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Swift detection of IGR16418-4532

ATel #4148; P. Romano (INAF-IASFPA), S. D. Barthelmy (GSFC), J. A. Kennea (PSU), P. Esposito (INAF-IASFMI), P. A. Evans (U Leicester), V. Mangano (INAF-IASFPA), D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU), H. Krimm (CRESST/GSFC/USRA), S. Vercellone (INAF-IASFPA), N. Gehrels (GSFC)
on 4 Jun 2012; 14:07 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Gamma Ray, Binary, Transient, Pulsar

Referred to by ATel #: 4939, 5398

Swift observed a new outburst from the SFXT IGR J16418-4532. The Swift/BAT triggered on it on 2012 Jun 03 at 18:08:48 UT (a 320-sec image trigger, 523489). This is the first detection using the new Sub-threshold Interesting Source BAT trigger criteria. Swift immediately slewed to the target.

Using the BAT data set from T-221 to T+963 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 lightcurve is consistent with it starting around T-100 s, broadly peaking at ~0.02 ph/cm2/s and then decreasing somewhat after ~T+300 s, and continuing out to T+980 s where the event-by-event data stop. The DPH data show emission out to at least T+1500 s (where that data stop). The time-averaged spectrum from T+0 to T+320 s is best fit by a simple power-law model with a photon index of 2.75+/-0.59. The fluence (same time interval) in the 15-150 keV band is (4.8+/-1.6)E-7 erg/cm2. All quoted errors are at the 90% c.l.

The XRT began observing the field at 18:16:15.4 UT, 447.7 seconds 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 ~0.2 count/s. The mean XRT spectrum (T+466 to 7318 s) can be fit by an absorbed power-law model with a photon index of 2.6(-0.8,+0.9) and an absorbing column density of NH=37(-8,+9)E22 cm-2. The average 2-10 keV observed (unabsorbed) flux is 9E-11 (5E-10) erg/cm2/s. We note that the NH is in excess of the Galactic value (1.59E22 cm-2; Kalberla et al. 2005) and of the values previously reported by Romano et al 2012 (MNRAS, 419, 2695).