We have, at last, an observatory dedicated to X-ray polarimetry that has been operational since December 9th, 2021. The Imaging X-ray Polarimetry Explorer (IXPE) is a NASA SMEX mission, in partnership with ASI, based on three X-ray telescopes, each equipped with a polarization-sensitive detector in the focus. An extending boom was deployed in orbit, positioning the detectors at the optimal distance from the optics, which have a 4-meter focal length. The spacecraft is three-axis stabilized, providing power, attitude determination and control, transmission, and commanding capabilities.
After two and a half years of observation, IXPE has detected positive polarization from nearly all classes of celestial sources that emit X-rays. In this report, we describe the IXPE mission, detailing the performance of the scientific instrumentation after 2.5 years of operation. We also present the main astrophysical results and a few examples of scientific performance during flight.
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.
Superconducting transition-edge sensors (TESs) carried by X-ray telescopes are powerful tools for the study of neutron stars and black holes. Several methods, such as optimal filtering or principal component analysis, have already been developed to analyse X-ray data from these sensors. However, these techniques may be hard to implement in space. Our goal is to develop a lower-computational-cost technique that optimizes energy and time resolution when X-ray photons are detected by a TES. TESs exhibit a non-linear response with photon energy. Therefore, at low energies we focus on the current-pulse height whereas at high energies we consider the current-pulse width, to retrieve energy and arrival time of X-ray photons. For energies between 0.1 keV and 30 keV and with a sampling rate of 195 kHz, we obtain an energy resolution (full width at half the maximum) between 1.32 eV and 2.98 eV. We also get an arrival-time resolution (full duration at half the maximum) between 163 ns and 3.85 ns. To improve the accuracy of these results it will be essential to get a thorough description of non-stationary noise in a TES, and to develop a robust on-board identification method of pile-up events.
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.
Superconducting transition-edge sensors (TESs) carried by x-ray telescopes are powerful tools for the study of neutron stars and black holes. Several methods, such as optimal filtering or principal component analysis, have already been developed to analyze x-ray data from these sensors. However, these techniques may be hard to implement in space. Our goal is to develop a lower-computational-cost technique that optimizes energy and time resolution when x-ray photons are detected by a TES. Current pulses, in TESs, exhibit a non-linear response to photon energy. Therefore, at low energies, we focus on the current-pulse height, whereas at high energies, we consider the current-pulse width, to retrieve energy and arrival time of x-ray photons. For energies between 0.1 and 30 keV and with a sampling rate of 195 kHz, we obtain an energy resolution (full-width at half-maximum) between 1.32 and 2.98 eV. We also get an arrival-time resolution (full-duration at half-maximum) between 163 and 3.85 ns. To improve the accuracy of these results, it will be essential to get a thorough description of non-stationary noise in a TES and develop a robust on-board identification method of pile-up events.
We propose a high-time-resolution, high-spectral-resolution X-ray telescope that uses transition-edge sensors (TES) as detectors and collector optics to direct the X-rays onto the focal plane, providing a large effective area in a small satellite. The key science driver of the instrument is to study neutron stars and accreting black holes. The proposed instrument is built upon two technologies that are already at high TRL: TES X-ray detectors and collector optics.
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.
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