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The Imaging X-ray Polarimetry Explorer is the Instrument on board the SMEX mission IXPE. The IXPE mission, announced on 13 January 2017, is a NASA mission in partnership with the Italian space agency, Agenzia Spaziale Italiana (ASI). It has been launched at 1 a.m. EST on December 9, 2021. This is the website of the IXPE Italian collaboration, which provided the mission with the ground-breaking focal plane instrument, plus several contributions from the Science and Data analysis to the IXPE primary ground station. The IXPE mission will fly three telescope systems capable of measuring the polarization of X-rays emitted by cosmic sources. ASI will contribute IXPE's three polarization-sensitive X-ray detectors which are designed, built and tested in Italy, and the use of its equatorial ground station located at Malindi, Kenya. The focal plane Detector Units (DUs) and the Detector Service Unit (DSU) were developed by INAF-IAPS and INFN and were manufactured by OHB-IT. NASA will supply the X-ray telescopes and use of its facilities to perform end-to-end X-ray calibration and science operations. Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. Ball Aerospace will also operate the flight system with support from LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado at Boulder. Other partners include Stanford University, Nagoya University and MIT (Massachusetts Institute of Technology). The figure below highlights the roles of the IXPE partners IXPE is designed as a 2-year mission with possible extension as a general-observer mission. IXPE will orbit at 600 km of altitude, with an orbital period of 5.76 ks and 35% of this time affected by Earth occultations of X-ray targets. This fraction of the orbital period can be used for calibration purposes. IXPE will be the first mission entirely dedicated to X-ray polarimetry. In addition to the direction, energy, and arrival time of every photon, polarimetry adds two observables: the degree and angle of polarization. These observables provide information on the emission mechanism and the geometry of the source. December 9, 2021 The Imaging X-Ray Polarimetry Explorer (IXPE) mission has been launched at 1 a.m. EST Thursday on a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center in Florida. December 11, 2021 Solar panels deployed. Contacts with Malindi and Singapore are okay. Temperatures are nominal. Orbit insertion is very accurate: 601 km and 0.22 deg. December 15, 2021 Boom Deployment orbit 98 has been completed. Everything is nominal. First calibration source starting 18 December; first celestial source 10 day later. First source of observing plan CasA first week of January.

IXPE is designed as a 2-year mission with possible extension as a general-observer mission. IXPE will orbit at 600 km of altitude, with an orbital period of 5.76 ks and 35% of this time affected by Earth occultations of X-ray targets. This fraction of the orbital period can be used for calibration purposes. IXPE will be the first mission entirely dedicated to X-ray polarimetry. In addition to the direction, energy, and arrival time of every photon, polarimetry adds two observables: the degree and angle of polarization. These observables provide information on the emission mechanism and the geometry of the source.

Spacecraft, mechanical and structural elements of the payload, observatory assembly, integration and test, and mission operations for IXPE has been carried out by the Ball Aerospace, at its Boulder facility. Ball Aerospace has completed the spacecraft and payload assembly integration for IXPE. All of its pre-launch testing have been completed. Now it has been delivered from Ball’s facilities in Boulder to Kennedy Space Center in Florida.

Selected in 2017 January, as a NASA Astrophysics Small Explorer (SMEX), IXPE is a NASA mission in partnership with the Italian space agency, Agenzia Spaziale Italiana (ASI). The IXPE mission will fly three telescope systems capable of measuring the polarization of X-rays emitted by cosmic sources. ASI will contribute IXPE's three polarization-sensitive X-ray detectors which were developed in Italy and the use of its equatorial ground station located at Malindi, Kenya. NASA will supply the X-ray telescopes and use of its facilities to perform end-to-end X-ray calibration and science operations. Ball Aerospace in Broomfield, Colorado, will provide the spacecraft and mission integration. Ball Aerospace will also operate the flight system with support from LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado at Boulder. Other partners include Stanford University, McGill University and MIT (Massachusetts Institute of Technology). The figure below highlights the roles of the IXPE partners.

