INTEGRAL observations of GRO J1008-57 in outburst provide hint of a cyclotron absorption line at ~ 79 keV

ATel #6664; V. Sguera (INAF-IASF Bologna, Italy), A. Bazzano, P. Ubertini, M. Fiocchi, L. Natalucci (INAF-IAPS Roma, Italy), L. Sidoli, A. Paizis (INAF-IASF Milano, Italy), A. J. Bird (Univ. of Southampton, UK), J. Chevenez (National Space Institute DTU, Denmark), E. Kuulkers (ESA/ESAC, Spain)
on 4 Nov 2014; 14:56 UT
Credential Certification: Eleonora Torresi (torresi@iasfbo.inaf.it)

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The transient Be HMXB GROJ1008-57, currently active in X-rays during an unexpected outburst with double peaked morphology (ATel #6465, ATel #6630, ATel #6656), was in the field of view of the IBIS/ISGRI instrument onboard INTEGRAL for 11 ks exposure on-source during recent Galactic Plane Scannings (GPS, PI: A. Bazzano) performed from Nov 02 11:42 (UTC) to Nov 03 05:40 (UTC). Unfortunately the source was never in the JEM-X field of view.

GRO J1008-57 was detected at about 93-sigma and 11-sigma level in the energy bands 20-60 and 60-100 keV respectively, no significant detection was obtained above 100 keV. A fit to the 23-100 keV spectrum with a power law model is unacceptable (reduced chi sq = 2.47 for 22 degrees of freedom d.o.f.) suggesting the presence of a possible cyclotron absorption line at energies above 70 keV. The fit significantly improves (reduced chi sq = 1.3 for 20 d.o.f.) by adding a multiplicative scattering cyclotron feature (cyclabs model in XSPEC) with energy centroid left free to vary in the energy range 70-90 keV and width frozen to 11 keV, according to the finding reported by Bellm et al. 2014 (ApJ, 792, 108). The photon index of the power law continuum is equal to 2.75 +/- 0.15 while the 20-100 keV flux is 3.7 x 10^-9 erg cm^-2 s^-1. The energy centroid as measured by INTEGRAL is 79.0 (+7.7, -5.5) KeV which is in agreement with that previously measured with NuSTAR at 78.0 (+3, -2) keV (Bellm et al. 2014, ApJ, 792, 108).

We encourage multiwavelengths follow-up observations of this currently very unusual X-ray activity, especially in the optical band.