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Swift observes an outburst of the high mass X-ray binary IGR J01572-7259 in the SMC

ATel #9021; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU), S. D. Barthelmy (GSFC), J. Cummings (CPI), N. Gehrels (GSFC), A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (Aoyama Gakuin Univ.), M. Stamatikos (OSU/GSFC), T. Ukwatta (LANL)
on 4 May 2016; 21:00 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: Optical, X-ray, Binary, Transient, Pulsar

The high-mass X-ray binary IGR J01572-7259 (IGR J015712-7259 = SXP 11.6) in the Small Magellanic Cloud is undergoing an outburst. An increase in counts was first detected in the BAT in a 4-day period beginning 15 April 2016 (MJD 57493), when it was detected at 0.0016 +/- 0.0004 ct cm-2 s-1 (~7 mCrab) . The flux peaked on 25 April 2016 (MJD 57503) at 0.0027 +/- 0.0005 ct cm-2 s-1 (~12 mCrab). The peak rate is about three times higher than the maximum count rate reported in Segreto et al., A&A 557, 113 (2013) for BAT observations between 2005 and 2012. The current rate is 0.0015 +/- 0.0005 ct cm-2 s-1. We note that the 25 April peak corresponds to a phase of 0.0, relative to the orbital period and epoch reported by Segreto, which is half an orbital period away from the Segreto peak phase of 1.50.

A 1000-second Swift ToO observation was carried out starting at 3 May 2016, 17:03:08 UT. The source is clearly detected in the XRT at an average rate of 1.4 +/- 0.06 ct s-1, corresponding to a an unabsorbed flux of 9.82 +/- 1.0 X 10-11 erg cm-2 s-1. This is about 5 times the maximum XRT count rate reported by Segreto. Analysis of the XRT data shows a broad peak seen at 11.6s, consistent with that reported by Coe et al., ATel #1882.

The source is also detected in the Swift/UVOT using the uvw1 filter. A preliminary estimate is that the magnitude is 12.57 +/- 1.09 (stat) +/- 0.03 (sys) in the Vega system.

Swift/BAT transient monitor light curve for IGR J01572-7259