The IXPE spacecraft is separated into two different parts. The first is the main spacecraft with the solar array, attitude control, and communication systems. The second part is attached with a deployable payload boom with its X-Ray shield and main Mirror Module Support Structure (MMSS) deck. The IXPE payload is a set of three identical, imaging, X-ray polarimetry systems mounted on a common optical bench and co-aligned with the pointing axis of the spacecraft. Each system, operating independently, comprises a 4-m-focal length Mirror Module Assembly (grazing incidence x-ray optics) that focuses X-rays onto a polarization-sensitive imaging detector. The focal length is achieved using a deployable boom. Each DU contains its own back-end electronics, which communicate with the DSU, which in turn interfaces with the spacecraft. Each DU has a multi-function filter calibration wheel (FCW) assembly for in-flight calibration checks and source flux attenuation. Three identical systems provide redundancy, a range of detector clocking angles to mitigate against any detector biases, shorter focal length for given mirror graze angles (i.e., given energy response) and thinner/lighter mirrors compared to a single telescope system. The figure below shows the IXPE observatory with key payload elements.

The instrument performances are listed below. Modulation factor, Efficiency (expected during flight), Gray filter transparency, UV Filter transparency, Spurious modulation, Systematic error on the P.A. determination, Energy resolution, Position resolution, Dead Time, Timing accuracy, Timing resolution, Common FoV without dithering, Common FoV with dithering, Expected background rate (2-8 keV), Expected Crab nebula rate (2-8 keV), Expected Crab nebula rate (1-12 keV)

The IXPE Ground System consists of the Mission Operations Center (MOC) at CU/LASP, the IXPE Science Operations Center (SOC) at MSFC, the Malindi ground station (with Singapore (KSAT) as a backup), and the Internet and other connections between the various elements. In addition, IXPE uses the Space Network (TDRS) for launch and early operations support for critical event monitoring and orbit determination using differential one-way Doppler (DOWD) tracking. Malindi is also used during early operations support, and during boom deployment. The Flight Dynamics Facility (FDF) will provide improved inter-range vectors (IIRV) to the MOC and the ground stations until TLE data has converged with the FDF provided solutions. The IXPE mission operations team performs mission planning, commanding, health and status monitoring, and sustaining engineering for both the Spacecraft and the payload from the MOC. The MOC will be located within LASP’s existing multi-mission satellite operations facility and will use hardware, software, procedures, and personnel that are already in place and used to operate AIM, QuikSCAT, K2 and SORCE.

The IXPE mission consists of 4 distinct phases of operation: Pre-launch, Launch and Commissioning, Science Operations, and End of Life decommissioning. Launch and commissioning operations are conducted during the first 30 days on-orbit. Launch begins with umbilical separation and continues through Observatory separation from the Falcon 9 second stage. At that point, the Observatory is free flying in its stowed configuration in the injected orbit. Commissioning occurs during the subsequent 30 days. Upon separation from the launch vehicle (LV), the Spacecraft autonomously detumbles, autonomously deploys the solar arrays, and performs solar acquisition. The Observatory communicates via the TDRS constellation post launch for critical event coverage. After first acquisition of the S-band telecommunications link with the Malindi ground station, spacecraft commissioning activities begin. Payload commissioning includes boom deployment, instrument activation, TTR activation (if needed), and instrument on-orbit calibration activities. Instrument calibration on-orbit includes pointing at a number of bright X-ray sources. Science operations (Phase E) are planned to last at least 2 years. Science operations phase includes specific target observations, data downlink, periodic astrometric calibrations, and target-to-target slews. X-ray targets are known in advance and observed with a single science mode. Phase E science operations commence with uplink of the first weekly science observation sequence. Malindi coverage transitions to 3-9 passes per day of 8 minutes each (7 minutes downlink time). Based on the target list, many of the targets can be observed using one continuous observation period with 2 ground contacts per day, while other targets are data intensive and require splitting the observations into 2 to 4 observing sequences, filling the recorder and downlinking up to 9 times per day. Science and calibration data are stored in the C&DH and downlinked daily during the scheduled passes. Downlinks are initiated and monitored by ground automation. Downlinks are through the Malindi station at a rate of 2.0 Mbit/s (Singapore is backup). The ground stations sync and decode the channels and send them to the MOC in real time and/or as files after each pass. Real time and stored state of health telemetry is monitored by the MOC for health and safety checking and trend analysis of the Spacecraft and Payload. The MOC transmits data to the Science Operations Center (SOC) for processing. The SOC at MSFC, with support from the ASI Space Science Data Center (SSDC), is responsible for IXPE science operations. The IXPE science team performs data processing and archiving of the data for community use through the HEASARC. Decommissioning (Phase F) occurs at the end of the mission once science operations are complete. The Observatory will be shut down (passivated) in accordance with NASA end of mission guidelines.

