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Swift observes a new outburst from the Supergiant Fast X-ray Transient AX J1845.0-0433

ATel #4095; P. Romano (INAF-IASF Palermo), S. D. Barthelmy (GSFC), M. M. Chester (PSU), S. R. Oates (UCL-MSSL), D. N. Burrows (PSU), P. Esposito (INAF-OAC), P. A. Evans (LU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASF Palermo), S. Vercellone (INAF-IASF Palermo), N. Gehrels (GSFC)
on 5 May 2012; 14:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Transient

Swift observed a new outburst from the SFXT AX J1845.0-0433/IGR J18450-0435. The Swift/BAT triggered on it on 2012 May 05 at 01:44:39 UT (image trigger=521567).

Using the BAT data set from T-239 to T+963 s from the recent telemetry downlink, we report that the mask-weighted light curve shows the source already in outburst when it came into the BAT FoV during a pre-planned slew. Emission continues at the 0.013 ph/cm2/s level out past T+963 s. At a low confidence level, there may be some variability between T+100 and T+200 s. 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 2.47 +- 0.53. The fluence in the 15-150 keV band is (9.7 +/- 3.0)E-07 erg/cm2. All quoted errors are at the 90% c.l.

Swift slewed to the target immediately, so that the XRT began observing the field at about 423 s after the BAT trigger. Utilizing the first 2775 s PC mode data, and correcting for the astrometric errors by utilizing Swift/UVOT data according to the method described by Evans et al. (2009, MNRAS, 397, 1177), we find AX J1845.0-0433 at: RA, Dec (J2000) = 281.25658,-4.56593, which is equivalent to:
RA(J2000) = 18h 45m 01.58s,
Dec(J2000) = -04d 33m 57.4s,
with an estimated error of 1.4 arcsec radius (90% confidence).

The XRT light curve shows an initial bright flare that reached ~20 c/s at ~T+1000 s, followed by several more flares. The XRT/WT spectrum (T+429 to T+846 s; 417 s net exposure) can be fit with an absorbed power law, with a photon index of 1.5+/-0.1, and an absorbing column density of NH=(1.8,-0.2+0.3)E+22 cm-2, in slight excess of the Galactic value of 1.30E+22 cm-2, Kalberla et al. 2005). The mean flux is ~7E-10 erg/cm2/s (2-10 keV, unabsorbed). The XRT/PC spectrum (T+848 to T+1891 s; 1043 s net exposure) shows a power law shape with NH= (1.8+/-0.4)E+22 cm-2, photon index of 1.1+/-0.2 and an average flux of ~8E-10 erg/cm2/s (2-10 keV, unabsorbed). These fluxes translate into a luminosity of 1E+36 erg/s (assuming the optical counterpart distance of 3.6 kpc, Coe et al. 1996, MNRAS, 281, 333).

The Swift/UVOT began settled observations 429 s after the BAT trigger. Preliminary photometry and 3-sigma upper limits of the catalogued USNO-B1 source at the centre of the XRT position using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 5.46 in the direction of the source (Schlegel et al. 1998).

Previously, Swift caught a flare from this source on 2005 November 4 and 2009 June 28 (Romano et al 2009, ATel #2102). 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/IGRJ18450-0435/ ).

Scaled Map Transient Analysis for IGR J18450-0435