The high energy emission of GRO J1655-40 as revealed with INTEGRAL spectroscopy of the 2005 outburst

TL;DR

This study analyzes INTEGRAL X-ray spectra of GRO J1655-40 during its 2005 outburst, revealing that non-thermal Comptonization dominates high energy emission, challenging previous models of black hole spectra.

Contribution

It demonstrates that high energy emission in the hard state can be modeled with an unbroken power-law and is dominated by non-thermal electron Comptonization, contrary to prior expectations.

Findings

High energy spectra are well fitted by an unbroken power-law.

Non-thermal electron Comptonization dominates the hard state.

No high energy cut-off is required in the spectra.

Abstract

We present broadband (3-500 keV) INTEGRAL X-ray spectra and X-ray/optical light curves of the luminous black hole X-ray transient and relativistic jet source GRO J1655-40. Our analysis covers four Target of Opportunity observations of the outburst that started in February 2005. We find that the high energy emission of GRO J1655-40 can be modelled well with an unbroken power-law (with photon indices of 1.72+-0.03,2.21+-0.04 for the first and the second observations, respectively). These correspond to hard and thermal dominant states, respectively. In contrast to many other black hole spectra, high energy complexity in the form of a break or cut-off is not required for the hard state, contrary to previous expectations for this state. We show for the first time that Comptonization by non-thermal electrons is the dominant process for the high energy emission in the hard state. We discuss our results in terms of models for broad-band emission and accretion flows in stellar-mass black holes.