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

M. Kühnel (1) , S. Müller (1) , I. Kreykenbohm (1) , F. Fürst (2) , K. Pottschmidt (3) , R. E. Rothschild , I. Caballero (4) , V. Grinberg (1) , G. Schönherr (5) , C. Shrader (3) , D. Klochkov , R. Staubert , C. Ferrigno , J.-M. Torrejón , S. Martínez-Núñez , J. Wilms (1)

1 ECAP - Erlangen Centre for Astroparticle Physics
2 CALTECH - California Institute of Technology
3 CRESST - Center for Research and Exploration in Space Science and Technology [GSFC]
4 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
5 AIP - Leibniz-Institut für Astrophysik Potsdam

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.

Keywords: X-rays: binaries; pulsars: individual: GRO J1008-57; accretion; accretion disks; ephemerides

Domains: Physics [physics]; Astrophysics [astro-ph]

M. Kühnel, S. Müller, I. Kreykenbohm, F. Fürst, K. Pottschmidt, et al.. GRO J1008−57: an (almost) predictable transient X-ray binary. Astronomy and Astrophysics - A&A, 2013, 555, pp.A95. ⟨10.1051/0004-6361/201321203⟩. ⟨cea-01135403⟩