Dottorato in ASTRONOMIA

Titolo della tesi: X-ray spectroscopy and polarimetry of black hole X-ray binaries

X-ray polarimetry will open a new window in X-ray astronomy that can revolutionize the current understanding of the accretion and ejection mechanisms onto black holes X-ray binaries (BHXBs). With the launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 9th December 2021, X-ray astronomy will be benefited with the additional polarimetry signal along the currently existing timing, imaging, and spectroscopic analysis. Since BHXBs are highly variable in time, the polarimetry signals might also vary depending upon various intrinsic properties and hence, it is necessary to have a thorough spectral analysis of different states of the source to make a meaningful comprehension of the polarimetric signal. In this work we performed a spectroscopic analysis of GRS 1915+105, with a special focus on the accretion disk winds. We find that a MHD model can be used to explain the wind in the soft and hard states of the source using Chandra HETG data. Further, using NuSTAR data, we also predict that the same wind could be the source of obscuration in the recent obscured state of the source. Then, we predicted the polarization properties from the system considering various geometrical and physical parameters left unconstrained by spectroscopy alone. Therefore, upcoming IXPE observations will be crucial in understanding the accretion and outflow geometry in this source. Being one of its first kind, the calibration of IXPE is important in order to understand the response of the detector to polarized and unpolarized radiation as well as other systematic effects in the detector while observing bright and variable sources. In particular, the response to unpolarized radiation, called the spurious modulation, cannot be modeled but only characterized experimentally and hence we performed experiments using mock versions of the detector which are identical to the flight version of the gas pixel detector (GPD) for calibration. We constrained the secular variation of pressure and therefore the variation in quantum efficiency. We also calibrated a model of GPD, and estimated its response to polarized and unpolarized radiation and hence the efficiency of the detector as a polarimeter. In order to validate the X-ray tubes to be used to generate the response of the GPD to unpolarized radiation, their polarization properties were also studied in detail.