The front page shows an artist’s impression of the XIPE spacecraft together with one of its primary targets, the Crab nebula. The pixelized detector in the background illustrates how XIPE will measure the polarization of X-ray photons from the trajectories of the photoelectrons they produce. XIPE Assessment Study – Mission Summary Key scientific Obtain unique physical and geometrical information on basically all classes of X-ray sources goals through two still unexplored observables – the degree and angle of polarization. Particle acceleration processes: map the magnetic field and locate acceleration regions in the Crab Nebula and other PWN, and in SNR; infer the magnetic field structure in jets, deriving jet composition in blazars and studying acceleration physics in GRBs. Emission in strong magnetic field: derive vital information on accretion geometry and physical parameters in white dwarfs and neutron stars; constrain the equation of state of ultra-dense matter; understand the mechanism that triggers bursts in magnetars. Scattering in aspherical geometries: definitely test if the black hole at the center of our Galaxy was active a few hundred years ago; constrain the geometry and origin of the X- ray emitting corona in AGN and Galactic accreting black holes. Study the geometry of wind flow in AGNs. Fundamental Physics: observe vacuum birefringence in highly magnetized neutron stars; take advantage of General Relativistic effects to derive the spin of Galactic accreting black holes; search for Quantum Gravity signatures and Axion-like particles. Reference The Payload consists of three identical X-ray telescopes with 4 m focal length and an core payload instrument control unit. The mirrors have a total effective area of about 1500 cm2 with an angular resolution better than 30”. The Gas Pixel Detectors are designed as gas proportional counters but with a revolutionary readout, i.e. an ASIC CMOS chip developed to this aim. This readout allows us to measure the X-ray polarization of cosmic sources in the 2-8 keV energy range with high sensitivity by imaging the photoelectron track of each event. The energy resolution is 20% at 5.9 keV. The sensitivity (Current Best Estimate) is 1.2% for an intensity of 2x10-10 erg/s/cm2 (10 mCrab) in 300 ks of net observing time (the requirement is 1.8%). Each Detector Unit comprises also a Filter and Calibration Wheel and the Back End Electronics that manages the ASIC, digitally convert the analogue signals, time-tag the events with a few s resolution and perform the zero suppression. The Instrument Control Unit produces on- board a set of Quick Look data to be downloaded with high priority, used to monitor the on- going observations. It also formats and stores the data and sends them to the on-board data handling unit. The countries involved in the design and provision of the payload items are: Germany, Italy, Spain, United Kingdom, Poland. Involved in the study phase were also Sweden, Switzerland, the Netherlands, Finland and, to a small extent, also China. Overall XIPE is designed to be launched with a VEGA-C vehicle into an orbit of 550-630 km and <6o mission inclination. With this orbit, eclipses last about 35 minutes. The decay time is well above the profile nominal life time, extendable with a small amount of delta-v. Due to remaining spacecraft fragments the mission is planned to end by controlled re-entry. The baseline ground station is Kourou either for S/S or S/X telemetry band, albeit Malindi (or Singapore) is additionally available, if necessary, for additional downloading capability in case of particularly bright sources. The mission duration is 3+2 years. XIPE will perform consecutive slews and long exposure pointings while the observing plan includes also snapshot pointings for monitoring purposes. Target of Opportunities are also foreseen with a reaction time below 12 hours in working days and hours. The data policy foresees that 25% of the time is dedicated to a Core Program while the remaining 75% is dedicated to a competitive Guest Observing program. Description of The Spacecraft is composed by a Payload module and a Service module. The first hosts the the spacecraft focal plane, the second hosts the three mirrors. They are connected by a telescope tube maintained at a constant temperature of 20o C. Two industrial studies assessed the feasibility of the mission design with this satellite configuration. The Service Module surrounds the 3 mirrors. The focal plane platform will be accommodated at the other end of the telescope tube. The solar panel, of about 6.7 m2, will be fixed and either configured as a single body-mounted panel or composed of two sections with one deployed after launch. The field of regard corresponds to ½ accessibility of the sky at any