirements. More than 700 CAT gratings will be produced using high-volume semiconductor industry tools and special techniques developed at MIT
The Chandra X-ray Observatory (CXO) was launched almost 19 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imager Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of -120°C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60°C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. All three have exhibited significant changes with time. There has been a dramatic decrease in the accumulation rate of the contaminant starting in 2017. The lower accumulation rate may be due to a decrease in the deposition rate or an increase in the vaporization rate or a combination of the two. We show that the current calibration file which models the additional absorption of the contamination layer is significantly overestimating that additional absorption by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC). In addition, spectral data from the cluster of galaxies known as Abell 1795 and the Blazar Markarian 421 are used to generate a model of the absorption produced by the contamination layer. The Chandra X-ray Center (CXC) calibration team is preparing a revised calibration file that more accurately represents the complex time dependence of the accumulation rate, the spatial dependence, and the chemical composition of the contaminant. Given the rapid changes in the contamination layer over the past year, future calibration observations at a higher cadence will be necessary to more accurately monitor such changes.
We describe an optical design and possible implementation of a broadband soft x-ray polarimeter. Our arrangement of gratings is designed optimally for the purpose of polarimetry with broadband focusing optics by matching the dispersion of the spectrometer channels to laterally graded multilayers (LGMLs). The system can achieve polarization modulation factors over 90%. We implement this design using a single optical system by dividing the entrance aperture into six sectors; high efficiency, blazed gratings from opposite sectors are oriented to disperse to a common LGML forming three channels covering the wavelength range from 35 to 75 Å (165 to 350 eV). The grating dispersions and LGML position angles for each channel are 120 deg to each other. CCD detectors then measure the intensities of the dispersed spectra after reflection and polarizing by the LGMLs, giving the three Stokes parameters needed to determine a source’s linear polarization fraction and orientation. The design can be extended to higher energies as LGMLs are developed further. We describe examples of the potential scientific return from instruments based on this design.
A simple design of a soft x-ray polarimeter using multilayer mirrors is presented. A multilayer mirror acts as a linear polarization analyzer for x-rays at incidence angles close to the Brewster angle. The instrument consists of an x-ray concentrator, a set of multilayer mirrors placed at 45 deg from the optical axis, and a detector at Nasmyth focus. The instrument rotating about its optical axis during observations can measure the linear polarization of 0.2- to 0.7-keV x-rays from astronomical sources. The use of a soft x-ray concentrator with geometrical area ∼630 cm2 provides sufficient sensitivity to address key scientific questions. Five different multilayer mirrors placed on a rotating wheel provide the option to measure polarization in any of the five narrow bands spanning the 0.2- to 0.7-keV range. Design and estimated performance of the design are discussed.
X-ray polarimetry offers a new window into the high-energy universe, yet there has been no instrument so far that could measure the polarization of soft X-rays (about 17-80 Å) from astrophysical sources. The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX Polarimeter) is a proposed sounding rocket experiment that uses a focusing optic and splits the beam into three channels. Each channel has a set of criticalangle transmission (CAT) gratings that disperse the x-rays onto a laterally graded multilayer (LGML) mirror, which preferentially reflects photons with a specific polarization angle. The three channels are oriented at 120 deg to each other and thus measure the three Stokes parameters: I, Q, and U. The period of the LGML changes with position. The main design challenge is to arrange the gratings so that they disperse the spectrum in such a way that all rays are dispersed onto the position on the multi-layer mirror where they satisfy the local Bragg condition despite arriving on the mirror at different angles due to the converging beam from the focusing optics. We present a polarimeteric Monte-Carlo ray-trace of this design to assess non-ideal effects from e.g. mirror scattering or the finite size of the grating facets. With mirror properties both simulated and measured in the lab for LGML mirrors of 80-200 layers we show that the reflectivity and the width of the Bragg-peak are sufficient to make this design work when non-ideal effects are included in the simulation. Our simulations give us an effective area curve, the modulation factor and the figure of merit for the REDSoX polarimeter. As an example, we simulate an observation of Mk 421 and show that we could easily detect a 20% linear polarization.
The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX Polarimeter) is a sounding rocket instrument that can make the first measurement of the linear X-ray polarization of an extragalactic source in the 0.2-0.8 keV band as low as 10%. We employ multilayer-coated mirrors as Bragg reflectors at the Brewster angle. By matching the dispersion of a spectrometer using replicated optics from MSFC and critical angle transmission gratings from MIT to three laterally graded multilayer mirrors (LGMLs), we achieve polarization modulation factors over 90%. We present a novel arrangement of gratings, designed optimally for the purpose of polarimetry with a converging beam. The entrance aperture is divided into six equal sectors; pairs of blazed gratings from opposite sectors are oriented to disperse to the same LGML. The LGML position angles are 120 degrees to each other. CCD detectors then measure the intensities of the dispersed spectra after reflection and polarizing by the LGMLs, giving the three Stokes parameters needed to determine a source’s linear polarization fraction and orientation. A current grant is funding further development to improve the LGMLs. Sample gratings for the project have been fabricated at MIT and the development team continues to improve them under separate funding. Our technological approach is the basis for a possible orbital mission
We present continued development of components for measuring linear X-ray polarization over the 0.2-0.8 keV (15-62 Angstrom) band. We present results from measurements of new laterally graded multilayer mirrors and critical angle transmission gratings essential to the approach. While the lab is designed to verify components to be used in a soft X-ray polarimeter, it is reconfigurable and has been used to verify grating efficiencies with our new CCD detector. Our development work is the basis for a sounding rocket mission (Rocket Experiment Demonstration of a Soft X-ray Polarimeter) and future orbital missions.
