Swift/BAT reports a new outburst from the HMXB XTE J1855-026

ATel #3964; H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. B. Markwardt (GSFC), D. Palmer (LANL), P. Romano (INAF-IASF Palermo), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMD), M. Stamatikos (OSU/GSFC), J. Tueller (GSFC), T. Ukwatta (GSFC/MSU)
on 13 Mar 2012; 17:30 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The Swift/BAT hard X-ray transient monitor reports a new outburst from the supergiant high-mass X-ray binary XTE J1855-026. The source has increased in intensity from near its normal level of approximately 0.0023 cts/cm^2/sec (~10 mCrab) in the 15-50 keV band on 2012 March 12 to a level of 0.011 +/- 0.001 (~50 mCrab) on 2012 March 13 (MJD 55999). However there is evidence of increased activity on two other recent dates: March 11, 0.0055 +/- 0.0006 cts/cm^2/s, March 7, 0.0049 +/- 0.0007 cts/cm^2/s. The brightest single pointing in recent days was in a 768-second observation beginning 2012 Mar 13 00:22:48 UT, with a rate of 0.039 +/- 0.003 cts/cm^2/s (~175 mCrab).

This is comparable to the highest daily average level seen in this source in the BAT monitor. At the time of the flares reported for XTE J1855-026 by INTEGRAL in March 2010 (Watanabe et al., ATel #2482), the BAT monitor saw sustained emission only at the ~20 mCrab level.

Note that XTE J1855-026 triggered BAT on 2011 Sep. 18 at 10:07:28.6 UT (trigger 503434). The outburst lasted approximately 900 seconds. The time-averaged spectrum from -56.0 to 825.0 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.48 +/- 0.12. The fluence in the 15-150 keV band is 3.8 +/- 0.2 x 10^-6 erg/cm2. All the quoted errors are at the 90% confidence level. The BAT transient monitor rate for the observation covering the trigger time was 0.050 +/- 0.005 cts/cm^2/s or ~230 mCrab. The average monitor rate for that day was ~55 mCrab.

BAT Transient Monitor light curve for XTE J1855-026