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.
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.
eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary goals are the determination of the equation of state of matter at supra-nuclear density, the measurement of QED effects in highly magnetized star, and the study of accretion in the strong-field regime of gravity. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of ∼0.9 m2 and 0.6 m2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of ∼3.4 m2, between 6 and 10 keV, and spectral resolution better than 250 eV; the Polarimetry Focusing Array (PFA) – a set of 2 X-ray telescope, for a total effective area of 250 cm2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees field of view. The eXTP international consortium includes major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.
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.
X-ray polarimetry is a hot topic and, as a matter of fact, a number of missions dedicated to the measurement of the polarization in the ∼2-8 keV energy range with photoelectric devices are under advanced study by space agencies. The Gas Pixel Detector (GPD), developed and continuously improved in Italy by Pisa INFN in collaboration with INAF-IAPS, is the only instrument able to perform imaging polarimetry; moreover, it can measure photon energy and time of arrival. In this paper, we report on the performance of a GPD prototype assembled with flight-like materials and procedures. The remarkably uniform operation over a long period of time assures a straightforward operation in orbit and support the high readiness level claimed for this instrument.
The Lightweight Asymmetry and Magnetism Probe (LAMP) is a micro-satellite mission concept dedicated for astronomical X-ray polarimetry and is currently under early phase study. It consists of segmented paraboloidal multilayer mirrors with a collecting area of about 1300 cm2 to reflect and focus 250 eV X-rays, which will be detected by position sensitive detectors at the focal plane. The primary targets of LAMP include the thermal emission from the surface of pulsars and synchrotron emission produced by relativistic jets in blazars. With the expected sensitivity, it will allow us to detect polarization or place a tight upper limit for about 10 pulsars and 20 blazars. In addition to measuring magnetic structures in these objects, LAMP will also enable us to discover bare quark stars if they exist, whose thermal emission is expected to be zero polarized, while the thermal emission from neutron stars is believed to be highly polarized due to plasma polarization and the quantum electrodynamics (QED) effect. Here we present an overview of the mission concept, its science objectives and simulated observational results.
The Gas Pixel Detector (GPD) is an imaging X-ray polarimeter with a moderate spectral resolution and a very good position resolution.1, 2 The GPD derives this information from the true 2-d charge image of the photoelectron track produced in gas and collected by an ASIC CMOS chip after its drift and its multiplication. In this paper we report on the experimental results of the study of the effect of a strong magnetic field in reducing the diffusion and increasing the sensitivity for a GPD filled with one bar of He-DME 20-80. We generated a magnetic field of about 1600 Gauss by means of commercial magnets made of an alloy of Neodymium-Iron-Boron configured as one ring and one cylinder. We compared the pixel size distributions and the modulation curves with and without magnets at two different drift fields, corresponding to different nominal diffusion properties, with both polarized and unpolarized sources. The results obtained show that a not sensitive improvement is present at this fields implying that a much larger magnetic field is necessary with this mixture, albeit a shift on the position angle of the modulation curve, derived from a polarized source, is observed.
We show that meaningful, highly sensitive x-ray polarimetry with imaging capability is possible with a small
mission tailored to the NASA Explorer program. Such a mission—derived from the Imaging X-ray Polarimetry
Explorer (IXPE) proposed to a previous NASA call—takes advantage of progress in light-weight x-ray optics
and in gas pixel detectors to achieve sensitive time-resolved, spectrometric, imaging polarimetry. We outline the
main characteristics and requirements of this mission and provide a realistic assessment of its scientific utility
for modeling point-like and extended x-ray sources and for studying physical processes (including questions of
We describe here the session of measurements that allowed the imaging capabilities of the Gas Pixel Detector
at the focus of an X-ray optics to be assessed. Firstly laboratory measurements and Monte Carlo simulations
were performed in order to study the intrinsic position resolution of the detector. Then a stand-alone test of the
JET-X FM-2 optics was performed at the PANTER X-ray test facility on November 2012, showing basically no
variation with respect to the results obtained in 1996. Finally a session of measurements performed at the same
facility allowed the imaging capability of the GPD at the focus of this JET-X optics to be calibrated.
The Gas Pixel Detector, developed and continuously improved by Pisa INFN in collaboration with INAF-IAPS, can visualize the tracks produced within a low Z gas by photoelectrons of few keV. By reconstructing the impact point and the original direction of the photoelectrons, the GPD can measure the linear polarization of X-rays, while preserving the information on the absorption point, the energy and the time of arrival of individual photons. The Gas Pixel Detector filled with He-DME mixture at 1 bar is sensitive in the 2-10 keV energy range and this configuration has been the basis of a number of mission proposals, such as POLARIX or XPOL on-board XEUS/IXO, or the X-ray Imaging Polarimetry Explorer (XIPE) submitted in response to ESA small mission call in 2012. We have recently improved the design by modifying the geometry of the absorption cell to minimize any systematic effect which could leave a residual polarization signal for non polarized source. We report on the testing of this new concept with preliminary results on the new design performance.