During its first 18 years of operation, the cold (about -60°C) optical blocking filters of the Advanced CCD Imaging Spectrometer (ACIS), aboard the Chandra X-ray Observatory, has accumulated a growing layer of molecular contamination, which attenuates low-energy x rays. Over the past several years, the accumulation rate, spatial distribution, and composition have changed. This evolution has motivated further analysis of contamination migration within and near the ACIS cavity, in part to evaluate potential bake-out scenarios intended to reduce the level of contamination. This paper, the fourth on this topic, reports the results of recent contamination-migration simulations and their relevance to a decision whether to bake-out the ACIS instrument.
P. Soffitta, R. Bellazzini, E. Bozzo, V. Burwitz, A. Castro-Tirado, E. Costa, T. Courvoisier, H. Feng, S. Gburek, R. Goosmann, V. Karas, G. Matt, F. Muleri, K. Nandra, M. Pearce, J. Poutanen, V. Reglero, D. Sabau Maria, A. Santangelo, G. Tagliaferri, C. Tenzer, J. Vink, M. Weisskopf, S. Zane, I. Agudo, A. Antonelli, P. Attina, L. Baldini, A. Bykov, R. Carpentiero, E. Cavazzuti, E. Churazov, E. Del Monte, D. De Martino, I. Donnarumma, V. Doroshenko, Y. Evangelista, I. Ferreira, E. Gallo, N. Grosso, P. Kaaret, E. Kuulkers, J. Laranaga, L. Latronico, D. Lumb, J. Macian, J. Malzac, F. Marin, E. Massaro, M. Minuti, C. Mundell, J. U. Ness, T. Oosterbroek, S. Paltani, G. Pareschi, R. Perna, P.-O. Petrucci, H. B. Pinazo, M. Pinchera, J. P. Rodriguez, M. Roncadelli, A. Santovincenzo, S. Sazonov, C. Sgro, D. Spiga, J. Svoboda, C. Theobald, T. Theodorou, R. Turolla, E. Wilhelmi de Ona, B. Winter, A. M. Akbar, H. Allan, R. Aloisio, D. Altamirano, L. Amati, E. Amato, E. Angelakis, J. Arezu, J.-L. Atteia, M. Axelsson, M. Bachetti, L. Ballo, S. Balman, R. Bandiera, X. Barcons, S. Basso, A. Baykal, W. Becker, E. Behar, B. Beheshtipour, R. Belmont, E. Berger, F. Bernardini, S. Bianchi, G. Bisnovatyi-Kogan, P. Blasi, P. Blay, A. Bodaghee, M. Boer, M. Boettcher, S. Bogdanov, I. Bombaci, R. Bonino, J. Braga, W. Brandt, A. Brez, N. Bucciantini, L. Burderi, I. Caiazzo, R. Campana, S. Campana, F. Capitanio, M. Cappi, M. Cardillo, P. Casella, O. Catmabacak, B. Cenko, P. Cerda-Duran, C. Cerruti, S. Chaty, M. Chauvin, Y. Chen, J. Chenevez, M. Chernyakova, C. C. Cheung, D. Christodoulou, P. Connell, R. Corbet, F. Coti Zelati, S. Covino, W. Cui, G. Cusumano, A. D’Ai, F. D’Ammando, M. Dadina, Z. Dai, A. De Rosa, L. de Ruvo, N. Degenaar, M. Del Santo, L. Del Zanna, G. Dewangan, S. Di Cosimo, N. Di Lalla, G. Di Persio, T. Di Salvo, T. Dias, C. Done, M. Dovciak, G. Doyle, L. Ducci, R. Elsner, T. Enoto, J. Escada, P. Esposito, C. Eyles, S. Fabiani, M. Falanga, S. Falocco, Y. Fan, R. Fender, M. Feroci, C. Ferrigno, W. Forman, L. Foschini, C. Fragile, F. Fuerst, Y. Fujita, J. L. Gasent-Blesa, J. Gelfand, B. Gendre, G. Ghirlanda, G. Ghisellini, M. Giroletti, D. Goetz, E. Gogus, J.-L. Gomez, D. Gonzalez, R. Gonzalez-Riestra, E. Gotthelf, L. Gou, P. Grandi, V. Grinberg, F. Grise, C. Guidorzi, N. Gurlebeck, T. Guver, D. Haggard, M. Hardcastle, D. Hartmann, C. Haswell, A. Heger, M. Hernanz, J. Heyl, L. Ho, J. Hoormann, J. Horak, J. Huovelin, D. Huppenkothen, R. Iaria, C. Inam Sitki, A. Ingram, G. Israel, L. Izzo, M. Burgess, M. Jackson, L. Ji, J. Jiang, T. Johannsen, C. Jones, S. Jorstad, J. J. E. Kajava, M. Kalamkar, E. Kalemci, T. Kallman, A. Kamble, F. Kislat, M. Kiss, D. Klochkov, E. Koerding, M. Kolehmainen, K. Koljonen, S. Komossa, A. Kong, S. Korpela, M. Kowalinski, H. Krawczynski, I. Kreykenbohm, M. Kuss, D. Lai, M. Lan, J. Larsson, S. Laycock, D. Lazzati, D. Leahy, H. Li, J. Li, L.-X. Li, T. Li, Z. Li, M. Linares, M. Lister, H. Liu, G. Lodato, A. Lohfink, F. Longo, G. Luna, A. Lutovinov, S. Mahmoodifar, J. Maia, V. Mainieri, C. Maitra, D. Maitra, A. Majczyna, S. Maldera, D. Malyshev, A. Manfreda, A. Manousakis, R. Manuel, R. Margutti, A. Marinucci, S. Markoff, A. Marscher, H. Marshall, F. Massaro, M. McLaughlin, G. Medina-Tanco, M. Mehdipour, M. Middleton, R. Mignani, P. Mimica, T. Mineo, B. Mingo, G. Miniutti, S. M. Mirac, G. Morlino, A. Motlagh, S. Motta, A. Mushtukov, S. Nagataki, F. Nardini, J. Nattila, G. Navarro, B. Negri, Matteo Negro, S. Nenonen, V. Neustroev, F. Nicastro, A. Norton, A. Nucita, P. O’Brien, S. O’Dell, H. Odaka, B. Olmi, N. Omodei, M. Orienti, M. Orlandini, J. Osborne, L. Pacciani, V. Paliya, I. Papadakis, A. Papitto, Z. Paragi, P. Pascal, B. Paul, L. Pavan, A. Pellizzoni, E. Perinati, M. Pesce-Rollins, E. Piconcelli, A. Pili, M. Pilia, M. Pohl, G. Ponti, D. Porquet, A. Possenti, K. Postnov, I. Prandoni, N. Produit, G. Puehlhofer, B. Ramsey, M. Razzano, N. Rea, P. Reig, K. Reinsch, T. Reiprich, M. Reynolds, G. Risaliti, T. Roberts, J. Rodriguez, M. Rossi, S. Rosswog, A. Rozanska, A. Rubini, B. Rudak, D. Russell, F. Ryde, S. Sabatini, G. Sala, M. Salvati, M. Sasaki, T. Savolainen, R. Saxton, S. Scaringi, K. Schawinski, N. Schulz, A. Schwope, P. Severgnini, M. Sharon, A