Swift detected outburst of the SFXT IGR J18410-0535/AX1841.0-0536

ATel #3453; V. Mangano (INAF-IASF Palermo), S. D. Barthelmy (GSFC/UMBC), P. Romano (INAF-IASF Palermo), T. Sakamoto (GSFC), M. M. Chester (PSU), M. Siegel (PSU), P. A. Evans (LU), P. Esposito (INAF-OAC), S. Vercellone (INAF-IASFPA), D. N. Burrows (PSU), G. Cusumano (INAF-IASFPA), R. Farinelli (INAF-IASFPA, U. Ferrara), J. A. Kennea (PSU), H. Krimm (CRESST/GSFC/USRA), V. La Parola (INAF-IASFPA), N. Gehrels (GSFC)
on 25 Jun 2011; 14:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Vanessa Mangano (vanessa@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 2011 June 24 at 14:27:04.9 UT (image trigger=455967). Swift did not slew immediately because the merit value of this new trigger was less than the merit value of the current target. The source was detected in a 320-second BAT image trigger (T+0 s) starting after settling (T-225 s) from a pre-planned slew. Ground processing reveals a weak detection in BAT (~6.8 sigma, 15-50 keV). The mask-weighted lightcurve shows some emission (~0.02 counts/cm2/s, 15-50 keV) in the T-200 to T+0 pre-trigger interval. The emission increases slightly to ~0.05 counts/cm2/s around T+130 to T+200 s. A simple power law fit to the spectrum (T+0 to T+320 sec) yields an index of 2.6 +/- 0.7 (chisq is 63.7, dof=57), and the fluence is (4.8,-1.9,+2.3)E-7 erg/cm2. The XRT began a single orbit follow up observation of the field at 15:52:47 UT, about 5100 s after the BAT trigger. The limited duration of the XRT follow up (~1 ks) was due to strict temperature constraints. The source count rate reached a peak at about 7 counts/s and decreased to about 2 counts/s at the end of the observation. The spectrum of PC mode data can be fitted (using Cash statistics) with an absorbed power-law with a photon index of 0.19 (+0.69,-0.30) and an absorbing column density of NH=[1.5(+2.2,-1.1) ]E22 cm-2. The average 2-10 keV unabsorbed flux is ~3.6E-10 erg/cm2/s. This is the second outburst of IGR J18410-0535 detected by the BAT for which Swift performed a follow up observation that clearly revealed an on-going flaring activity in the XRT. Observation results on the previous Swift-detected outburst, which occurred on 2010 June 5, are reported in Romano et al. 2011 (MNRAS,412,L30). Compared to the 2010 June 5 event, this flare shares consistent spectroscopic characteristics. Several on-board BAT 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.