Sw J1725-26 still brightening in Swift XRT observations
ATel #4436; B. Sbarufatti (INAF-OAB/PSU), J. A. Kennea (PSU), M. C. Stroh (PSU), D. N. Burrows (PSU), P. A. Evans, A. P. Beardmore (U. Leicester), H. A. Krimm (CRESST/GFSC), N. Gehrels (GFSC)
on 28 Sep 2012; 18:33 UT
Credential Certification: Boris Sbarufatti (boris.sbarufatti@brera.inaf.it)
Subjects: X-ray, Transient
We report on the continued monitoring of Sw J1745-26 (ATel #4383) by Swift. While the BAT light-curve shows the source reaching peak flux on MJD 56187 and then steadily decreasing, the XRT count rate continues to increase. Excluding the first day of observations the XRT count rate light-curve can be modeled by a power-law with index 0.65+/-0.05. The last observation we analyzed was performed on MJD 56197, and the rate was in the range of 500 counts/s, showing a ~330% increase since discovery.
The 0.5-10 keV XRT spectrum can be fit with an absorbed power-law model. The absorbing column is consistent with being constant at a level of ~1.2E22 cm^-2, significantly in excess of the galactic value of 6.52E21 cm^-2 (Kalberla et al. 2005). The 2.5-10 keV / 1.0-2.5 keV Hardness Ratio and the Photon Index clearly show a hard to soft evolution, going from 1.36+/-0.02 on MJD 56186 to 2.28+/-0.02 on MJD 56197. The absorption corrected flux in the 0.5-10 keV band increased from 1.8E-9 erg cm^-2 s^-1 to 5.5E-8 erg cm^-2 s^-1. The softening of the source accounts for both the XRT flux flattening and the BAT flux decrease. We find no evidence of a thermal component in the spectrum at any epoch: the low temperature disk component mentioned in ATel #4393 is likely a residual effect associated with the low energy redistribution of photons that is observed in Windowed Timing mode spectra of bright, highly absorbed sources (see http://www.swift.ac.uk/analysis/xrt/digest_cal.php#abs for a description of this issue).
All quoted errors are at 90% confidence level.