Shaw, A. Shearer, X. Shesheng, I. -C. Shih, K. Silva, R. Silva, E. Silver, A. Smale, F. Spada, G. Spandre, A. Stamerra, B. Stappers, S. Starrfield, L. Stawarz, N. Stergioulas, A. Stevens, H. Stiele, V. Suleimanov, R. Sunyaev, A. Slowikowska, F. Tamborra, F. Tavecchio, R. Taverna, A. Tiengo, L. Tolos, F. Tombesi, J. Tomsick, H. Tong, G. Torok, D. Torres, A. Tortosa, A. Tramacere, V. Trimble, G. Trinchieri, S. Tsygankov, M. Tuerler, S. Turriziani, F. Ursini, P. Uttley, P. Varniere, F. Vincent, E. Vurgun, C. Wang, Z. Wang, A. Watts, J. Wheeler, K. Wiersema, R. Wijnands, J. Wilms, A. Wolter, K. Wood, K. Wu, X. Wu, W. Xiangyu, F. Xie, R. Xu, S.-P. Yan, J. Yang, W. Yu, F. Yuan, A. Zajczyk, D. Zanetti, R. Zanin, C. Zanni, L. Zappacosta, A. Zdziarski, A. Zech, H. Zhang, S. Zhang, W. Zhang, A. Zoghbi
XIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of
writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the
polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently
exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrally-spatially-
resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics.
Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective
area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega
launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest
Observer competitive program and it is organized as a consortium across Europe with main contributions from
Italy, Germany, Spain, United Kingdom, Poland, Sweden.
We study a critical angle transmission (CAT) grating spectrograph that delivers a spectral resolution significantly above any X-ray spectrograph ever own. This new technology will allow us to resolve kinematic components in absorption and emission lines of galactic and extragalactic matter down to unprecedented dispersion levels. We perform ray-trace simulations to characterize the performance of the spectrograph in the context of an X-ray Surveyor or Arcus like layout (two mission concepts currently under study). Our newly developed ray-trace code is a tool suite to simulate the performance of X-ray observatories. The simulator code is written in Python, because the use of a high-level scripting language allows modifications of the simulated instrument design in very few lines of code. This is especially important in the early phase of mission development, when the performances of different configurations are contrasted. To reduce the run-time and allow for simulations of a few million photons in a few minutes on a desktop computer, the simulator code uses tabulated input (from theoretical models or laboratory measurements of samples) for grating efficiencies and mirror reflectivities. We find that the grating facet alignment tolerances to maintain at least 90% of resolving power that the spectrometer has with perfect alignment are (i) translation parallel to the optical axis below 0.5 mm, (ii) rotation around the optical axis or the groove direction below a few arcminutes, and (iii) constancy of the grating period to 1:105. Translations along and rotations around the remaining axes can be significantly larger than this without impacting the performance.
The Chandra X-ray Observatory (CXO) was launched 16 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imager Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of -120 C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60 C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. All three have exhibited significant changes with time. The calibration team within the Chandra X-ray Center (CXC) generates calibration files that describe the additional absorption produced by the contamination layer as a function of time, position, and energy. We have verified the accuracy of this contamination file for the on-axis aimpoints using the standard model spectrum for the Supernova Remnant 1E 0102.2-7219 in the Small Magellanic Cloud developed by the International Consortium for High Energy Calibration (IACHEC), but we show the model is less accurate for the off-axis positions after 2013. In 2015, the ACIS Detector Housing heater was turned on to increase the temperature of the OBF in the hope that the accumulation rate of the contamination layer would decrease. We show that the accumulation rate of the contaminant is unchanged since the DH heater was turned on.
