INTEGRAL detection of GRO J1008-57 in outburst

ATel #10418; J.-G. Victor (ISDC, Switzerland) C. Sanchez (ESA/ESAC, Spain), L. Sidoli (IASF-Milano/INAF, Italy) K. Watanabe (Florida Gulf Coast University, USA) R. Diehl (MPE, Germany) L. Pavan, V. Savchenko C. Ferrigno (ISDC, Switzerland)
on 23 May 2017; 17:00 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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GRO J1008-57 is a High Mass X-Ray Binary, composed of a transient neutron star and a Be-star companion (B0e). This source shows periodical type I outbursts roughly spaced by the orbital period of the binary (P~247.8 d, Kühnel et al. 2013, A&A 555, A95).

During an observation of the Carina Region from 2017 May 22, at 14:30 to 2017-05-23 at 13:31, INTEGRAL detected renewed activity from GRO J1008-57 with its instruments IBIS and JEM-X: in the 20-40(3-10) keV band, the observed flux is 22.8±0.9(17.9±0.6) mCrab (statistical uncertainty at one sigma confidence level).

The spectrum is well described (χ²_red=1.2) by a power law with exponential cutoff at high energy. The best-fit parameters are: Γ= 0.9±0.3, E_C= 8_-2³ and 20-100 keV flux of 3.5e-10 erg/s/cm²; the cutoff energy is slightly lower than in previous outbursts, while the power-law index is consistent with the source dim phase (Kühnel et al. 2013). This outburst occurs at orbital phase -0.087, indicating that it is most probably linked to the neutron star periastron passage. If this is confirmed to be a regular type I outburst, the maximum flux is expected at phase -0.0323, i.e., in about 12 days.