Swift observation during the current type II outburst of GRO J1008-57

ATel #4577; M. Kuehnel, S. Mueller, I. Kreykenbohm, J. Wilms (Remeis-Observatory Bamberg), K. Pottschmidt (CRESST-UMBC/NASA-GSFC), H. Krimm (NASA-GSFC), F. Fuerst (Caltech-SRL), R. E. Rothschild (UC San Diego), I. Caballero (CEA Saclay), D. Klochkov, R. Staubert, S. Suchy (IAA Tuebingen), P. Kretschmar (ESAC/ESA)
on 15 Nov 2012; 21:26 UT
Credential Certification: Ingo Kreykenbohm (ingo.ik.kreykenbohm@fau.de)

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The transient X-ray binary GRO J1008-57 is currently undergoing a very bright outburst (ATEL #4561), which triggered the BAT instrument on 2012 November 13 (ATEL #4573). The current flux of the source, which is around 0.5 Crab in Swift/BAT, is about 5 times higher than the strongest outburst detected before (ATEL #3805), which, along with the orbital phase of ~0.3 (ATELs #4561, #4564), strongly suggests that this outburst is of Type II. Usually, GRO J1008-57 shows regular type I outbursts at the periastron. Archival spectra from earlier outbursts are well described by a powerlaw with an exponential cut-off at higher energies and a black body at energies below 10 keV. We applied this model to the Swift/XRT and Swift/BAT spectra recorded on MJD 56245 (2012 November 13). The fit parameters of Gamma = 0.65, E_fold = 17.7 keV and kT = 1.8 keV are consistent with previous type I outbursts. The 15-50 keV flux is 7.5e-9 erg/s/cm^2. Thus, there is no overall spectral change from a regular type I to a type II outburst for GRO J1008-57. There are, however, differences in the absorption column density and iron fluorescence lines compared to our previous analysis (Kuehnel et al., 2012, 9th Integral Workshop). We detected a significantly lower column density of about 1.25e22 cm^-2, which at first glance would imply less material available in the system. There is also weak evidence for three narrow emission lines from neutral, H-like, and He-like iron. Considering the very high luminosity of GRO J1008-57 compared to previous type I outbursts this means that the apparently lower neutral hydrogen column density is probably due to strong photoionization of the circumstellar material. The XRT-lightcurve shows clear pulsations around 93.65 s, which is slightly lower than the 93.75 s detected during the 2007 outburst (ATEL #1304). Using using the orbital parameters given in ATEL #4564 shows that the Doppler effect is not able to explain this difference. Thus, the neutron star has spun up.