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A type-I X-ray burst detected by Swift/XRT from the direction of IGR J18245-2452

ATel #4959; A. Papitto (IEEC-CSIC Barcelona), E. Bozzo, C. Ferrigno, L. Pavan (ISDC Geneve), P. Romano (INAF-IASF Palermo), S. Campana (INAF-OAB)
on 8 Apr 2013; 14:09 UT
Distributed as an Instant Email Notice Transients
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

Subjects: Binary, Neutron Star, Transient

Referred to by ATel #: 4960, 4961, 4964, 4981, 5031, 5068, 5069, 5086

During a Swift/XRT observation starting on 2013-04-07 at 20:32:59, and pointed at the newly discovered X-ray transient IGR J18245-2452 (ATel #4925, #4927, #4929, #4924), located in the globular cluster M28, a burst with rise time of less than 10 s and exponential decay of 38.9+/-0.5 s, was observed by the XRT, while operated in Windowed Timing mode. By fitting the XRT spectrum observed during the peak 6 s of the burst with a 2.5(2) keV blackbody, we measure a 1-10 absorbed flux of 3.2(2)E-8 erg cm^-2 s^-1, corresponding to an unabsorbed X-ray luminosity of 9.6(6)E37 erg/s in the same energy band, for a distance of 5kpc and an absorption column of 0.3(1)E22 cm^-2. The observed flux level is a factor ~16 higher than the persistent flux observed during the rest of the observation. The shape, duration and energetics of the burst suggest it is most likely a burst of thermonuclear origin. To firmly assess the nature of the event, analysis of the spectral evolution throughout the burst is ongoing.
During the rest of the observation the XRT was operated in Photon Counting mode and the position of the source was estimated as:
RA = 18h 24m 32.20s
DEC = -24° 52' 05.7"
with an uncertainty of 3.5 arcsec. Such position is consistent with that determined previously for IGR J18245-2452 (ATel #4927, #4929), suggesting how most probably the burst came from this source, which would be then identified as a neutron star. It should be noted that the burst could also come from other sources in the globular cluster. However, no other accreting source was reported to be active since the onset of the outburst of IGR J18245-2452, and only one source from Becker et al. (2003) is consistent with the position of IGR J18245-2452 (ATel #4927), so we estimate this chance probability as marginal.

We are grateful to the Swift team for the rapid planning of the currently ongoing observational monitoring of IGR J18245-2452.

The lightcurve of the burst can be seen HERE.