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Swift J174510.8-262411 (to be known as Sw J1745-26): 0.5 Crab and rising

ATel #4383; B. Sbarufatti (INAF-OAB/PSU), J. A. Kennea (PSU), D. N. Burrows (PSU), S. Campana (INAF-OAB), N. Gehrels (GFSC), C. B. Markwardt (GFSC), J. R. Cummings (GSFC/UMBC), M. Siegel (PSU), H. A. Krimm (CRESST/GFSC), F. E. Marshall (NASA/GSFC)
on 17 Sep 2012; 19:44 UT
Credential Certification: Boris Sbarufatti (boris.sbarufatti@brera.inaf.it)

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

Referred to by ATel #: 4385, 4388, 4394, 4401, 4436, 4450, 4456, 4782

Swift BAT triggered on Swift J174510.8-262411 (which we will refer to as Sw J1745-26) on 2012/09/16 at 09:16 UT, and again at 12:36 UT. A third trigger happened on 2012/09/17 at 16:23 UT (see Cummings et al, GCN #13774 for a detailed analysis of the first two triggers). The source continues to brighten in BAT. As of 16:00 UT on Sept 17, 2012, the count rate due to this source was approximately 500 mCrab. Since the third trigger did not result in an automated followup observation, a Swift ToO observation has been scheduled to get more XRT observations of the source.
Examination of archival images from the Swift/BAT hard X-ray monitor shows no activity from Sw J1745-26 in the 15-50 keV band during the month prior to the Sep. 16 outburst. The average daily 1-sigma limit is 0.0008 ct/s/cm^2. The count rate rose from 0.038 +/- 0.001 ct/s (daily average; ~175 mCrab) on Sep. 16 to 0.14 +/- 0.007 ct/s/cm^2 (~650 mCrab) on Sep. 17.

The BAT monitor light curve for this source can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/SwiftJ1745.1-2624/

Swift XRT began observing Sw J1745-26 at 09:36:59.0 UT, 1.25 ks after the BAT trigger (Cummings et al, GCN #13774). The observations consist of 6.5 ks in Window Timing mode, from 09:37 UT to 14:25 UT.
A later ToO observation of 1.0 ks in Photon Counting mode was performed from 19:26 UT to 20:35 UT, in order to get a more accurate position for the source: the UVOT astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue, see Goad et al. 2007) is RA,Dec (J2000) 266.29421, -26.40407, i.e.

RA =17h 44m 10.6 s
Dec=-26d 24' 14.7''


with an error radius of 2.2'' (90% confidence). This is 3.4'' away from the candidate IR counterpart reported by Rau et al. (ATel 4380).

The source shows an increasing count rate during the observations, starting at ~20 ct/s at the beginning of the XRT observations up to ~40 ct/s at the end of the PC snapshot. A power law fit to the lightcurve gives a time index of 0.20+/-0.01, with the PC observations significantly higher than the model prediction. The timing analysis does not show strong evidence for periodicity or QPOs. The hardness ratio is consisted with no spectral variations.

The average WT spectrum is best fit (red. chi sq. = 1.1690) by a black body plus power law model with the following parameters:
BB temp.: 1.2+/-0.1 keV
Photon Index: 1.4+/-0.1
Abs. Column: (9.1+/-0.5)E21 cm^-2 in slight excess with respect to the Galactic value of 6.52E21 cm^-2 (Kalberla et al. 2005)

Alternatively, an accretion disk model with inner temperature 2.69+/-0.06 keV (red chi sq. = 1.1545) gives a comparably good fit. Both models are compatible with a NS-LMXB nature of the source, but we remark that the lack of X-ray bursts indicates the possibility of this being an X-ray nova.

The observed (unabsorbed) flux in the 0.3-10 keV band, averaged over the WT observation, is 1.22 (1.59)E-09 ergs/cm^2/s, corresponding to a luminosity of 1.00 (1.30)E37 erg/s of assuming the source is at the distance of the Galactic Center.

Swift UVOT began observations of Sw J1745-26 at 09:37 UT, 1.25 ks after the trigger. No source is detected by UVOT inside the Sw J1745-26 XRT error circle to a limiting magnitude in the white filter of 21.94.  This may be caused by the high reddening (E(B-V)=3.22) in the direction of the source.

All quoted errors are at 90% confidence level.