The IXPE focal plane Detector Units (DUs) and the Detector Service Unit (DSU) were supported by the Italian Space Agency and developed by INAF-IAPS and INFN and were manufactured by OHB-IT. The IXPE payload consists of a set of three identical telescope systems. Each system, while operating independently, comprises of a 4-m-focal length Mirror Module Assembly (MMA) that focuses X-rays onto the respective Detector Units (DUs). The DU housing is composed of two boxes. The top one is the ”GPD housing”. The bottom item is the ”Back-End Housing”. Each of the four DUs (included one spare unit), comprises the following sub-Units: Gas Pixel Detector (GPD), which is an X-ray detector with gas as the absorption medium and a custom ASIC as the readout electrode, specifically developed by INFN in collaboration with INAF-IAPS for X-ray polarimetry. Filter & Calibration Wheel (FCW), which hosts the calibration set comprising calibration sources and filters for specific observations to be placed in front of the GPD when needed. Back End Electronics (BEE), which comprises of electronics boards (DAQ, Data Acquisition Board to manage the GPD ASIC), the required High Voltage lines and the Low Voltages lines. Stray-light Collimator (STC), already mentioned above. DU Housing (DUH), which provides the mechanical and thermal interface of the DU to the S/C. DU wiring (DUW), which provides the electrical interfaces (internal to the DU) between the BEE and the GPD. The DSU is the unit which provides the DU with the needed secondary power lines, controls and powers the FCW, formats and forwards the scientific data of the three DUs to the spacecraft. The DSU comprises of the following sub-Units: a DSU Board Set (DBS), which is the set of electronic boards (both nominal and redundant) which perform the DSU tasks; the DSU Software, which comprises the software which runs in the DSU; a DSU Case (DSC), which includes a back-plane that provides the electrical interface among the DSU boards. Further, the DSU Case provides the mechanical and thermal interface of the unit; the Harness DSU to DU, which comprises the cables necessary to electrically interface the three DUs to the DSU.

The filter and the calibration wheel (FCW) has been designed to monitor, during the observational life of the mission, the performances of the detector. In particular we intend to check the low and high energy modulation factor and the level of absence of spurious modulation. Also we monitor the gain that depends both on temperature and on charging. The FCW is placed on the top lid of the DU, as shown in the figure above. By rotating around its central axis, the FCW allows the placement of one of the four calibration sources or a gray filter in front of the GPD, in addition to the open and closed position, depending on the observational requirements. The FCW hosts the filters and the calibration sources to calibrate the detector during flight or to perform peculiar observations. The FCW has 7 positions commanded by the DSU. The positions correspond to open position, closed position, gray filter and calibration source A, B, C and D. The stepper motor is a Phytron phySPACETM 42-2. It is a COTS component with 25 years of space flight heritage. The phySPACETM series is developed and built to resist vacuum, vibrations, low/high temperature and radiation while maintaining high performance, precise positioning long life. The motor pinion is made of Vespel®. The double bearing system that connects the fixed part to the rotating one is made of stainless steel. The positioning is performed and monitored by two different systems. One system is based on the use of Hall sensors as non-contact switches to stop the wheel rotation to the requested position. A combination of three hall sensors allows for setting and monitoring the 7 wheel positions. In order to have an independent measurement of the wheel position a second, analog, device, a Novotechnik PRS65/S152 potentiometer, is implemented. A qualification campaign allowed for declaring this component as flight compatible. The FCW positioning can be set digitally or on a step-by-step base allowing, for example for spanning the entire sensitive area for measuring the gain and modulation factor at different positions from the center. The calibration set is composed of the X-ray sources and of their holders. It comprises the items which can be put in front of the GPD by rotating the FCW.

A stray-light collimator (SLC) is mounted on the top of the DU to shield the GPD from background X-rays not coming from the optics. At the bottom of the SLC, an ions-UV filter is mounted to reduce the thermal load and to prevent ions and UV from entering in the DU. An extensible boom connects the mirror modules to the spacecraft top-deck. A tapered stray-light carbon-fiber (CF) collimator collects only the photons reflected by the optics, thanks to fixed X-ray shields around the mirror modules, too. The total thickness is 1.25 mm and includes the 1-mm thick CF with an external 50-µm molybdenum coating of thickness and 20 µm gold in the intersections.

