INTEGRAL catches an exceptional bright outburst from the HMXB XTE J1855-026

ATel #2482; K. Watanabe (FGCU), E. Bozzo (ISDC/University of Geneva), S. Mereghetti (IASF-Mi/INAF), C. Baldovin (ISDC/University of Geneva) Diego Gotz (Ces-Saclay)
on 16 Mar 2010; 17:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

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During the observation of the inner Galactic disc (PI Georg Weidenspointner) that were performed between 2010-03-12 14:56 and 2010-03-14 23:04 (UTC), the Integral Burst Alert System (IBAS; Mereghetti et al. A&A, 411, L291, 2003) detected a bright flare form the Supergiant High Mass X-ray Binary XTE J1855-026 (Corbet & Mukai, ApJ 577, 923, 2002, Negueruela et al. 2008, ATEL #1876 ). The unexpected flare was first misinterpreted as a GRB by the automatic IBAS pipeline and later retracted by Mereghetti et al. in the GCN 10438. Off line analysis showed the presence of three different flares from XTE J1855-026 separated approximatively by one spin period (360 s). The first flare started on 2010-03-14 at 19:02:27 (UTC) and lasted for about 130 s. We estimated for this flare a 20-40 keV peak luminosity of 0.9+/-0.1 Crab (we integrated in bins of 5 s). The average IBIS/ISGRI spectrum of the flare could be fitted by a power-law with index 2.5+/-0.3 and resulted in a 20-40 keV mean flux of (2.2+/-0.3) x 1e-9 erg/s/cm2 (0.29+/-0.04 Crab). The second flare started on 2010-03-14 at 19:09:39 (UTC) and lasted about 55 s. The peak luminosity in the 20-40 keV energy band was of 0.7+/-0.1 Crab. The average spectrum of the flare could be fit by a power-law of index 2.2+/-0.4, and the corresponding flux was ~2e-9 erg/s/cm2 (~0.3 Crab; 20-40 keV). The third flare started on 2010-03-14 at 19:15:16 and lasted for about 80 s with a peak flux of 0.30+/-0.09 mCrab (20-40 keV). Its average spectrum could be fit by a power-law of index 2.2+/-0.4, and the corresponding flux was ~1.2e-9 erg/s/cm2 (~0.2 Crab; 20-40 keV). We also extracted the spectrum during the whole burst (from 2010-03-14 18:51 to 2010-03-14 19:46, UTC). This could be well fit by a power-law of photon index 2.5 ± 0.2 and the corresponding 20-40 keV X-ray flux was of (6.5+/-0.5) x 1e-10 erg/s/cm2 (~85 mCrab). Pulsations with a period of ~360 s were clearly detected using the entire observation. During the 5 hours before the outburst and the following 2.5 hours, the 20-40 keV flux of XTE J1855-026 was stable at about 30 mCrab. During the previous INTEGRAL observations performed between 2010-03-12 14:56 and 2010-03-13 04:33 in the direction of the source, XTE J1855-026 presented hour-scale variations in the 20-40 keV X-ray flux ranging from 10 to 60 mCrab. The source was outside the JEM-X field of view during the burst. A version of the telegram including figures can be found here.