Swift J1753.7-2544: Continued Swift observations

ATel #4818; H. A. Krimm (CRESST/GSFC/USRA), S. T. Holland (STScI), J. A. Kennea (PSU)
on 16 Feb 2013; 13:14 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Tweet

The newly discovered galactic transient source Swift J1753.7-2544 (Krimm et al, ATels #4769, #4789) continues to be observed with all of the Swift telescopes.

In the BAT, the 15-50 keV light curve shows a drop in intensity from its peak on 2013 January 29. The count rate has leveled off at ~45 mCrab. The rate on 2013 February 15 (MJD 56338) was 0.010 +/- 0.003 ct/s/cm^2.

In the XRT, the count rate is also falling, to 1.96 +/- 0.05 ct/s in the 0.3-10 keV band in a 464-second observation starting at 2013 February 14 13:22:58 UT. The fit to an absorbed power law gives a photon index of 1.5 +/- 0.3, NH = 5.0 +/- 1.1 X 10^22 cm-2, consistent with what was reported in Krimm et al (ATel #4789). The unabsorbed flux is 4.2 +/- 1.0 X 10^-10 erg cm-2 s-1. Pile-up corrected spectra were extracted using the methods described by Evans et al (2009, MNRAS, 2009, 397, 1177).

The UVOT detected the a star within the XRT error circle at a position of RA=268.416500 (17h 53m 39s.96), dec=-25.753470 (-25d 45m 12s.5), with an error (statistical plus systematic of 0.55 arcsec). This is consistent with the position of a field star found in several astronomical catalogs and reported in Krimm et al, ATel #4789. The magnitudes in the UVOT B filter are consistent with the reported B magnitude of USNO-A2.0 0600-29362287 (B=17.2) and show no signs of fading. The UVOT position is also consistent with the brighter archival near-infrared source reported by Rojas, Masetti & Minniti (ATel #4802).

The UVOT magnitudes are:

------------------------------------------------------
Filter Date Exposure Mag Err Sig OBSID
------------------------------------------------------
uvw1 2013-02-04 2135 19.55 0.13 8.7 00032698002
b 2013-02-08 1486 17.31 0.04 58.5 00032698002
b 2013-02-11 660 17.37 0.05 36.7 00032698003
b 2013-02-14 776 17.26 0.04 45.8 00032714001
------------------------------------------------------

Given the high X-ray absorption, the proximity of the source to the Galactic Center, and large expected Galactic reddening, it is likely that the UVOT optical source in the XRT error circle of Swift J1753.7-2544 is not related to the X-ray source, but is rather a chance alignment. The lack of variability of the UVOT source also supports this conclusion.

Near infrared observations at the current epoch would be able to show if any of the archival NIR sources is the optical counterpart to Swift J1753.7-2544.

Swift/BAT Hard X-ray Transient light curve for Swift J1753.7-2544