Due to the fact that the beryllium window of the detector and the supporting titanium frame is at high potential (about -2800 V) positive ions may interact with the top GPD structure producing eventual secondary photons and a possible failure of the high voltage system. For this reason, photons from the optics cross a UV-ion filter made by LUXEL corp. composed of 1059 nm of LuxFilm® (based on kapton) with an external coating of 50 nm of aluminum and an internal coating of 5 nm of Carbon. The scope of UV-ions filter is to prevent that UVs and low velocity plasma present in orbit reach the beryllium window of the GPD.

Ions-UV filter transmittance measurements were performed at INAF-IAPS with the Instrument Calibration Equipment (ICE). This is a customized facility developed at INAF-IAPS to perform the on-ground calibration of the IXPE DUs, and it is installed in an ISO 7 (10,000 class) cleanroom. ICE provides X-ray beams of know energy, polarization degree and angle. The X-ray beam and the DU are aligned by means of micrometric movement stages. The ICE comprises o-the-shelf auxiliary detectors: a Silicon Drift Detector (SDD) spectrometer and a Charge Coupled Device (CCD) to fully characterize the energy and the shape of the beam. The IXPE DU ions-UV filters underwent acceptance and qualification at unit level in compliance with the Environmental Design and Test Specification (EDTS) for IXPE Spacecraft components. As required by the EDTS, the acoustic qualification was done through the application of random vibrational loads. The protoflight vibration test on the units was concluded successfully and the structural requirements were verified. No damage has been detected by the visual inspection performed after each axis vibration. The X-rays transmittance was measured at INAF-IAPS for all the filters. Luxel Corp. measured the transmission of visible light of ions-UV filters with its Transmission Imaging Photometer (TIP). The TIP directly compares the intensity of a diuse broadband visible (400 nm to 700 nm) light source both unattenuated and attenuated by each ions-UV filter for a high-sensitivity, spatially-resolved transmittance measurement.

A variation of the count rate of the source could aect the measurement of transmittance. During the test, the X-ray source was turned on and o. Thus, we estimated its stability. During the on ground calibration activities of the IXPE detector units performed at INAF-IAPS, the X-ray transparencies of the DU-FM ions-UV filters were measured with monochromatic radiation at 2.7 keV and 6.4 keV. The transparencies are compatible with respect to the nominal values, including the mechanical tolerances of the thicknesses of the materials.

IXPE will be the first instrument to perform spatially resolved X-ray polarimetry on several astronomical sources in the 2–8 keV energy band. These measurements are made possible owing to the use of a gas pixel detector (GPD) at the focus of three X-ray telescopes. The GPD allows simultaneous measurements of the interaction point, energy, arrival time, and polarization angle of detected X-ray photons. The GPD, located in the GPD housing, is a gas detector for which the charge-signal is readout by a matrix of 105k pixels (300 352 pixels arranged in a 50 µm pitch hexagonal pattern) of a dedicated, custom CMOS ASIC. The custom ASIC has self-triggering capabilities thanks to local triggers defined each group of four pixels called mini-clusters. Each event consists of a Region of Interest (ROI) made by all the mini- clusters that trigger plus a selectable additional fiducial region of 10/20 pixels. The charge content of each pixel in the ROI is readout serially from a single buffer as differential current output by means of a 5 MHz clock. The ASIC, also, provides the absolute position of the ROI as digital coordinates of two opposite vertices and the global trigger output, about 1µs after the arrival of the charge. The threshold is set low enough to collect signals coming from events which release about 300 eV. The charge amplification is provided by a Gas Electron Multiplier (GEM), a thin 50 µm dielectric (liquid crystal polymer) foil with 5-µm (TBC) copper metallization on both sides. Through holes are laser etched and disposed on a regular triangular pattern with vertexes 50µm apart. These cylindrical holes have a diameter of 30 µm. The basic building blocks of the GPD assembly and their functions are schematically illustrated in the figure. The small pitch of the ASIC and the GEM are responsible for the good image of the photoelectron track. In summary, the GPD is composed by the following subassemblies: mechanical interface (GPD Mech. I/F), made of titanium, which supports the GPD unit, connects it to the focal plane and provides references for alignment with the MMA; printed circuit board (GPD Board), which connects the GPD to the readout electronics; ceramic spacer (GEM Support Frame), supporting and insulating the GEM from the GPD Board; this item defines a lower gas gap of 500 µm for the drift of the amplified electrons. the GEM foil (GEM), including four soldering pads for the high voltage connections; ceramic support (Drift Spacer), which defines the absorption gas cell above the GEM and isolates the GEM from the top electrode; titanium frame (Ti frame and Be window) which closes the gas cell and allows X-rays into the GPD through the integrated thin, optical-grade 50 µm one-side aluminized beryllium window. The titanium frame and the beryllium window serve, also, as a drift electrode; filling tube of OFHC copper and its fixture to the titanium frame (tube and fixture); The GPD is sealed and does not require any gas cycling systems. The keystone of the assembly is the Kiocera package. In fact the ASIC is designed to fit into a commercial package that acts as the bottom parts of the gas cell and connects the internal wiring and the clean gas volume to the external Printed Circuit Board (PCB), onto which the package is soldered.The GPD assembly is done at INFN, whereas the filling procedure is performed at Oxford Instruments Technologies Oy (OIT, ESPOO, Finland). The performances of the GPD are, in principle, dependent on temperature to some extent. The GPD temperature is controlled during the operation by means of a Peltier cooler and a heater. A thermal strap connects the cooler directly to the spacecraft radiator. The GPD thermal control is performed directly by the DSU and is designed to be as independent as possible from the BEE temperature, since the latter has much looser requirements.

