GRO J1008-57: an (almost) predictable transient X-ray binary

TL;DR

This paper presents a detailed analysis of GRO J1008-57, demonstrating that its outbursts are highly predictable based on orbital phase and flux-dependent spectral properties, enabling precise timing and spectral evolution forecasts.

Contribution

It introduces a new orbital ephemeris for GRO J1008-57 and establishes flux-dependent spectral correlations that allow prediction of outburst timing and spectral evolution.

Findings

Outbursts occur near periastron with high timing precision.

Spectral shape depends on source flux, enabling predictions.

The spectral evolution model applies even during a type II outburst.

Abstract

A study of archival RXTE, Swift, and Suzaku pointed observations of the transient high mass X-ray binary GRO J1008-57 is presented. A new orbital ephemeris based on pulse arrival timing shows times of maximum luminosities during outbursts of GRO J1008-57 to be close to periastron at orbital phase -0.03. This makes the source one of a few for which outburst dates can be predicted with very high precision. Spectra of the source in 2005, 2007, and 2011 can be well described by a simple power law with high energy cutoff and an additional black body at lower energies. The photon index of the power law and the black body flux only depend on the 15-50 keV source flux. No apparent hysteresis effects are seen. These correlations allow to predict the evolution of the pulsar's X-ray spectral shape over all outbursts as a function of just one parameter, the source's flux. If modified by an additional soft component, this prediction even holds during GRO J1008-57's 2012 type II outburst.