The Imaging X-ray Polarimetry Explorer (IXPE) expands observation space by simultaneously adding polarization measurements to the array of source properties currently measured (energy, time, and location). IXPE will thus open new dimensions for understanding how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions—such as neutron stars and black holes. Polarization singularly probes physical anisotropies—ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin—that are not otherwise measurable. Hence, IXPE complements all other investigations in high-energy astrophysics by adding important and relatively unexplored information to the parameter space for studying cosmic X-ray sources and processes, as well as for using extreme astrophysical environments as laboratories for fundamental physics.
During its first 16 years of operation, the cold (about -60°C) optical blocking filter of the Advanced CCD Imaging Spectrometer (ACIS), aboard the Chandra X-ray Observatory, has accumulated a growing layer of molecular contamination that attenuates low-energy x rays. Over the past few years, the accumulation rate, spatial distribution, and composition have changed. This evolution has motivated further analysis of contamination migration within and near the ACIS cavity, in part to evaluate potential bake-out scenarios intended to reduce the level of contamination.
We present continued development of laterally graded multilayer mirrors (LGMLs) for a telescope design capable of measuring linear X-ray polarization over a broad spectral band. The multilayer-coated mirrors are used as Bragg reflectors at the Brewster angle. By matching to the dispersion of a spectrometer, one may take advantage of high multilayer reflectivities and achieve modulation factors near 100%. In Phase II of the polarimetry beam- line development, we demonstrated that the system provides 100% polarized X-rays at 0.525 keV (Marshall et al. 2013). In Phase III of the polarimetry beam-line development, we installed an LGML in the source to polarize a wide range of energies between 0.15 and 0.70 keV (Marshall et al. 2014). Here, we present results from continued development of the LGMLs to improve reflectivity in the band of interest, a blazed reflection grating that is suitable for a small flight instrument, and a new detector with a directly deposited optical blocking filter. We also present updated plans for a suborbital rocket experiment designed to detect a polarization level of better than 10% for an active galactic nucleus.
ABSTRACT We present continued development of laterally graded multilayer mirrors (LGMLs) for a telescope design capable of measuring linear X-ray polarization over a broad spectral band. The multilayer-coated mirrors are used as Bragg re ectors at the Brewster angle. By matching to the dispersion of a spectrometer, one may take advantage of high multilayer re ectivities and achieve modulation factors over 50% over the entire 0.2-0.8 keV band. In Phase II of the polarimetry beam-line development, we demonstrated that the system provides 100% polarized X-rays at 0.525 keV (Marshall et al. 2013). Here, we present results from phase III of our development, where a LGML is used at the source and laterally manipulated in order to select and polarize X-rays from emission lines for a variety of source anodes. The beam-line will then provide the capability to test polarimeter components across the 0.15-0.70 keV band. We also present plans for a suborbital rocket experiment designed to detect a polarization level of better than 10% for an active galactic nucleus.
During its first 14 years of operation, the cold (about -60°C) optical blocking filter of the Advanced CCD Imaging
Spectrometer (ACIS), aboard the Chandra X-ray Observatory, has accumulated a growing layer of molecular
contamination that attenuates low-energy x rays. Over the past few years, the accumulation rate, spatial distribution, and
composition have changed. This evolution has motivated further analysis of contamination migration within and near the
ACIS cavity. To this end, the current study employs a higher-fidelity geometric model of the ACIS cavity, detailed
thermal modeling based upon temperature data, and a refined model of the molecular transport.
We are developing instrumentation for a telescope design capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics. Multilayer-coated mirrors are key to this approach, being used as Bragg reflectors at the Brewster angle. By laterally grading the multilayer mirrors and matching to the dispersion of a spectrometer, one may take advantage of high multilayer reflectivities and achieve modulation factors over 50% over the entire 0.2-0.8 keV band. We present progress on laboratory work to demonstrate the capabilities of an existing laterally graded multilayer coated mirror pair. We also present plans for a suborbital rocket experiment designed to detect a polarization level of 12-17% for an active galactic nucleus in the 0.1-1.0 keV band.
MARX is a portable ray-trace program that was originally developed to simulate event data from the trans-
mission grating spectrometers on-board the Chandra X-ray Observatory (CXO). MARX has since evolved to
include detailed models of all CXO science instruments and has been further modified to serve as an event
simulator for future X-ray observatory design concepts. We first review a number of CXO applications of MARX to demonstrate the roles such a program could play throughout the life of a mission, including its design and calibration, the production of input data products for the development of the various software pipelines, and for observer proposal planning. We describe how MARX was utilized in the design of a proposed future X-ray spectroscopy mission called ÆGIS (Astrophysics Experiment for Grating and Imaging Spectroscopy), a mission concept optimized for the 0.2 to 1 keV soft X-ray band. ÆGIS consists of six independent Critical Angle Transmission Grating Spectrometers (CATGS) arranged to provide a resolving power of 3000 and an effective area exceeding 1000 cm2 across its passband. Such high spectral resolution and effective area will permit ÆGIS to address many astrophysics questions including those that pertain to the evolution of Large Scale Structure of the universe, and the behavior of matter at very high densities. The MARX ray-trace of the ÆGIS spectrometer yields quantitative estimates of how the spectrometer’s performance is affected by misalignments between the various system elements, and by deviations of those elements from their idealized geometry. From this information, we are able to make the appropriate design
tradeoffs to maximize the performance of the system.