The GPD assembly — including the wire-bonding of the readout ASIC in its package and the positioning of the latter on the GPD board, the assembly of the gas cell, the metrological verifications and the initial leak test — was entirely performed in house, using the INFN facilities. The assembly procedure was developed in collaboration with Oxford Instruments Technologies Oy in Finland (which also performed the final bake-out and filling of the detectors) and further refined through phases A and B of the mission.

INFN produced 9 flight GPD—out of which four were chosen to be installed in the (three plus one spare) Detector Units (DU). All of them were extensively tested to verify their basic performance as focal-plane detectors using a dedicated test setup at INFN and the calibration facility at IAPS later used for the calibration of the DUs. We found all the detectors to show very similar performance metrics, as we shall detail in the remainder of this section.

The calibration equipment was designed and built at INAF-IAPS in Rome, Italy, for the calibration of the polarization-sensitive focal plane detectors on-board IXPE. Equipment includes calibration sources, both polarized and unpolarized, stages to align and move the beam, test detectors and their mechanical assembly The focal plane detectors on-board IXPE are based on the Gas Pixel Detector (GPD) design, which have been developed in Italy for nearly 20 years by a collaboration of Istituto Nazionale di Fisica Nucleare (INFN) and Istituto Nazionale di Astrofisica/Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS) in Rome. These detectors are the main Italian contribution to IXPE, which also includes the electronics to interface them to the spacecraft, the primary ground station and several contributions for the data processing pipeline, scientific analysis and data exploitation. IXPE polarimeters, named Detector Units (DUs), have been manufactured in INFN, whereas the Detector Service Unit (DSU) which interfaces them to the spacecraft was built by OHB-Italia. A flight DSU and four DUs have been produced, three for flight plus one spare. The DUs and the DSU, comprehensively named the IXPE Instrument, were delivered to INAF-IAPS, including both the flight and the spare units, for extensive tests with X-rays before the integration on the spacecraft at Ball Aerospace, in Boulder, CO. DUs were also integrated to the DSU and illuminated with X-ray sources to effectively test the operation of the whole IXPE Instrument in a configuration equivalent to the flight one.

