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Optical observations of GRO J1008-57

ATel #10561; Coe MJ (Southampton University), Townsend LJ (UCT), Buckley D (SAAO), McBride V (IAU/OAD), Monageng I (UCT/SAAO)
on 7 Jul 2017; 15:35 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

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

Referred to by ATel #: 10617, 10626, 10902

The Be/X-ray binary system GRO J1008-57 was reported undergoing unusual enhanced X-ray activity by MAXI/GSC (ATel #10529) and Sguera et al 2017 (ATel #10533) following a regular Type I outburst. The optical counterpart to the system was observed by the High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) on MJD 57933.724 (29 June 2017). The resulting spectrum revealed both H alpha and H beta Balmer lines in emission. The former shows a primarily single-peaked shape with possible small features in the peak. The latter shows a weaker clear double-peaked structure. The equivalent width of the H alpha has a value of -29.4 +/- 1.1A making it the highest reported value for this system. A review of previous observations by Coe et al (2007 MNRAS 378, 1427) presented evidence for the large dynamic changes that take place in the circumstellar disk of this system, with H alpha EW values ranging from -22A to almost zero. The value reported here suggests an exceptionally large circumstellar disk is currently present which may go some way to explaining the recent exceptional X-ray behaviour. SALT will continue to monitor this system for the near future.