An exceptionally bright outburst from the HMXB XTE J1855-026

On 14 March 2010, INTEGRAL observed three exceptionally bright flares from the high-Mass X-ray Binary XTE J1855-026. The first flare lasted only about 2 minutes and reached a peak intensity of almost that of the Crab nebula. Watanabe et al. 2010, ATel 2482

INTEGRAL image of the field around XTE J1855-026
20-40 keV significance map of the region containing XTE J1855-026 taken between 2010-03-12 14:56 and 2010-03-14 23:04 for an exposure of 66 ks.
Credits: ISDC/C. Ferrigno

Lightcurve of XTE J1855-026 on 14 March 2010
Source light curve adaptively binned to obtain a minimum S/N = 6. The Crab count rate is 96 cts/s for the adopted extraction method.
Credits: ISDC/C. Ferrigno

INTEGRAL spectrum of XTE J1855-026 during outburst
Count rate average IBIS/ISGRI spectrum during the outburst of XTE J1855-026
Credits: ISDC/C. Ferrigno

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) detected a bright flare form the Supergiant High Mass X-ray Binary XTE J1855-026 (Corbet & Mukai 2002, ApJ 577, 923; 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 flares separated by about the pulse period of 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 using 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 during the entire event.

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.