During the mission lifetime, the GPD response will be monitored using a filter and calibration set (FCS) hosted on a filter and calibration wheel (FCW) included in each DU. Each FCS consists of four calibration sources, namely, CalA, CalB, CalC, CalD, and filters for special observations. The sets are denoted as flight model (FM)1, FM2, FM3, and FM4, and they are assigned to the three DUs that will fly onboard the IXPE spacecraft and one DU that will act as a spare. The FCS includes both polarized and unpolarized calibration sources, capable of illuminating the whole detector or just a part of it, for mapping and monitoring of the GPD modulation factor (i.e., the detector response in terms of modulation to 100% polarized radiation), quantum efficiency, and energy resolution at different energies. In-orbit calibrations will also allow us to check for the presence of spurious polarization, as well as to map and monitor the gain and its uniformity across the 15x15 mm2 detector surface. These information will help improve our understanding of the detector performance and asses the reliability of scientific results. The calibration sources successfully assess and verify the functionality of the GPD and validate its scientific results in orbit; this improves our knowledge of the behavior of these detectors in X-ray polarimetry. The FCS is composed of four calibration sources (a polarized source, CalA; a collimated unpolarized source, CalB; two uncollimated, unpolarized sources, CalC and CalD), a gray filter, and an open and closed position. The FCS is hosted in the FCW which is placed on the top lid of the DU, as shown in the figure above. By rotating around its central axis, the FCW allows the placement of one of the four calibration sources or a gray filter in front of the GPD, in addition to the open and closed position, depending on the observational requirements. The FCW also hosts other elements that ensure the stability and positional accuracy of the calibration sources. A rotary potentiometer is used to determine the wheel angular position accurately. Further, for redundancy, three radially placed Hall effect sensors and twelve magnets (positioned to realize a unique binary coding for the wheel’s seven positions) function as position reference points. The calibration sources can thus be positioned with an accuracy greater than ± 500 μm with respect to their nominal positions. The angular position of the polarized calibration source with respect to the DU coordinate system is known with an uncertainty below 20 arcmin. The fixed parts of the FCW (e.g., the cover lid) are connected to the rotating parts (the wheel itself) by a bearing sub-assembly. Finally, a ballast mass is installed to balance the weights and the momentum of inertia on the wheel. Each calibration source inside the FCW contains a radioactive source constituted of a 55Fe nuclide, which, following a K electron capture, emits X-rays at 5.9 and 6.5 keV, i.e., the Mn Kɑ and Mn Kɓ emission lines, respectively. The activity of 55Fe naturally decays with a half-life of 2.7 years, which provides sufficient time to cover the entire operative life of IXPE. This solution removes the problem of including X-ray tubes on board of the spacecraft, as their installation would have been complex, as well as mass- and power-demanding, especially on a moving support. The items of the FCS were tested first with a commercial SDD and CCD to verify their operation. The calibration sources were then tested in TV with the flight DU to derive their spectra, images on the detectors, and polarimetric performance. The morphology of the sources, studied independently with CCD and GPD, are consistent. The expected counting rates are comparable across the different FMs, with differences that can be ascribed to the different energy resolution of each DU. The counting rates satisfy the requirements, and the modulation of the polarized sources is consistent with the one expected from Bragg diffraction.

The Instrument Calibration Equipment (ICE) has been built at INAF-IAPS in Rome (Italy) to produce both polarized and unpolarized radiation, with a precise knowledge of direction, position, energy and polarization state of the incident beam. Inflight, a set of four calibration sources based on radioactive material and mounted on a filter and calibration wheel will allow for the periodic calibration of all of the three IXPE focal plane detectors independently. A highly polarized source and an unpolarized one will be used to monitor the response to polarization; the remaining two will be used to calibrate the gain through the entire lifetime of the mission. The ICE comprises the items which are used for Instrument calibration and functional tests. In particular, it includes: The X-ray sources used for illuminating the detector. Each source emits X-ray photons at known energy and with known polarization degree and angle. The direction of the beam, the direction of polarization for polarized sources, and its position can be measured with respect to the GPD inside the DU and aligned and moved as necessary. The test detectors which are used to characterized the beam before DU calibration and as a reference for specific measurements (e.g., the measurement of quantum efficiency). All the electrical and mechanical equipment required to support the DU and the calibration sources, monitor the relevant diagnostic parameters and assure safe operations during calibrations. This includes also the clean environment (class better than 100,000) in which DU will be calibrated. The DU will be mounted in the ICE without the stray-light collimator and the UV filter, to limit the distance between the X-ray source and the GPD and hence air absorption and beam divergence. The DU is placed on the top of a tower which allows to: Move the DU on the plane orthogonal to the incident beam with an accuracy of ±2 μm (over a range of 100 mm) to map the GPD sensitive surface. rotate the DU on the plane orthogonal to the incident beam with an accuracy of ±7 arcsec, to test the response at different polarization angle values and to average residual polarization of unpolarized sources, if necessary. Tip/tilt align the orthogonal direction of the GPD to the incident beam. Two out of the three feet of the tip/tilt plate will be manual micrometers, but one will be motorized to carry out automatically measurements with the beam off-axis of a series of known angles, between <1 degree and about 5 degrees, e.g., to simulate the focusing of X-ray mirror shells. A cut-out and a detailed view of the polarized source with Oxford Series 5000 X-ray tube mounted is reported in the figures below. Testing and characterization of the ICE sources will be carried out with three commercial X-ray detectors: (i) a CCD imager (model Andor iKon-M SY) to measure and map the beam spot and center the diaphragm of polarized source; (ii) a SDD spectrometer (model Amptek FAST SDD) to characterize the spectrum (and hence the polarization) and the counting rate of the beam. This detector will be also used as a reference for the efficiency measurements of the DU efficiency; (iii) a Si-PIN detector (model Amptek XR100CR) as a spare spectrometer.

