X-ray observations of black hole sources

TL;DR

This paper reviews the role of X-ray observations in understanding black hole sources, highlighting past developments, recent polarisation measurements, and emphasizing the need for more sensitive future instruments.

Contribution

It provides a historical overview of X-ray studies of black holes and discusses recent polarisation measurements, proposing future directions for more detailed observations.

Findings

X-ray observations confirmed black holes in X-ray binaries and AGN.

Recent polarisation measurements of Cygnus X-1 offer new insights.

Highlights the necessity for more sensitive X-ray instruments.

Abstract

X-ray astronomy is closely related to the study of black hole sources. The discovery that some unseen objects, more massive than any degenerate star, emit huge amounts of X-rays helped accept the concept that back holes are present in X-ray binaries. The detection of copious amounts of highly variable X-rays helped the emergence of the paradigm that all Active Galactic Nuclei harboured a supermassive black hole. Since the bulk of the emission in these sources are in X-rays and X-rays are thought to be originating from regions closest to the black holes, it was expected that X-ray observations would yield significant inputs for our understanding of the physical phenomena happening close to black holes like the disk-jet connection and help measure many important parameters like the mass and spin of the black holes. I will trace the developments in this area for the past several decades and, noting the relatively limited success, stress the need for more sensitive measurements. I will highlight the recent X-ray polarisation measurements of the Galactic black hole candidate source Cygnus X-1 using the CZT Imager instrument of AstroSat and sketch possible future developments.