Swift detects a flare from IGR J16418-4532

ATel #4939; P. Romano (INAF-IASFPA), S. D. Barthelmy (GSFC), V. D'Elia (ASDC), P. Esposito (INAF-IASF Milano), P. A. Evans (LU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. Gehrels (NASA/GSFC), A. Y. Lien (NASA/GSFC/ORAU), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL), E. Sonbas (NASA/GSFC/Adiyaman Univ.)
on 3 Apr 2013; 13:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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The Swift Burst Alert Telescope (BAT) triggered on the supergiant fast X-ray transient (SFXT) candidate IGR J16418-4532 on 2013 April 2 at 11:56:29 UT (image trigger, 552677). This is the second detection using the new Sub-threshold Interesting Source BAT trigger criteria. Swift immediately slewed to the target.

Using the data set from T-119 to T+183 s from recent telemetry downlinks, we report that this is a very weak source in BAT, and as such a description of the mask-weighted light curve is problematic. The time-averaged spectrum from T+0.0 to T+183.04 s is best fit by a simple power-law model with a photon index of 2.7 +/- 1.1. The fluence (same time interval) in the 15-150 keV band is (2.3 +/- 1.3)E-7 erg/cm2. All quoted errors are at the 90% c.l.

The XRT began observing the field at 12:16:29.0 UT, 1200 s after the BAT trigger. The XRT light curve shows an initial flare reaching ~2 count/s, while the second orbit has an average count rate of ~4 count/s. We used 1935 s of PC mode data and simultaneous Swift/UVOT images to obtain an astrometrically corrected position (see Goad et al. 2007, A&A, 476, 1401; Evans et al. 2009, MNRAS, 397, 1177) of RA, Dec 250.46231, -45.54069, which is
RA(J2000) = 16h 41m 50.95s
Dec(J2000) = -45d 32' 26.5"
with an uncertainty of 1.7 arcsec (radius, 90% c.l.). This position is 2.1 arcsec from the source 2MASS J16415078-4532253, the proposed optical counterpart of IGR J16418-4532. The mean XRT/PC spectrum (T+1214 to 7566 s) can be fit by an absorbed power-law model with a photon index of 1.1+/-0.3 and an absorbing column density of NH=(8+/-1)E22 cm-2. The average 2-10 keV observed (unabsorbed) flux is 2E-10 (3E-10) erg/cm2/s. We note that the NH is in excess of the Galactic value (1.59E22 cm-2; Kalberla et al. 2005).

Previously, Swift observed flares from this source on 2009 March 21 and on 2012 June 03 (Romano et al. 2012, Atel #4148). 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/IGRJ16418-4532 .

Scaled Map Transient Analysis for IGR J16418-4532