INTEGRAL detection of type II outburst onset from GRO J1008-57

ATel #10626; V. Sguera (INAF/IASF Bologna, Italy)
on 9 Aug 2017; 14:11 UT
Credential Certification: Vito Sguera (sguera@iasfbo.inaf.it)

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The transient Be HMXB GRO J1008-57 is known to display regular type I outburst (duration of about 20 days) at intervals consistent with the orbital period of about 250 days. The latest type I burst was detected by INTEGRAL (ATels #10418, #10466), it began on 2017 May 22 (MJD 57895) and reached its peak on 2017 June 4 (MJD 57908). Subsequently, rather than declining to deep quiescence, the source underwent a peculiar re-brightening starting from 2017 June 18 (MJD 57922) as detected by both MAXI/GSC and INTEGRAL (ATels #10529, #10533). After such re-brightening, the source flux remained at a roughly constant value of about 45 mCrab in the IBIS/ISGRI energy band 22-60 keV. Follow-up observations in the optical band indicated the presence of an exceptionally large circumstellar disk around the Be star (ATel #10561). Recently MAXI/GSC detected on 2017 August 3 (MJD 57968) a further re-brightening from GRO1008-57 in the 4-10 keV energy band (ATel #10617).

GRO J1008-57 was recently in the field of view of INTEGRAL during public observations targeted at GX 301-2. The source was detected by IBIS/ISGRI at about 34 sigma level (22-60 keV) during a time interval starting on 03 Aug 2017 at 11:51 UTC (MJD 57968.5) and ending on 04 Aug 2017 at 05:23 UTC (MJD 57969.22). No significant detection was obtained in the energy band 60-100 keV. The measured average flux was 286+/-11 mCrab (22-60 keV), this strongly suggests that IBIS/ISGRI likely detected the onset of a new type II outburst. This would be the third recorded giant type II outburst from the source, the previous ones date back to 2012 and 2014.

We note that the Swift/BAT daily light curve of GRO J1008-57 also shows a strong and rapid flux increase (Swift/BAT transient monitor results provided by the Swift/BAT team, Krimm et al., 2013, ApJSS 209,14). The Swift/BAT measured average fluxes in the energy band 15-50 keV (i.e. similar to the above IBIS/ISGRI one) are about 213+/-10 mCrab and 267+/-11 mCrab on MJD 57968 and 57969, respectively (i.e. date close to the above INTEGRAL observation date). At the time of writing, the source reached a 15-50 keV flux of about 515 mCrab as measured by Swift/BAT on 2017 Aug 09 (MJD 57974).