The background of the Gas Pixel Detector is expected to be negligible for polarimetry of point sources due
to the intrinsic low atomic number and density of the He-DME mixtures and to its imaging properties. Also
the background for extended sources is expected to be negligible at least down to the smallest flux for sensitive
polarimetry in a reasonable observing time. However in the spatial distribution of the background in a laboratory
environment we observed an accumulation on the edges of the sensitive plane due to the presence of the nearby
cell walls. We recently developed gas pixel detectors with a new design of the gas cell having a larger distance of
the walls from the sensitive plane. In this paper we compare the spatial distribution of the measured background
for the two design and their residual systematics. Also the impact of the background in the case of SgrB2 a faint
extended source in the galactic center region is evaluated.
The possibility to perform polarimetry in the soft X-ray energy band (2-10 keV) with the Gas Pixel Detector, filled with low Z mixtures, has been widely explored so far. The possibility to extend the technique to higher energies, in combination with multilayer optics, has been also hypothesized in the past, on the basis of simulations. Here we present a recent development to perform imaging polarimetry between 6 and 35 keV, employing a new design for the GPD, filled with a Ar-DME gas mixture at high pressure. In order to improve the efficiency by increasing the absorption gap, while preserving a good parallel electric field, we developed a new configuration characterized by a wider gas cell and a wider GEM. The uniform electric field allows to maintain high polarimetric capabilities without any decrease of spectroscopic and imaging properties. We present the first measurements of this prototype showing that it is now possible to perform imaging and spectro-polarimetry of hard X-ray sources.
The New Hard X-ray Mission (NHXM) is conceived to extend the grazing-angle reflection imaging capability up to 80
keV energy. The payload of the mission consists of four telescopes: three of the them having at their focal plane an
identical spectral-imaging camera operating between 0.2 and 80 keV, while the fourth one is equipped with a X-ray
imaging polarimeter. The three cameras consist of two detection layers: a Low Energy Detector (LED) and a High
Energy Detector (HED) surrounded by an Anti Coincidence (AC) system. Here we present the preliminary design and
the solutions that we are currently studying to meet the requirements for the high energy detectors. These detectors will
be based on Cadmium Telluride (CdTe) pixel sensors coupled to pixel read-out electronics using custom CMOS ASICs.
The New Hard X-Ray Imaging and Polarimetric Mission makes a synergic use of Hard X-Ray Imaging, Spectroscopy
and Polarimetry, as independent diagnostic of the same physical systems. It exploits the technology of
multi-layer optics that, with a focal length of 10 m, allow for spectroscopic and imaging, with a resolution from
15 to 20 arcseconds, on the band 0.2 - 80 keV. One of the four telescopes is devoted to polarimetry. Since the
band of a photoelectric polarimeter is not that wide, we foresee two of them, one tuned on the lower energy band
(2-10 keV) and another one tuned on higher energies (6 - 35 keV). The blurring due to the inclined penetration
of photons in the gas , thanks to the long focal length is practically negligible. In practice the polarimeters fully
exploit the resolution the telescope and NHXM can perform angular resolved simultaneous spectroscopy and
polarimetry on the band 2 - 35 keV. We are also studying the possibility to extend the band up to 80 keV by
means of a focal plane scattering polarimeter.
The Gas Pixel Detector (GPD) is a new generation device which, thanks to its 50 μm pixels, is capable of imaging
the photoelectrons tracks produced by photoelectric absorption in a gas. Since the direction of emission of the
photoelectrons is strongly correlated with the direction of polarization of the absorbed photons, this device has
been proposed as a polarimeter for the study of astrophysical sources, with a sensitivity far higher than the
instruments flown to date. The GPD has been always regarded as a focal plane instrument and then it has been
proposed to be included on the next generation space-borne missions together with a grazing incidence optics.
Instead in this paper we explore the feasibility of a new kind of application of the GPD and of the photoelectric
polarimeters in general, i.e. an instrument with a large field of view. By means of an analytical treatment
and measurements, we verify if it is possible to preserve the sensitivity to the polarization for inclined beams,
opening the way for the measurement of X-ray polarization for transient astrophysical sources. While severe
systematic effects arise for inclination greater than about 20 degrees, methods and algorithms to control them
The development of micropixel gas detectors, capable to image tracks produced in a gas by photoelectrons,
makes possible to perform polarimetry of X-ray celestial sources in the focus of grazing incidence X-ray telescopes.
