Abstract Details

Name: Tanmoy Chattopadhyay
Affiliation: Penn State University
Conference ID: ASI2017_1225
Title : Observational Aspects of Hard X-ray Polarimetry
Authors and Co-Authors : NA
Abstract Type : Thesis
Abstract Category : Thesis
Abstract : Sensitive X-ray polarization measurements may address a wealth of astrophysical phenomena which so far remain beyond our understanding through available X-ray spectroscopic, imaging and timing studies. However, there has not been any significant advancement in this field for the last four decades primarily due to poor sensitivity of the polarimeters. Recently with the advancement in detection technology, X-ray polarimetry may see a significant progress in near future, especially in soft X-rays with the invention of photoelectron tracking polarimeters. However, photoelectric polarimeters are sensitive only in soft X-rays where the radiation from the sources is dominated by thermal radiation and therefore expected to be less polarized. On the other hand, in hard X-rays, sources are expected to be highly polarized due to the dominance of nonthermal emission over its thermal counterpart. Compton polarimeters provide better sensitivity than photoelectric polarimeters in hard X-rays with a broad energy band of operation. Recently, with the development of hard X-ray focusing optics e.g. NuSTAR, it is now possible to conceive Compton polarimeters at the focal plane of such hard X-ray telescopes. Such a configuration is expected to be very sensitive for polarization experiments. In this context, we initiated the development of a focal plane Compton polarimeter, consisting of a plastic scatterer surrounded by a cylindrical array of CsI(Tl) scintillators. We carried out extensive experiments to characterize the plastic with normal PMT readout and the CsI(Tl) scintillators with Si photomultipliers (SiPM) readout. Finally, we integrated the Compton polarimeter and tested its response to polarized and unpolarized radiation and compared the experimental results with Geant−4 simulation. Besides the efforts in developing sensitive X-ray polarimeters, it is equally important to attempt polarization measurements from the existing or planned instruments which are not meant for X-ray polarization measurements but could be sensitive to it. Cadmium Zinc Telluride Imager (CZTI) onboard Astrosat is one of such instruments which is expected to provide sensitive polarization measurements for bright X-ray sources due to its pixelation nature and significant Compton scattering efficiency beyond 100 keV. Detailed Geant−4 simulations and polarization experiments with the flight configuration of CZTI show that CZTI does have significant polarization measurement capability for bright sources in hard X-rays. CZTI is primarily a spectroscopic instrument and to properly utilize its spectroscopic capabilities, it is important to generate accurate response matrix which in turn requires precise modelling of the CZT lines shapes. We have developed a model to predict the line shapes from CZTI modules taking into account the mobility and lifetime of the charge carriers and charge sharing fractions. The model predicts the line shape quite well and has been used to generate pixel-wise response matrix for CZTI.