Evidence for different accretion regimes in GRO J1008-57

TL;DR

This study analyzes the spectral evolution of GRO J1008-57 across a wide luminosity range, revealing transitions between different accretion regimes and providing insights into neutron star accretion physics.

Contribution

It provides the first comprehensive spectral analysis over multiple outbursts, identifying accretion regime transitions and comparing observed luminosities with theoretical predictions.

Findings

Spectral parameters evolve with luminosity, with photon index hardening at intermediate luminosities.

Breakdown of correlation at extreme luminosities suggests regime transitions.

Detailed spectroscopy of a unique outburst challenges existing accretion models.

Abstract

We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE data. We find significant evolution of the spectral parameters with luminosity. In particular the photon index hardens with increasing luminosity at intermediate luminosities between $10^{36}$ $-$ $10^{37}$ erg s$^{-1}$. This evolution is stable and repeatedly observed over different outbursts. However, at the extreme ends of the observed luminosity range, we find that the correlation breaks down, with a significance level of at least $3.7\sigma$. We conclude that these changes indicate transitions to different accretion regimes, which are characterized by different deceleration processes, such as Coulomb or radiation breaking. We compare our observed luminosity levels of these transitions to theoretical predications and discuss the variation of those theoretical luminosity values with fundamental neutron star parameters. Finally, we present detailed spectroscopy of the unique "triple peaked" outburst in 2014/15 which does not fit in the general parameter evolution with luminosity. The pulse profile on the other hand is consistent with what is expected at this luminosity level, arguing against a change in accretion geometry. In summary, GRO J1008-57 is an ideal target to study different accretion regimes due to the well constrained evolution of its broad-band spectral continuum over several orders of magnitude in luminosity.