Evidence for a change in the X-ray radiation mechanism in the hard state of Galactic black holes

TL;DR

This study investigates spectral variability in two black hole X-ray binaries, revealing a change in the X-ray radiation mechanism at low luminosities, possibly involving seed photon sources or jet dominance.

Contribution

It provides new observational evidence of a spectral transition in black hole binaries and discusses potential physical mechanisms behind this change.

Findings

Photon index decreases with luminosity down to 0.01 L_Edd

Below this luminosity, the photon index increases again

The ratio of Compton to seed photon luminosity varies with luminosity

Abstract

We present results on spectral variability of two Galactic black hole X-ray binaries, GRO J1655-40 and GX 339-4, in the hard state. We confirm a transition in behaviour of the photon index with luminosity, such that the well known decrease in X-ray photon index with decreasing luminosity only continues down to L_bol ~ 0.01 L_Edd . Below this point the photon index increases again. For Comptonisation models, this implies that the ratio of the Compton luminosity to seed photon luminosity, lh/ls, changes with bolometric luminosity, consistent with a scenario where seed photons change from cyclo-synchrotron at the lowest luminosities to those from a truncated disc. Alternatively, the transition could mark the point below which the non-thermal jet starts to dominate, or where reprocessed photons replace the viscous ones in an outflowing corona model.