2011ATel.3182....1R

triggered (image trigger=446475) on the supergiant fast X-ray transient

XTE J1739-302/IGR J17391-3021 (Smith et al. 1998, ApJ 501, L101). Swift immediately slewed to the target, so that the narrow-field instruments began observing the field 141 s after the BAT trigger.

Using the BAT data from T-79 to T+167 s, the mask-weighted lightcurve shows the burst to have started at about T-70 sec (possibly sooner) with

a slow rise and continuing emission out past the current of the downlinked

data at T+167 s. The time-averaged spectrum from T+0.0 to T+64.0 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.50±0.45. The fluence in the 15-150 keV band

is (4.5±1.1)E-7 erg/cm2. The 1-s peak photon flux measured from T+0.00 in the 15-150 keV band is 0.141±0.026 ph/cm2/s. All the quoted errors

are at the 90% confidence level.

The XRT began observing the field at 07:23:58 UT. The Swift/XRT light curve shows the descending part of one bright flare that reached about 30 counts/s. The mean XRT/WT spectrum

(T+147 to T+1321s since the trigger) can be fit with an absorbed powerlaw

with a photon index of Gamma=1.18±0.06 and an absorbing column density

of NH=(1.5±0.2)E+22 cm-2 (chi^2_red=1.1 for 492 dof). The mean flux is 1.6E-9 erg/cm2/s (unabsorbed, 2-10 keV), which translates

into a luminosity of 1.3E+36 erg/s (assuming the optical counterpart distance of 2.7 kpc, Rahoui et al. 2008, A&A, 484, 801).

The previous outbursts that triggered Swift/BAT occurred on 2009 March 10 (Atel #1961), 2008 Aug 13 (ATel #1659), and 2008 Apr 8 (ATel #1466). The IGR J17391-3021 light curve from the BAT Transient monitor can be found

http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17391-3021/

BAT Transient Monitor page: IGRJ17391-3021/: http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17391-3021/