New insights from the best-studied black hole systems: Cyg X-1 and M87*

Abstract:
I will present recent observational results for Cyg X-1 and M87* and critically examine their theoretical interpretations. These two sources are canonical examples of black-hole accretion systems and play a central role in shaping our understanding of the physics operating near accreting black holes. M87* is the best-studied active galactic nucleus, thanks to its rich spectral coverage and the ability to image its central region with VLBI techniques. Its broadband spectrum is often attributed to emission from a stratified jet. However, this interpretation appears inconsistent with Event Horizon Telescope (EHT) imaging and with basic physical expectations for the EHT-emitting region. I will discuss an alternative scenario - motivated by GRMHD simulations of black-hole accretion - in which the bulk of the observed radiation originates in the immediate vicinity of the black hole. I will also highlight the similarity between the electron-acceleration parameters required to explain the gamma-ray flares in M87* and the X-ray flares in Sgr A*. Cyg X-1 is the longest- and best-studied black-hole X-ray binary, and its extensive timing and spectral data have recently been complemented by precise X-ray polarization measurements in both the hard and soft spectral states. I will focus on the soft-state measurement, which was initially interpreted as evidence for strong gravitational lensing of X-rays near the black hole - an argument that, if correct, would strongly support a rapid black-hole rotation. However, we show that GR effects can only make a minor contribution to the observed polarization, which can instead be explained by Comptonization in the X-ray–emitting corona.

Al. Lotników 32/46

CFT PAN, room 203