The efficiencies of the gratings in the High Energy Transmission Grating Spectrometer (HETGS) were
updated using in-light observations of bright continuum sources. The procedure first involved verifying that fluxes obtained from the +1 and -1 orders match, which checks that the contaminant model and the CCD quantum efficiencies agree. Then the fluxes derived using the high energy gratings (HEGs) were compared to those derived from the medium energy gratings (MEGs). The flux ratio was fit to a low order polynomial, which was allocated to the MEGs above 1 keV or the HEGs below 1 keV. The resultant efficiencies were tested by examining fits to blazar spectra.
We developed an instrument design capable of measuring linear X-ray polarization over a broad-band using
conventional spectroscopic optics, using a method previously described by Marshall (2008) involving laterally
graded, multilayer-coated flat mirrors. We present possible science investigations with such an instrument and
two possible configurations. This instrument could be used in a small orbiting mission or scaled up for the
International X-ray Observatory. Laboratory work has begun that would demonstrate the capabilities of key
components.
High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central
to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows
in Active Galactic Nuclei, coronal emission from stars, etc. The large collecting area, long focal length, and 5
arcsecond half power diameter telescope point-spread function of the International X-ray Observatory will present
unprecedented opportunity for a grating spectrometer to address these areas at the forefront of astronomy and
astrophysics. We present the current status of a transmission grating spectrometer based on recently developed
high-efficiency critical-angle transmission (CAT) gratings that combine the traditional advantages of blazed
reflection and transmission gratings. The optical design places light-weight grating arrays close to the telescope
mirrors, which maximizes dispersion distance and thus spectral resolution and minimizes demands on mirror
performance. It merges features from the Chandra High Energy Transmission Grating Spectrometer and the
XMM-Newton Reflection Grating Spectrometer, and provides resolving power R = E/ΔE = 3000 - 5000 (full
width half max) and effective area >1000 cm2 in the soft x-ray band. We discuss recent results on ray-tracing
and optimization of the optical design, instrument configuration studies, and grating fabrication.
Multilayer-coated optics can strongly polarize X-rays and are central to a new design of a broad-band, soft X-ray
polarimeter. We have begun laboratory work to verify the performance of components that could be used in
future soft X-ray polarimetric instrumentation. We have reconfigured a 17 meter beamline facility, originally
developed for testing transmission gratings for Chandra, to include a polarized X-ray source, an X-ray-dispersing
transmission grating, and a multilayer-coated optic that illuminates a CCD detector. The X-rays produced from
a Manson Model 5, multi-anode source are polarized by a multilayer-coated flat mirror. The current configuration
allows for a 180 degree rotation of the source in order to rotate the direction of polarization. We will present
progress in source characterization and system modulation measurements as well as null and robustness tests.
We present a high-resolution soft x-ray grating spectrometer concept for the International X-Ray Observatory
(IXO) that meets or exceeds the minimum requirements for effective area (> 1, 000 cm2 for E < 1 keV) and
spectral resolution (E/▵E > 3, 000). At the heart of the spectrometer is an array of recently developed highefficiency
blazed transmission gratings, the so-called critical-angle transmission (CAT) gratings. They combine
the advantages of traditional transmission gratings (very low mass, extremely relaxed alignment and flatness tolerances)
with those of x-ray reflection gratings (high efficiency due to blazing in the direction of grazing-incidence
reflection). In addition, a CAT grating spectrometer is well-suited for co-existence with energy-dispersive highenergy
focal plane detectors, since most high-energy x rays are neither absorbed, nor diffracted, and contribute
to the effective area at the telescope focus. Since our initial successful x-ray demonstrations of the CAT grating
concept with large-period and lower aspect-ratio prototypes, we have now microfabricated 200 nm-period silicon
CAT gratings comprised of grating bars with the required dimensions (6 micron tall, 40 nm wide, aspect ratio
150), optimized for the 0.3 to 1.0 keV energy band. Preliminary analysis of recent x-ray tests show blazing
behavior up to 1.28 keV in accordance with predictions.
An approach for measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics is
described. A set of multilayer-coated flats reflect the dispersed X-rays to the instrument detectors. The intensity
variation as a function of energy and position angle is measured to determine three Stokes parameters: I, Q,
and U. By laterally grading the multilayer optics and matching the dispersion of the gratings, one may take
advantage of high multilayer reflectivities and achieve modulation factors over 80% over the entire 0.2 to 0.8 keV
band. A sample design is shown that could be used with a small orbiting mission.
We present a spectrometer design based on a novel nanofabricated blazed X-ray transmission grating which is modeled
to have superior efficiency. Here we outline a full instrument design proposed for Constellation-X which is expected to
give resolving powers ~2000 (HEW). The spectrometer advantages include lower mass budget and smaller diffractor
area, as well as order-of-magnitude more relaxed alignment tolerances for crucial degrees of freedom than reflection
grating schemes considered in the past1,2,3. The spectrometer readout is based on conventional CCD technology adapted
to operate with very high speed and low power. This instrument will enable high resolution absorption and emission line
spectroscopy in the critical band between 0.2 and 1.5 keV.
