Hard (State) Problems

TL;DR

This paper reviews recent developments and challenges in understanding the hard state of microquasars, focusing on the disk/jet connection and the controversy over broad iron features in X-ray spectra.

Contribution

It critically examines current models of the hard state, highlighting issues with the truncated disk picture and relativistic reflection models based on recent observational data.

Findings

Detection of broad iron features challenges the truncated disk model.

Uncertainty whether the problem lies with the disk model or reflection modeling.

Recent results question the standard understanding of the hard state in microquasars.

Abstract

For microquasars, the one time when these systems exhibit steady and powerful jets is when they are in the hard state. Thus, our understanding of this state is key to learning about the disk/jet connection. Recent observational and theoretical results have led to questions about whether we really understand the physical properties of this state, and even our basic picture of this state is uncertain. Here, I discuss some of the recent developments and possible problems with our understanding of this state. Overall, it appears that the strongest challenge to the standard truncated disk picture is the detection of broad iron features in the X-ray spectra, and it seems that either there is a problem with the truncated disk picture or there is a problem with the relativistic reflection models used to explain the broad iron features.