Analysis of Swift data of the June 5 outburst of the SFXT IGR J18410-0535/AX1841.0-0536

ATel #2662; P. Romano (INAF-IASFPA), G. Cusumano (INAF-IASFPA), W. H. Baumgartner (GSFC/UMBC), H. A. Krimm(CRESST/GSFC/USRA), T. Sakamoto (GSFC/UMBC), M. De Pasquale (UCL-MSSL), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), M. M. Chester (PSU), J. A. Kennea (PSU), P. A. Evans (LU), P. Esposito (OAC), N. Gehrels (GSFC), D. M. Palmer (LANL), V. La Parola (INAF-IASFPA), S. Vercellone (INAF-IASFPA)
on 6 Jun 2010; 17:14 UT
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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The Swift Burst Alert Telescope (BAT) triggered on an outburst from the Supergiant Fast X-ray Transient (SFXT) IGR J18410-0535/AX1841.0-0536 on 2010 June 5 at 17:23:30 UT (image trigger=423958, ATel #2661). Swift slewed to the source immediately after its detection.

The source was detected in a 22.4 minute BAT image trigger starting when the spacecraft finished a pre-planned slew (T+0s) and ending when the spacecraft executed another slew due to observing constraints (T+1344s). We find that the source was already in outburst when it came into the BAT FOV during a pre-plannned target observation slew at T+0s. There is also an indication that there is emission in the BAT energy range out past T+963s. Using the BAT data set from T+0 to T+963s, we find the source light curve to be flat and weak but significant, especially in the lowest bands (15-50 keV). The time-averaged spectrum from T+0.0 to T+963s is best fit by a simple power-law model with a photon index of 2.55+/-0.45. All the quoted errors are at the 90% confidence level.

The XRT began observing the field at 17:51:50 UT, about 1700 s after the BAT trigger. The source count rate reached about 5 counts/s. Then its flux decreased steadily during the following seven orbits, down to about 1E-2 counts/s. The spectrum of the first orbit (PC mode data, T+1707 to 2390 s) can be fitted with an absorbed power-law with a photon index of 0.7(+0.5,-0.4) and an absorbing column density of NH=[3(+2,-1)]E22 cm-2. The average 2-10 keV unabsorbed flux is ~7E-10 erg/cm2/s. A timing analysis on the arrival times of XRT events (corrected to the Solar system barycenter) does not reveal any significant evidence of periodicity.

This is the first outburst of IGR J18410-0535 detected by the BAT for which Swift performed a slew, although several on-board detections have been reported (Romano, et al. 2009,MNRAS,399,2021). The historical light curve from the BAT hard X-ray transient monitor (Krimm et al, 2006, ATel #904; 15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ18410-0535/ .

We would like to thank the Swift Team for making these observations possible, in particular the duty scientists as well as the science planners.