GRO J1008-57: an (almost) predictable transient X-ray binary
Klochkov, D.; Wilms, J.; Ferrigno, C.; Pottschmidt, K.; Grinberg, V.; Staubert, R.; Caballero, I.; Fürst, F.; Müller, S.; Kreykenbohm, I.; Rothschild, R. E.; Schönherr, G.; Kühnel, M.; Martínez-Núñez, S.; Shrader, C.; Torrejón, J. -M.
Germany, United States, France, Switzerland, Spain
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.
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