time. The spacecraft is 3-axis stabilized with reaction wheels. Science and housekeeping data are stored in a 228 Gbit memory. The dry mass at launch is 1430 kg, including contingency. The required power in the observation mode is 1050 W. XIPE Assessment Study Report page 3 Foreword Despite the many spectacular successes in the more than fifty years long history of X-ray Astronomy, our knowledge of X-ray emitting sources is still sorely incomplete. For almost all of them, in fact, and with the notable exception of the two brightest sources in the X-ray sky (the Crab Nebula and the accreting neutron star Sco X-1) we lack information on one fundamental characteristic of the radiation – polarization. Without knowing the polarization degree and angle, vital physical and geometrical information is missing, often leading to severe model degeneracies. Given that many X-ray sources are characterized by non-thermal emission processes and/or by radiation transferred in highly asymmetric systems, it is immediately clear that X-ray polarimetric observations would often be crucial, much more than at longer wavelengths. Indeed, as discussed in the Scientific Objectives section, almost all classes of X-ray sources are expected to benefit from polarimetric measurements. Key information on phenomena like e.g. particle acceleration, radiative transfer in strong magnetic fields (including vacuum birefringence, a QED effect predicted 80 years ago but still to be unambiguously verified) and in deep gravitational potential wells, and scattering in aspherical geometries, is encoded in the polarization of X-ray photons. X-ray polarimetry may even be used to test Quantum Gravity theories and to search for Axion-like Particles. The only two currently available measurements, mentioned above, were obtained in the 1970s with non- imaging, narrow-band Bragg polarimeters. No X-ray polarimeters were part of the payload of space missions afterwards. Fortunately enough, highly efficient X-ray polarimeters based on the photoelectric effect have recently became available which, coupled with high throughput, long-heritage, focusing mirrors, result in a dramatic increase in sensitivity. The X-ray Imaging Polarimetry Explorer (XIPE) is designed to bring X- ray polarimetry into full maturity by providing astrophysically significant polarimetric measurements (time, spatially and spectrally resolved) for hundreds of targets. This will allow us to extend the observations to all classes of X-ray sources of interest, and to observe many sources in each of them, searching for correlations of the polarimetric properties with the main parameters of the class. Such an approach is crucial to fully understand the nature of these sources. The XIPE proposal was submitted in response to the ESA call for M4 missions and selected, in June 2015, for a phase A study. The activities included an internal ESA study and two parallel industrial studies, which were completed in early 2017. These studies did not only confirm the feasibility and high Technology Readiness Level of XIPE, they also led to several improvements with respect to the original proposal. Among them: a larger effective area (30 mirror shells instead of the original 27), a longer focal length (4 meters instead of 3.5), a larger Field of Regard (50% of the sky instead of 30%). Scientific performances were correspondingly enhanced, and XIPE now has a better sensitivity over the whole band, an improved capability to observe transient phenomena and to perform coordinated observations with ground-based facilities. The original Science Case has been revised, extended and sharpened by a Science Team, composed by more than 350 scientists worldwide, and structured in thematic Working Groups. Extensive simulations were performed to assess quantitatively the ability of polarization measurements to extract physical and geometrical information and to distinguish between competing models. One of the main results of this work is the confirmation that the baseline mission duration of 3 years is adequate, with some margin, to reach the main scientific goals. A possible extension of 2 years will permit to explore more challenging and/or uncertain science objectives, as well as to increase the statistical quality of the population studies. It is worth mentioning that a number of our targets need to be observed in a particular flux or spectral state. In the pessimistic case that no external facilities will be operating at the time of XIPE to provide the relevant triggers, a monitoring program of up to 12-14 sources, to be observed at regular cadence with very short exposures, is foreseen to assess their state. The 12 h repointing time will then allow us to go promptly to the target, if needed, as well as to point to unexpected,