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INTEGRAL observations of GRO J1008-57 in outburst

ATEL # 1995; J.-C. Leyder (IAG, Liege), C. Ferrigno (IAAT, Tuebingen & ISDC, Geneva), M. Tuerler (ISDC, Geneva), R. Walter (ISDC, Geneva)
on 30 Mar 2009; 21:00 UT
Password Certification: Roland Walter (Roland.Walter@obs.unige.ch)

Subjects: X-ray, Gamma Ray, Binaries, Pulsars, Transients

The Be X-ray binary GRO J1008-57 is known to display regular outburts, with a period of 248 days (ATel #940; Coe et al. 2007, MNRAS 378, 1427). In the past, it has been detected by INTEGRAL at a level of 30 mCrab in October 2005 (ATel #647; in the 18-45 keV energy range), but also in June 2004 (Coe et al. 2007). Swift/BAT detected a brighter than usual flare in November 2007 (ATel #1298), with a level of 100 mCrab up to 400 mCrab (in the 15-50 keV energy range; see the Swift/BAT lightcurve).

Recent observations taken with INTEGRAL show that GRO J1008-57 is brigthening again, as expected every 248 days. In the latest revolution (2009-03-29 02:45:55 UT to 2009-03-30 00:25:41 UT), GRO J1008-57 is detected with a flux of 100 mCrab. The ISGRI lightcurve is stable, staying between 14-18 counts/s (in the 20-40 keV energy range). This outburst is thus much brighter than all the previous detections with INTEGRAL, and is of the same level as the November 2007 flare detected by Swift/BAT. It is interesting to note that the November 2007 and March 2009 outburst of GRO J1008-57 are much above the flux usually observed during the previous flares, but can still be considered Type I outbursts. Indeed, during the last Type II outburst (July 1993), the source reached a flux up to 1.4 Crab (Stollberg et al. 1993, IAU Circ. 5836; Wilson et al. 1994, AIP Conf. Proc. 304, 390).

The combined ASCA and OSSE-BATSE spectra were modeled using a cyclotron absorption feature at 88 keV (Shrader et al. 1999, ApJ 512, 920). Our preliminary spectral analysis does not confirm the presence of such a line, although further observations would be needed to check for its presence.

As a side note, the HMXB 1A 1118-615 is now returning to a much lower state after the outburst it underwent earlier (ATel #1949), but it is still detected at the 8-sigma level.


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