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Unexpected flaring activity of the Be/X-ray binary GRO J1008-57

ATel #4355; M. Kuehnel, S. Mueller, I. Kreykenbohm, J. Wilms (Remeis-Observatory Bamberg), K. Pottschmidt (CRESST-UMBC/NASA-GSFC), H. Krimm (NASA-GSFC), F. Fuerst (Caltech-SRL), R. E. Rothschild (UC San Diego), I. Caballero (CEA Saclay), G. Schoenherr (AIP), D. Klochkov and R. Staubert (Institut fuer Astronomie und Astrophysik Tuebingen)
on 7 Sep 2012; 02:55 UT
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 4561, 4564, 4573, 6465, 10529

During the decay of the present outburst of the transient Be/X-ray binary GRO J1008-57 (ATel #4319), the source showed a ~1 day long flare around MJD 56172 in both, the Swift/BAT and MAXI light curves. The luminosity more than doubled and was comparable to the peak flux of ~160 mCrab (in the BAT energy range of 15-50 keV), which the source reached at MJD 56163. During the 2-3 days before and after this flare the flux did not decay further but stayed consistent with ~70 mCrab.

From this level the luminosity started increasing again and is, at the moment (MJD 56176), around 130 mCrab. Such strong flaring activity in the decay of an outburst of GRO J1008-57 has to our knowledge not been observed to date.

The MAXI light curve shows indications for a possible delay of ~1.5 days of the flares as well as the full outburst compared to the BAT light curve. Since MAXI is sensitive between 2 and 20 keV, this implies the soft X-rays are delayed compared to the hard ones (between 15 and 50 keV).

The flaring activity might be caused by magnetospheric instabilities, which is proposed as explanation in, e.g., EXO 2030+375, or an increased mass transfer rate onto the compact object. The material needed has to be provided by the companion star, especially the Be-disk. To reveal the dynamics and state of this disk, we encourage optical observations of GRO J1008-57.