A novel approach for measuring linear X-ray polarization over a broad-band using conventional imaging optics
and cameras is described. A new type of high efficiency grating, called the critical angle transmission grating is
used to disperse soft X-rays radially from the telescope axis. A set of multilayer-coated paraboloids re-image the
dispersed X-rays to rings in the focal plane. The intensity variation around these rings is measured to determine
three Stokes parameters: I, Q, and U. By laterally grading the multilayer optics and matching the dispersion of
the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors over 50%
over the entire 0.2 to 0.8 keV band. A sample design is shown that could be used with the Constellation-X
optics.
The sensitivity of the Advanced CCD Imaging Spectrometer (ACIS)
instrument on the Chandra X-ray Observatory (CXO) to low-energy X-rays
(0.3 - 2.0 keV) has been declining throughout the mission. The most
likely cause of this degradation is the growth of a contamination
layer on the cold (-60 C) filter which attenuates visible and near-visible light incident on the CCDs. The contamination layer is still increasing 4 years after launch, but at a significantly lower rate than initially. We have determined that the contaminant is composed mostly of C with small amounts of O and F. We have conducted ground experiments to determine the thermal desorption properties of candidate materials for the contaminant. We have conducted experiments to determine the robustness of the thin filter to the thermal cycling necessary to remove the contaminant. We have modeled the migration of the contaminant during this bake-out process to ensure that the end result will be a reduction in the thickness of the contamination layer. We have considered various profiles for the bake-out consisting of different temperatures for the ACIS focal plane and detector housing and different dwell times at these temperatures. The largest uncertainty which affects our conclusions is the volatility of the unknown contaminants. We conclude that bakeout scenarios in which the focal plane temperature and the detector housing temperature are raised to +20~C are the most likely to produce a positive outcome.
The low-energy sensitivity of the Advanced CCD Imaging Spectrometer
(ACIS) instrument on board the Chandra X-ray Observatory (CXO) has been continuously degrading since launch due to the accumulation of a layer of contamination on the ACIS optical blocking filter (OBF). This contamination layer, the result of out-gassing and off-gassing within the observatory, introduces a new, energy dependent absorption into the ACIS response. The thickness of this layer has been increasing with time and its spatial distribution across the OBF has been continually changing with time. We utilize multiple observations of the SMC Supernova Remnant 1E0102.2-7219 to verify the models for the spectral, temporal, and spatial dependence of the contamination layer. We also use this source to investigate cross-calibration between the front illuminated (FI) and back illuminated (BI) CCDs. 1E0102.2-7219 has a soft, line-dominated spectrum which is very sensitive to the additional absorption of the contamination layer. The extensive calibration observations of 1E0102.2-7219 over the course of the mission at several different locations on the ACIS Imaging (I) and Spectroscopy (S) arrays allows for a verification of the temporal and spatial dependence of the contamination model.
We present results from in-flight calibration of the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray
Observatory. Basic grating assembly parameters such as orientation and average grating period were measured using emission line sources. These sources were also used to determine the locations of individual CCDs within the flight detector. The line response function (LRF) was modeled in detail using an instrument simulator based on pre-flight measurements of the grating alignments and periods. These LRF predictions agree very well with in-flight observations of sources with narrow emission lines. Using bright continuum sources, we test the consistency of the detector quantum efficiencies by comparing positive orders to negative orders.
Chandra X-ray Observatory (CXO) -- the third of NASA's Great
Observatories -- has now been successfully operated for four years and has brought us fruitful scientific results with many exciting
discoveries. The major achievement comparing to previous X-ray
missions lies in the heart of the CXO -- the High Resolution Mirror
Assembly. Its unprecedented spatial resolution and well calibrated
performing characteristics are the keys for its success. We discuss
the effective area of the CXO mirrors, based on the ground calibration measurements made at the X-Ray Calibration Facility in Marshall Space Flight Center before launch. We present the derivations of both on-axis and off-axis effective areas, which are currently used by Chandra observers.
The Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory is suffering a gradual loss of low energy sensitivity due to a buildup of a contaminant. High resolution spectra of bright astrophysical sources using the Chandra Low Energy Transmission Grating Spectrometer (LETGS) have been analyzed in order to determine the nature of the contaminant by measuring the absorption edges. The dominant element in the contaminant is carbon. Edges due to oxygen and fluorine are also detectable. Excluding H, we find that C, O, and F comprise >80%, 7%, and 7% of the contaminant by number, respectively. Nitrogen is less than 3% of the contaminant. We will assess various candidates for the contaminating material and investigate the growth of the layer with time. For example, the detailed structure of the C-K absorption edge provides information about the bonding structure of the compound, eliminating aromatic hydrocarbons as the contaminating material.
We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral
redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the
calibration source and also because of the inherent non-linear
dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219
(the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.
