INTEGRAL detection of re-brightening from GRO J1008-57

ATel #10533; V. Sguera (INAF/IASF Bologna, Italy), A. Bazzano, P. Ubertini, M. Fiocchi (INAF/IAPS Roma, Italy), J. Chenevez (DTU Space, Denmark), L. Sidoli (INAF/IASF Milano, Italy), E. Kuulkers (ESA/ESAC, Spain), V. A. Lepingwell, A. J. Bird (Univ. of Southampton, UK)
on 26 Jun 2017; 15:31 UT
Credential Certification: Vito Sguera (sguera@iasfbo.inaf.it)

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The transient Be HMXB GRO J1008-57, recently detected during a type I X-ray outburst (ATels #10418, #10466) with a predicted peak on MJD 57909.37 or 5 June 2017 (based on the times of maximum flux, epoch=54416.65 MJD and Porb=249.48 days; Kuhnel et al. 2013), is currently undergoing a peculiar enhanced X-ray activity starting on MJD 57922 (17 June 2017) as detected by MAXI/GSC (ATel #10529).

GRO J1008-57 was in the field of view of INTEGRAL during recent Galactic Plane Scan observations (GPS) regularly performed in the last week.

The source was detected by IBIS/ISGRI at about 9 sigma level (22-60 keV, 5.8 ks of effective exposure time) during GPS observations in revolution 1828 starting on MJD 57920.8 (16 June 2017 at 19:12 UTC). The measured average flux was 25.3 +/- 2.8 mCrab. The source was also in the JEM-X monitor field of view for 1.8 ks effective exposure, the measured average fluxes were 25 +/- 4 mCrab (3-10 keV) and 15 +/- 6 mCrab (10-25 keV).
During the subsequent GPS observations (revolution 1829, starting on MJD 57922), the source was detected by IBIS/ISGRI at 11.8 sigma level (22-60 keV, 4.3 ks of effective exposure time) with an enhanced average flux of 33.1 +/- 2.8 mCrab. The source was also in the JEM-X monitor field of view for 3.3 ks effective exposure, the measured average fluxes were 25 +/- 3 mCrab (3-10 keV) and 38.0 +/- 10.5 mCrab (10-25 keV).
Finally, during the most recent GPS observations (revolution 1831, starting on MJD 57928.7 or 24 June at 16:48 UTC) the source was detected by IBIS/ISGRI at 15.6 sigma level (22-60 keV, 4.3 ks of effective exposure time) with a further enhanced average flux of 43.6+/-2.8 mCrab. The source was also in the JEM-X monitor field of view for 3.4 ks effective exposure, the measured average fluxes were 30 +/- 3 mCrab (3-10 keV) and 52 +/- 12 mCrab (10-25 keV).

GRO J1008-57 is clearly undergoing a peculiar re-brightening especially above 20 keV, confirming the enhanced X-ray activity detected by MAXI/GSC below 20 keV.