Black-hole binaries: life begins at 40 keV

TL;DR

This paper discusses the evolution of high-energy spectra in black-hole transients during state transitions, emphasizing the potential of Simbol-X to improve spectral measurements and understanding of black hole environments.

Contribution

It highlights the limitations of current observations and demonstrates how Simbol-X can accurately determine spectral shapes during black-hole state transitions.

Findings

RXTE observed the high-energy cutoff decrease then increase during transitions.

Simbol-X will significantly improve spectral shape measurements.

Understanding of extreme physical conditions near black holes will be enhanced.

Abstract

In the study of black-hole transients, an important problem that still needs to be answered is how the high-energy part of the spectrum evolves from the low-hard to the high-soft state, given that they have very different properties. Recent results obtained with RXTE and INTEGRAL have given inconsistent results. With RXTE, we have found that the high-energy cutoff in GX 339-4 during the transition first decreases (during the low-hard state), then increases again across the Hard-Intermediate state, to become unmeasurable in the soft states (possibly because of statistical limitations). We show Simbol-X will be able to determine the spectral shape with superb accuracy. As the high-energy part of the spectrum is relatively less known than the one below 20 keV, Simbol-X will provide important results that will help out understanding of the extreme physical conditions in the vicinity of a stellar-mass black hole.