DU calibrations will be performed in orbit with the set of calibration sources mounted on the FCW with the aim of: monitoring the modulation factor value of the GPD for monochromatic photons and hence the stability of polarimetric response at two energies; monitoring the energy resolution of the GPD; check for the presence of spurious polarization due to, e.g., any anisotropy in the distribution of the background; map and monitor the gain of the GEM and its non-homogeneities. Calibration sources are hosted on a Filter and Calibration Wheel (FCW) which is included in the DU.

The items of the FCS were tested first with a commercial SDD and CCD to verify their operation. The calibration sources were then tested in TV with the flight DU to derive their spectra, images on the detectors, and polarimetric performance. The morphology of the sources, studied independently with CCD and GPD, are consistent. The expected counting rates are comparable across the different FMs, with differences that can be ascribed to the different energy resolution of each DU. The counting rates satisfy the requirements, and the modulation of the polarized sources is consistent with the one expected from Bragg diffraction. During the mission, the FCS will help validate the scientific results of IXPE by checking the detector response to point-like and extended sources. In summary, the results obtained on-ground, when extrapolated to the ones expected in flight, provide us with confidence that the FCS will be able to properly monitor the performance of the DUs.

IXPE introduces the capability for X-ray polarimetric imaging, uniquely enabling the measurement of X-ray polarisation with scientifically meaningful spatial, spectral, and temporal resolution. During its baseline two-year operation, IXPE will have polarisation measurements on a few dozen sources, including various types of neutron stars, blackhole systems, active galactic nuclei, and supernova remnants, help to probe the origin and destiny of our universe. TECHNICAL AND SCIENCE OBJECTIVES The primary technical and science objectives of IXPE are: Improving polarization sensitivity by two orders of magnitude over the X-ray polarimeter aboard the Orbiting Solar Observatory OSO-8. Providing simultaneous spectral, spatial, and temporal measurements. Determining the geometry and the emission mechanism of Active Galactic Nuclei and microquasars. Finding the magnetic field configuration in magnetars and determining the magnitude of the field. Finding the mechanism for X ray production in pulsars (both isolated and accreting) and the geometry. Determining how particles are accelerated in Pulsar Wind Nebulae. IXPE uses X-ray polarimetry to dramatically expand X-ray observation space, which historically has been limited to imaging, spectroscopy, and timing. This advance will provide new insight as to how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions—such as neutron stars and black holes. Polarization uniquely probes physical anisotropies—ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin—that are not otherwise easily measurable. IXPE complements all other investigations in high-energy astrophysics by adding the important and relatively unexplored dimensions of polarization to the parameter space for exploring cosmic X-ray sources and processes, and for using extreme astrophysical environments as laboratories for fundamental physics.

IXPE scientific requirements descend from the expected polarization properties of the celestial sources and their temporal, spectral and angular characteristics. The sensitivity of IXPE, as shown in the same table, is set for meaningful polarimetry, with realistic observing time, of the brightest AGNs. The following table flows down the scientific requirements into instrument requirements. Modulation factor, Modulation factor, Spurious modulation, GPD quantum efficiency, GPD quantum efficiency, Energy resolution, Knowledge of the spurious modulation, Systematic error on angle, Position resolution (HEW), Dead time, Maximum counting rate, Time accuracy, Background