We present the in-flight effective area calibration of the Low Energy Transmission Grating Spectrometer (LETGS), which comprises the High Resolution Camera Spectroscopic readout (HRC-S) and the Low Energy Transmission Grating (LETG) aboard the Chandra X-ray Observatory. Previous studies of the LETGS effective area calibration have focused on specific energy regimes: 1) the low-energy calibration for which we compared observations of Sirius B and HZ 43 with pure hydrogen non-LTE white dwarf emission models; and 2) the mid-energy calibration for which we compared observations of the active galactic nuclei PKS 2155-304 and 3C 273 with simple power-law models of their seemingly featureless continua. The residuals of the model comparisons were taken to be true residuals in the HRC-S quantum efficiency (QE) model. Additional in-flight observations of celestial sources with well-understood X-ray spectra have served to verify and fine-tune the calibration. Thus, from these studies we have derived corrections to the HRC-S QE to match the predicted and observed spectra over the full practical energy range of the LETGS. Furthermore, from pre-flight laboratory flatfield data we have constructed an HRC-S quantum efficiency uniformity (QEU) model. Application of the QEU to our semi-empirical in-flight HRC-S QE has resulted in an improved HRC-S on-axis QE. Implementation of the HRC-S QEU with the on-axis QE now allows for the computation of effective area for any reasonable Chandra/LETGS pointing.
The High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory is a powerful tool for studying the
astrophysical properties of X-ray emitting objects. Emission and
absorption lines and features can probe the physical properties of
stellar winds, relativistic jets, supernova remnants (SNRs), active
galactic nuclei and the intergalactic medium. The capabilities of
this instrument are illustrated through highlights of HETGS science,
with examples including the jet structure in SS433, a Doppler velocity
map of SNR E0102-72, and evidence of abundance anomalies in the
microquasar GRS1915+105.
Using multilayer coated mirrors to provide high reflectivity at large graze angles, we have proposed to launch a small telescope that is capable of measuring the linear polarization of the soft x-ray fluxes from many astronomical sources. Three identical mirror-detectoer assemblies are designed for maximum efficiency at 0.25 keV, where the photon spectra of many celestial targets peak. In observations lasting 1-3 days using this low risk instrument with proven heritage, we can detect polarizations of 5-10% at 5σ due to Compton scattering or synchrotron processes in the relativistic jets of BL Lac objects, accretion disks or jets in active galactic nuclei and atmospheres of isolated pulsars. Pulsar data can be binned by pulse phase to measure the orientation of the neutron star rotation and magnetic field axes and constrain the mass to radius ratio. This project has been selected for technology development funding by the NASA Explorer Program.
The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present preliminary results from in- flight calibration of the low energy response of the High Resolution Camera Spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrometer (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (Teff equals 25000 K and log g equals 9.0) for comparison with the Chandra observations of Sirius B. Pre-flight calibration of the LETGS effective area was conducted only at wavelengths shortward of 45 angstroms (E > 0.277 keV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths overestimates the effective area on average by a factor of 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 45 - 185 angstroms. We make an independent test of our results by comparing a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.
The Chandra X-ray Observatory was launched in July 1999, and is returning exquisite sub-arc second X-ray images of star groups, supernova remnants, galaxies, quasars, and clusters of galaxies. In addition to being the premier X-ray observatory in terms of angular and spectral resolution, Chandra is the best calibrated X-ray facility ever flown. We discuss here the calibration of the on-axis effective area of the High Resolution Mirror Assembly. Because we do not know the absolute X-ray flux density of any celestial source, this must be based primarily on ground measurements and on modeling. We use celestial sources which may be assumed to have smoothly varying spectra, such as the BL Lac object Markarian 421, to verify the continuity of the area calibration as a function of energy across the Ir M-edges. We believe the accuracy of the HRMA area calibration is of order 2%.
In order to probe for small scale spectral features of the High Energy Transmission Grating Spectrometer (HETGS) and the low energy transmission grating, we performed test at the AXAF X-Ray Calibration Facility (XRCF) using a very bright continuum source. THE Electron Impact Point Source (EIPS) was used with the Cu anode and operated at high voltage and low current in order to provide a bright continuum at high energies. The AXAF CCD Imaging Spectrometer Spectroscopy detector (ACIS-S) was used to discriminate orders and to provide high throughput when operate in continuous clocking mode. Many spectral features are observed but most of them are emission lines attributable to the source spectrum. We find that the current models for the HETG efficiency, the LETG efficiency and the AXAF High Resolution Mirror Assembly effective area predict very well the observed fine structure near the Au and Ir M edges where the models are most complex. Edges in the detector filter and quantum efficiency (QE) curves are somewhat more sharply defined in the data than in the current modes. By comparing the positive and negative dispersion regions, we find no significant efficiency asymmetry attributable to the gratings and we can further infer that the QEs of the ACIS-S frontside illuminated (FI) chips are consistent to +/- 10 percent. On the other hand, we derive the ratio of the QE for the backside illuminated (BI) chips relative to that of the FI chips and show that it deviates for the expected ratio. This deviation may result from grade differences due to operation in CC mode while most calibration data are obtained in timed event mode.
Tests of the High-Energy Transmission Grating Spectrometer (HETGS) for the Advanced X-Ray Astrophysics Facility (AXAF) showed anomalous scattering of monochromatic radiation. The grating resolving power (E/dE) is of the order 1000, but test designed to search for small-angle scattering by the gratings showed events with significant deviations from the dispersed grating orders and concentrated along the dispersion direction. In this paper, we present a general overview of grating scatter as a result of fluctuations in the grating bar geometry. The grating scatter observed at the AXAF-X-Ray Calibration Facility is shown to be consistent with what one expects from scatter due to deviations in the grating bar geometry form perfect bars. For the purposes of modeling, a rectangular bar mode is adopted and bar parameters are deduced via fitting the model to the scattering data. The correlations deduce from this model lead to a simple physical picture of grating bar fluctuations where a small fraction of the bars, e.g., 1 in 200 are leaning.
