GRO J1008 57: a laboratory for accretion physics

We present timing and spectroscopic results of three outbursts of the transient high mass X-ray binary GRO J1008−57 in 2005, 2007, and 2011. The orbital parameters from the literature are not in agreement with the measured pulse arrival times. We therefore updated the orbital solution, specifically the orbital period and the time of periastron passage, using pointed observations with RXTE, Swift, and Suzaku. We confirmed our results with an analysis of RXTE-ASM lightcurves. We show that GRO J1008−57’s outbursts occur mostly at the same orbital phase and therefore make predictions of outbursts and, thus, scheduled observations possible. The X-ray spectrum of GRO J1008−57 during an outburst can be well described by a cutoff power law with an additional black body at energies below 10 keV. We found that the same spectral model describes GRO J1008−57 during the rise and the decline of the outburst at fluxes changing by two orders of magnitude. In particular, the photon index of the power law and the black body flux show a correlation with the total X-ray flux. Other parameters such as the black body temperature and the folding energy are independent of flux and remain the same over all analyzed outbursts