The Imaging X-ray Polarimetry Explorer (IXPE): prospects for spatially-resolved X-ray polarimetry of extended sources and in-orbit calibrations

Context: X-ray polarimetry provides two missing observables in the high energy domain, namely the polarization degree and angle, making it possible to obtain information on the geometry and emission processes of high energy celestial sources. Unfortunately, the field has been dormant for decades because of technological limitations and competition with other experiments. The launch of the NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE) mission in December 2021 opens a new era for X-ray polarimetry. Thanks to the imaging and the polarimetric capabilities of the Gas Pixel Detectors, IXPE will investigate the polarimetric properties of complex fields and extended sources. Aims: This thesis has been focused on two relevant tasks among those foreseen in the pre-launch phase of a new mission: the plan for in-orbit calibrations and for observations of faint extended sources. Since no X-ray celestial sources are available for in-orbit calibrations (the only known source in the X-rays, the Crab Nebula, cannot be employed because of its variability), IXPE will have on board a set of polarized and unpolarized calibration sources. Since the celestial sources will be observed by IXPE for long and segmented integration times, monitoring the detector performance during the mission lifetime will be of fundamental importance, because the characteristics of the GPD are expected to slightly evolve in time. I will present the acceptance tests of the Flight Models of the polarized and unpolarized calibration sources and their validation in thermal vacuum when combined with the Flight Models of the IXPE detectors. While negligible for the observation of point sources, the depolarizing effect of unpolarized instrumental and diffuse sources of backgrounds will be another challenge for the X-ray polarimetric observation of faint, extended sources. I will describe the effect of the main sources of background (instrumental, diffuse Galactic plane emission, and cosmic X-ray background) on the X-ray polarimetric observations of faint, extended sources. Finally, I will present a feasibility study of the IXPE observation of two extended sources: the Tycho supernova remnant, and the molecular clouds of the Sgr A complex. Methods: The acceptance and validation tests of the Flight Models of the polarized and unpolarized calibration sources were performed with calibrated commercial detectors such as Charged Couple Devices and Silicon Drift Detectors, and with the IXPE Flight detector Units harboring the Gas Pixel Detectors in a thermal vacuum chamber. The evaluation of the impact of the sources of background on the detectability of the polarization for a subset of faint extended sources in the IXPE observing plan, and the simulations of the IXPE observations were performed with the Monte Carlo software ixpeobssim. Results: I obtained the counting rates, spectra, image and polarization information, from each Flight Model of the on board calibration sources, and determined the time necessary to achieve the needed precision. I demonstrated that the on-board calibration system will enable us to assess and verify the functionality of the GPD and validate its scientific results in orbit. In particular, the calibration sources illuminate the whole detector, or just a part of it, measuring properties such as the detector response to both polarized and unpolarized radiation, and gain variation in time and space. This information will be used to perform the on orbit calibration to check the performance in time of the GPD. For the faintest extended sources, such as SN1006 and the molecular clouds of the Sgr A complex, background mitigation techniques will be necessary, while for other sources such as Cas A, Tycho, and the PSW MSH 15-52, the effects will be negligible. For the former, the impact of the instrumental background will require the application of rejection techniques based on the event properties in order to discriminate between real events and background produced by the interaction of cosmic rays with the detector. The feasibility studies presented for the IXPE observation of Tycho and the molecular clouds of the Sgr A complex show that IXPE will be able to (a) distinguish between magnetic field geometries and detect polarization of synchrotron-emitting structures, (b) determine the recent past of our Galactic center, and (c) put constraints on the position of the clouds along the line of sight. The data analysis techniques presented here make it possible to reconstruct the intrinsic, undiluted polarization degree of these sources. Conclusions: With the successful launch of IXPE, we can finally perform spatially resolved polarimetry in X-rays, we can add two observables (polarization angle and degree), and finally answer questions about source and magnetic field geometry and determine emission processes. The preliminary data acquired in the first weeks after the launch show that the the on-board calibration sources are performing as expected, that the residual instrumental background level is very close to the anticipated value, and that the imaging capabilities are fully compliant with the requirements. The work done during this PhD project has been crucial for the IXPE mission, gathering information about the possibility of performing calibrations in space with the on board calibration sources, and observing extended sources (supernova remnants and faint molecular clouds) by means of realistic simulations, and careful data analysis