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Swift/BAT detects continued brightening of Be/X-ray binary pulsar, GRO J1008-57

ATel #4573; H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), N. Gehrels (GSFC), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (Aoyama Gakuin Univ.), M. Stamatikos (OSU/GSFC), J. Tueller (GSFC), T. Ukwatta (MSU)
on 14 Nov 2012; 22:10 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 4577, 5004, 6465, 6917, 10529

The Swift/BAT hard X-ray transient monitor continues to show an increase in the flux from the Be/X-ray binary system GRO J1008-57. In the 15-50 keV band, the source had a count rate of 0.12 +/- 0.005 ct/s/cm^2 (~500 mCrab) in the daily average for 2012 November 14 (MJD 56245). There is no sign yet of the flux leveling off. This is the brightest that this source has been in the eight years of the Swift mission.

GRO J1008-57 triggered the BAT instrument on 2012 November 13 at 18:47:12.49 UT (Swift trigger #538290). This was a rate trigger with a 2.048-sec duration. The mask-weighted light curve clearly shows the ~93.8 s pulse period of the neutron star, with some variation (~20%) in brightness among the pulse peaks. Fitting the combined BAT event data from T-209 s to T+695 s, we see comparably good fits to cut-off power-law (CPL) and double blackbody models (DBB). The fit parameters are:

CPL:
Photon index: -0.22 +/- 0.72
Epeak [keV] : 24.3468 +/- 2.5
Chi-Squared = 44.2 for 56 dof.
Energy fluence (15-150 keV): (5.99 +/- 0.46) X 10^-6 ergs/cm2

DBB:
kT1 [keV] : 3.54 +/- 1.58
kT2 [keV] : 7.77964 +/- 0.97
Chi-Squared = 42.8 for 55 dof.
Energy Fluence 90% Error
[keV] [erg/cm2] [erg/cm2]
15- 25 2.502 X 10^-6 1.047 X 10^-6
25- 50 2.890 X 10^-6 8.648 X 10^-7
50-150 6.385 X 10^-7 2.158 X 10^-7
15-150 6.031 X 10^-6 1.741 X 10^-6

The Swift satellite made a delayed slew to the source position. However, at this time, the data from Swift/XRT is not yet available for analysis.

We note that the current activity of GRO J1008-57 is quite unusual. This source has had regular outbursts since late 2007 at roughly 8 month intervals, consistent with the orbital period of 249.46 days reported by Kuehnel et al (ATel #4564) and with outburst durations of ~20 days. The current outburst (reported by Jenke & Finger, ATel #4319) began on 2012 August 17, but rather than fading to quiescence, remained at a rate of > 50 mCrab in the BAT 15-50 keV range, as reported by Kuehnel et al (ATel #4355). The recent re-brightening was first reported by the MAXI team (Nakajima et al, ATel #4561). GRO J1008-57 has been brightening since around November 5.

BAT hard X-ray transient monitor light curve for GRO J1008-57