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Swift Detection of Flaring X-ray Activity from IGR J16479-4514

ATel #599; J. A. Kennea (PSU), C. Pagani (INAF-OAB/PSU), C. Markwardt (UMD/GSFC), A. Blustin (UCL-MSSL), J. Cummings (GSFC/NRC), J. Nousek (PSU), N. Gehrels (GSFC).
on 30 Aug 2005; 21:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: Optical, X-ray, Transient

Referred to by ATel #: 816, 1435

On August 30th at 04:08:48 UT, the Swift Burst Alert Telescope (BAT) triggered on a source near the location of IGR J16479-4514 (ATEL #176). The Swift spacecraft promptly slewed to this position and the Swift X-ray Telescope (XRT) began observations of the field at 04:10:56 UT. A bright variable X-ray source was found in the field at the following position: RA(J2000): 16:48:07.0, Dec(J2000): -45:12:05.8, with an estimated error radius of 6 arcseconds (90% containment). This position is 1.7 arcminutes from the position of IGR J16479-4514 reported by Bird et al. (2004), and 3 arcminutes from the position reported in ATEL #172.

We have performed a preliminary analysis of archival XMM-Newton data from March 21st, 2004 and have found an X-ray source at a position consistent with the Swift/XRT position above. We can therefore rule out the possibility that this source is a new transient or GRB event, and is most likely the X-ray counterpart to IGR J16479-4514.

During the Swift observations of this source it displayed considerable variability. The flare in the BAT light curve lasted ~400 seconds in the 15-50 keV band. There was no significant emission above 50 keV. The XRT light-curve, which covers ~30ks (7.2ks exposure) shows 2 long bright flares: The first is ongoing when the XRT started taking data, and presumably is the tail end of the peak that caused the BAT trigger; The second starts approximately 4000s after the observation began, and lasts approximately 1000s. The overall light-curve appears to be decaying.

The spectrum of the source is well fit by an absorbed power-law, with N_H = 6.4 +/- 0.9 x 10^22 cm^-2 and a photon index of 1.09 +/- 0.25. The mean flux in the 0.5-10 keV range is 3.8 x 10^-11 erg/s/cm^2, not corrected for absorption. This mean flux is approximately 10 times brighter than the mean flux seen in the archival XMM-Newton data for this source.

The Swift UV/Optical Telescope (UVOT) detects a faint source within 3 arcseconds of the XRT position: RA(J2000): 16:48:06.8, Dec(J2000): -45:12:08.0. It is visible only in the V-band at a magnitude of 20.4 +/- 0.4 (in a co-added image with a total of 1068 s of exposure time), and appears to be present at a low significance in the DSS. Currently there are not enough data from the UVOT to determine if this possible optical counterpart is varying.

Follow-up observations of this source are planned with Swift.