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 (email@example.com)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
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.