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Swift detects bursting activity from GRO J1744-28

ATel #5883; M. Linares (IAC), J. Kennea (PSU), H. Krimm (GSFC), C. Kouveliotou (MSFC)
on 12 Feb 2014; 13:33 UT
Credential Certification: Manuel Linares (linares@mit.edu)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 5895, 5901, 5904, 5910, 5963, 5997, 5999

Swift-BAT triggered on a burst from GRO J1744-28 (aka 'the bursting pulsar', BP; ATels #5790, #5810, #5845, #5858) on 2014-02-11 at 22:24:49 UTC. The BAT burst is clearly detected in the 15-25 and 25-50 keV bands, lasts for about 20 s and shows a double-peaked structure. The average spectrum is reasonably well fit with a blackbody model of temperature 5.4+/-0.5 keV, which yields a 15-50 keV fluence of (3.7+/-0.5)E-7 erg/cm2. Using this average spectrum, we estimate a 25-50 keV flux of ~1.6E-8 erg/s/cm2.

Swift-XRT slewed automatically and started observing the BP at 2014-02-11 22:26:13 UTC. Only the BP is detected in the short (173 s), piled-up PC mode image of the field. The WT mode lightcurve (containing two gaps) shows the tails of two additional bursts, each of them less than ~100s long, about 400 s apart. The second and brightest XRT burst tail shows a smooth decay from ~150 c/s (0.3-10 keV) down to the persistent level. We do not detect spectral softening or "cooling" during the flux decay, arguing against a thermonuclear origin of this burst (see Linares et al. 2011, ApJL,733,17, for details).

The previous XRT observation of the BP (1 ksec-long on 2014-02-11 06:18:51 UTC) showed no bursts and a similar persistent flux (2.9+/-0.2 E-9 erg/s/cm2 in the 0.5-10 keV band, not corrected for absorption). However, the latest XRT observation where two burst tails are detected seems to show stronger variability on long (~100s) timescales.

These results show that the BP is bursting again, 24 days after the first detection of the ongoing outburst (ATel #5790). The peak flux, fluence, duration, recurrence times and spectral properties of the bursts detected by Swift are fully consistent with the type II X-ray bursts discovered by CGRO-BATSE in 1995 (Kouveliotou et al. 1996, Nature, 379, 799).