HXMT is a mission by the Chinese Space Agency aimed to survey the Hard X-ray Sky with Phoswich detectors, by
exploitation of the direct demodulation technique. Since a fraction of the HXMT time will be spent on dedicated
pointing of particular sources, it could host, with moderate additional resources a pair of X-ray telescopes, each
with a photoelectric X-ray polarimeter (EXP2, Efficient X-ray Photoelectric Polarimeter) in the focal plane. We
present the design of the telescopes and the focal plane instrumentation and discuss the performance of this
instrument to detect the degree and angle of linear polarization of some representative sources. Notwithstanding
the limited resources, the proposed instrument can represent a breakthrough in X-ray Polarimetry.
The XEUS mission incorporates two satellites: the Mirror Spacecraft with 5 m2 of collecting area at 1 keV and
2 m2 at 7 keV, and an imaging resolution of 5" HEW and the Payload Spacecraft which carries the focal plane
instrumentation. XEUS was submitted to ESA Cosmic Vision and was selected for an advanced study as a
large mission. The baseline design includes XPOL, a polarimeter based on the photoelectric effect, that takes
advantage of the large effective area which permits the study of the faint sources and of the long focal length,
resulting in a very good spatial resolution, which allows the study of spatial features in extended sources. We
show how, with XEUS, Polarimetry becomes an efficient tool at disposition of the Astronomical community.
We devised and built a light, compact and transportable X-ray polarized source based on the Bragg diffraction
at nearly 45 degrees. The source is composed by a crystal coupled to a small power X-ray tube. The angles of
incidence are selected by means of two orthogonal capillary plates which, due to the small diameter holes (10
μm) allow good collimation with limited sizes. All the orders of diffraction defined by the crystal lattice spacing
are polarized up to the maximum order limited by the X-ray tube voltage. Selecting suitably the crystal and the
X-ray tube, either the line or the continuum emission can be diffracted, producing polarized photons at different
energies. A very high degree of polarization and reasonable fluxes can be reached with a suitable choice of the
capillary plates collimation.
We present the source and test its performances with the production of nearly completely polarized radiation
at 2.6, 5.2, 3.7 and 7.4 keV thanks to the employment of graphite and aluminum crystals, with copper and calcium
X-ray tubes respectively. Triggered by the very compact design of the source, we also present a feasibility study
for an on-board polarized source, coupled to a radioactive Fe55 nuclide and a PVC thin film, for the calibration
of the next generation space-borne X-ray polarimeters at 2.6 and 5.9 keV.
We discuss a new class of Micro Pattern Gas Detectors, the Gas Pixel Detector (GPD), in which a complete integration between the gas amplification structure and the read-out electronics has been reached. An Application-Specific Integrated Circuit (ASIC) built in deep sub-micron technology has been developed to realize a monolithic device that is, at the same time, the pixelized charge collecting electrode and the amplifying, shaping and charge measuring front-end electronics. The CMOS chip has the top metal layer patterned in a matrix of 80 μm pitch hexagonal pixels, each of them directly connected to the underneath electronics chain which has been realized in the remaining five layers of the 0.35 μm VLSI technology. Results from tests of a first prototype of such detector with 2k pixels and a full scale version with 22k pixels are presented. The application of this device for Astronomical X-Ray Polarimetry is discussed. The experimental detector response to polarized and unpolarized X-ray radiation is shown. Results from a full MonteCarlo simulation for two astronomical sources, the Crab Nebula and the Hercules X1, are also reported.
We report on a large active area (15x15mm2), high channel density (470 pixels/mm2), self-triggering CMOS analog chip that we have developed as pixelized charge collecting electrode of a Micropattern Gas Detector. This device, which represents a big step forward both in terms of size and performance, is the last version of three generations of custom ASICs of increasing complexity. The CMOS pixel array has the top metal layer patterned in a matrix of 105600 hexagonal pixels at 50μm pitch. Each pixel is directly connected to the underneath full electronics chain which has been realized in the remaining five metal and single poly-silicon layers of a standard 0.18μm CMOS VLSI technology. The chip has customizable self-triggering capability and includes a signal pre-processing function for the automatic localization of the event coordinates. In this way it is possible to reduce significantly the readout time and the data volume by limiting the signal output only to those pixels belonging to the region of interest. The very small pixel area and the use of a deep sub-micron CMOS technology has brought the noise down to 50 electrons ENC.
Results from in depth tests of this device when coupled to a fine pitch (50μm on a triangular pattern) Gas Electron Multiplier are presented. The matching of readout and gas amplification pitch allows getting optimal results. The application of this detector for Astronomical X-Ray Polarimetry is discussed. The experimental detector response to polarized and unpolarized X-ray radiation when working with two gas mixtures and two different photon energies is shown. Results from a full MonteCarlo simulation for several galactic and extragalactic astronomical sources are also reported.