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Swift observations of IGR J18245-2452

ATel #4929; P. Romano (INAF-IASFPA) , S. D. Barthelmy (GSFC), D. N. Burrows (PSU), V. D'Elia (ASDC), N. Gehrels (GSFC), S. T. Holland (STScI), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester)
on 30 Mar 2013; 20:47 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Gamma Ray, Globular Cluster, Transient

Referred to by ATel #: 4934, 4959, 4960, 4961, 4964, 4981, 5003, 5031, 5068, 5069, 5086

The newly discovered transient source IGR J18245-2452 (Eckert et al, ATel #4925), triggered the Swift Burst Alert Telescope (BAT) on 2013-03-30 at 02:22:21 (image trigger=552336, Barthelmy et al, GCN 14355), and subsequently at 15:10:37 UT (image trigger=552369) and 15:17:33.61 UT (image trigger=552370).

The BAT ground-calculated position for trigger 552336 is RA, Dec = 276.111, -24.857 deg which is
RA(J2000) = 18h 24m 26.6s
Dec(J2000) = -24d 51' 25.6"
with an uncertainty of 3.9 arcmin, (radius, sys+stat, 90% c.l.). Considering the event data, the outburst started at T+10 s and went on to ~T+220 s, but using the DPH data, a much larger increase is observed starting at ~T+800 s and continuing out past T+2200 s (where that data stop). The time-averaged spectrum from T+0.00 to T+63.04 s can be fit by a simple power-law model with a photon index of 2.14 +/- 1.26. The fluence in the 15-150 keV band is (1.4 +/- 0.9) E-07 erg/cm2. All the quoted errors are at the 90% c.l.

Following trigger 552336, XRT began observing the field T+191 s. We find IGR J18245-2452 at RA, Dec 276.1343, -24.8682 which is equivalent to:
RA(J2000) = 18h 24m 32.24s
Dec(J2000) = -24d 52' 05.7"
with an uncertainty of 3.5 arcseconds (radius, 90% c.l.), consistent with Heinke et al. (ATel #4927). The XRT/WT (T+191 to 2132 s, on-source exposure 1351 s) and PC (T+387 to 1502 s, on-source exposure 582s) spectra can be fit well with absorbed power laws with Gamma(WT)=1.35+/-0.04, NH=(4.4+/0.3) E21 cm-2, average observed 0.3-10 keV flux F=8.4E-10 erg/cm2/s, Gamma(PC)=1.21+/-0.16, NH=(3.3-0.8+1.0) E21 cm-2, F=4.8E-10 erg/cm2/s, respectively.
The XRT light curve shows either two large flares reaching count rates in excess of 20 and 40 counts/s, or a roughly constant count rate (at about 10 counts/) and one 300 s-long dip centered at about T+500 s. Given the shape of the light curve in observations taken on 2013-03-29, (roughly constant at about 10 counts/s) the second alternative seems more likely, although only more observations will be able to discriminate between these two cases. Further Swift observations are indeed ongoing as a follow up of the second and third trigger as well as a GI program (PI P. Romano).