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:
| Filter | T_start(s) | T_stop(s) | Exp(s) | Mag(s)+/-
|
| White_FC | 429 | 578 | 147 | 15.61 | 0.03 |
| White | 429 | 14604 | 1282 | 15.78 | 0.01 |
| v | 586 | 1751 | 136 | 14.35+/-0.02 |
| v | 2129 | 7428 | 216 | 14.33+/-0.02 |
| b | 685 | 1850 | 117 | 16.30+/-0.03 |
| b | 6611 | 13788 | 1082 | 16.31+/-0.01 |
| u | 660 | 1825 | 136 | 17.36+/-0.07 |
| u | 6405 | 20352 | 1803 | 17.40+/-0.02 |
| w1 | 635 | 1801 | 136 | >18.47 |
|
| m2 | 610 | 1600 | 78 | >19.03 |
| w2 | 1014 | 1894 | 110 | >18.86 |
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