XRCF measurements of the flight AXAF High Energy Transmission Grating Spectrometer throughput were used to determine absolute effective areas. The result are compared with component models of the HRMA, HETG and the ACIS-S. The comparison provides an independent view on HETG efficiencies as well as the detector efficiencies along the dispersion direction. Using the XRCF double crystal monochromator measurements in the range from 0.9 to 8.7 keV, the effective areas in the 1st order MEG were determined with an accuracy of better than 10 percent, in the 1st order HEG better than 15 percent throughout most of the energy range. This is within the goal set for the XRCF measurements to refine state of the art composite component model predictions, which in the future will allow us to draw conclusions on the in-flight HETGS absolute effective area.
Herman Marshall, Daniel Dewey, Kathryn Flanagan, C. Baluta, Claude Canizares, D. Davis, John Davis, T. Fang, D. Huenemoerder, Joel Kastner, Norbert Schulz, Michael Wise, Jeremy Drake, Jiahong Juda, Michael Juda, A. Brinkman, C. Gunsing, Jelle Kaastra, Gisela Hartner, Peter Predehl
The high-energy transmission grating for AXAF was tested with the AXAF HRMA during December 1996 through April 1997 at NASA's MSFC X-Ray Calibration Facility. This first-use of the complete HETG spectrometer (HETGS) produced some low-level surprises in the line response function (LRF) and indicate that the HETG is meeting or exceeding its resolving-power specifications. This paper reviews the ingredients of the HETGS LRF, describes the pre-XRCF HETG sub-assembly measurements, presents an overview of the XRCF LRF-related measurements and data, and summarizes our knowledge of the HETG contribution to the HETGS line response function. Two low-level effects, grating scatter and grating misalignment, were uncovered in this testing.
The AXAF-payload consisting of a high resolution telescope, two different transmission gratings and two imaging detection systems, has been extensively tested between mid December 1996 and the end of April 1997. In this paper we report a few preliminary results on the resolution of the low energy transmission grating spectrometer. The measurements reported here utilize different x-ray sources and different detector systems. The resolving power at long wavelength ((Delta) (lambda) at 130 angstrom) equals 0.074 angstrom.
The low energy transmission grating spectrometer (LETGS) on board the Advanced X-ray Astrophysics Facility provides high resolution dispersive spectroscopy between 70 eV and more than 7 keV. The LETG contains 180 grating modules, each equipped with 3 grating facets. The freestanding gold gratings have 1008 lines per mm. Early 1997, the AXAF telescope underwent extended calibrations in the long beam X-Ray Calibration Facility at the NASA/Marshall Space Flight Center. As part of the telescope, also the performance of the LETGS with respect of spectral resolving power and effective area was measured. At more than 50 individual energies we have measured the grating efficiency or the effective area of the spectrometer, respectively. All these energies were chosen in order to cover the numerous spectral features due to absorption edges of filters, detector coatings, mirror reflectivities, and grating efficiency variations. Although preliminary, the performance of the gratings is close to the predictions made on the basis of subassembly measurements of individual grating elements. In particular, the first order efficiency is about 15% (both sides including vignetting effects) outside the energy regime of partial transparency of the grating wires; inside the efficiency gains from constructive interference effects. Both first diffraction orders are symmetric within less than 1%. The second order is suppressed by a factor of about 200 relative to the first order.
Daniel Dewey, Kathryn Flanagan, Herman Marshall, C. Baluta, Claude Canizares, D. Davis, John Davis, T. Fang, D. Huenemoerder, Joel Kastner, Norbert Schulz, Michael Wise, Jeremy Drake, Jiahong Juda, Michael Juda, A. Brinkman, C. Gunsing, Jelle Kaastra, Gisela Hartner, Peter Predehl
The high-energy transmission grating for AXAF was tested with the AXAF high resolution mirror assembly during December 1996 through April 1997 at NASA's MSFC X-Ray Calibration Facility. This first-use of the HETG confirms sub-assembly measurements and demonstrates the power of this AXAF grating spectrometer. This paper discusses calibration goals, summarizes the pre- XRCF performance predictions, describes the XRCF data taken, and outlines the general approach to their analysis -- concentrating on the HETG contribution to the HETGS effective area. Very preliminary examples of the analysis of the XRCF data are presented. At a crude level (approximately equal to 30%) the data are in agreement with sub-assembly predictions. Future detailed analysis will result in a definitive instrument calibration.
Multilayer coatings make it possible to create soft x-ray (0.25 keV) optics with large graze angles which are necessary for designing efficient polarimeters. If the detector is placed out of focus, then stellar images will take the shape of the front aperture, which would show intensity variations if the source is polarized. Two basic designs are considered. The first uses a single parabolic optic with graze angles from 25 degree(s) to 40 degree(s) and the second uses a Cassegrain telescope with angles centered at 45 degree(s). The former gives higher reflectivity at the expense of wide-field imaging, which may be sacrificed when the source region is not crowded, while the second telescope allows for off-axis use as a spectropolarimeter but has reduced throughput. A simple imaging proportional counter would be used as a detector and the overall sensitivity of the first design would be high enough to measure the polarization of the bright BL Lac object, PKS 2155-304, to an accuracy of 3% during a rocket flight. A small satellite would be capable of measuring several hundred polarizations with an uncertainty of 1% per year.
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