Probing the Vicinity of Compact Sources: Galactic Black Holes and X-ray Pulsars

Abstract

Compact objects---black holes and neutron stars---serve as natural laboratories for exploring matter and radiation under extreme conditions shaped by strong gravity, magnetic fields, and relativistic effects. This dissertation presents a systematic study of accretion and emission processes in Galactic black hole binaries and X-ray pulsars, focusing on Cygnus X-1, GRS 1915+105, and 4U 1907+09. Using broadband spectral-timing analysis and recent X-ray polarimetry, this work aims to build a multi-dimensional picture of the innermost regions around compact objects.

We begin with Cygnus X-1, analyzing Insight-HXMT data to trace timing and spectral properties across different accretion states. The results reveal how reflection features, variability, and coherence evolve with state transitions.

Next, GRS 1915+105 is studied through a multi-year NICER and HXMT campaign, capturing its transition from a soft to an obscured state. This evolution includes changes in disk wind ionization, shedding light on the coupling between accretion flow geometry and wind launching.

Finally, we investigate the X-ray pulsar 4U 1907+09 with IXPE, presenting the first detailed polarization measurements. Phase-averaged and phase-resolved analyses reveal a stable magnetospheric geometry and highlight the effects of short-term variability on emission.

Together, these studies deepen our understanding of accretion, variability, and energy release in compact objects. By integrating spectral, timing, and polarimetric tools, this work demonstrates the power of multi-dimensional diagnostics and paves the way for future missions like eXTP and Athena.