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

¹ Dr. Karl Remeis-Observatory & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: matthias.kuehnel@sternwarte.uni-erlangen.de
² Space Radiation Lab, California Institute of Technology, MC 290-17 Cahill, 1200 E. California Blvd., Pasadena, CA 91125, USA
³ CRESST, Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
⁴ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Center for Astronomy and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
⁶ CEA Saclay, DSM/IRFU/SAp – UMR AIM (7158) CNRS/CEA/Université Paris 7, Diderot, 91191 Gif sur Yvette, France
⁷ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Universities Space Research Association, Columbia, MD 21044, USA
⁹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁰ ISDC Data Center for Astrophysics, Chemin d’Écogia 16, 1290 Versoix, Switzerland
¹¹ Instituto de Física Aplicada a las Ciencias y las Tecnologías, Universidad de Alicante, 03080 Alicante, Spain

Received: 31 January 2013
Accepted: 9 May 2013

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 the 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 us 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.