Increased soft X-ray activity but no optical response in Swift J1753.5-0127
ATel #7216; A. W. Shaw, P. A. Charles, P. Gandhi, D. Altamirano (U. Southampton)
on 12 Mar 2015; 21:57 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)
Subjects: Optical, X-ray, Binary, Black Hole
Referred to by ATel #: 7697
We report on new X-ray and optical observations of the unusual black hole candidate Swift J1753.5-0127. As previously found (ATel #7196), the source has been declining significantly in hard X-rays (15-50 keV) as seen in Swift-BAT light curves (http://swift.gsfc.nasa.gov/results/transients/SWIFTJ1753.5-0127/), with a similar response reported in the MAXI-GSC light curves (http://maxi.riken.jp/top/index.php?cid=1&jname=J1753-014).
We requested Swift ToO observations which commenced on March 11 2015 with a 1 ks pointing. The resultant 0.6-10 keV spectrum can be well constrained (χ2/ndf = 1.05) by an absorbed disk blackbody with an additional power law component. We found a column density of (4.51 +/- 0.05)E21 cm-2 and a power law photon index of 4.1 +/- 0.2, which is in agreement with ATel #7196, whereas the inner disc temperature was found to be 0.36 +/- 0.04 keV. The absorbed 0.6-10 keV flux is (1.10 +/- 0.01)E-9 erg/cm2/s, lower than during the last hard-to-soft state transition (Yoshikawa et al. 2015, PASJ, 67, 11), however we must note the much larger column density in the latest spectrum. Fixing the column density to 2.8E21cm-2 as in Yoshikawa et al. also yielded a formally acceptable fit though with an increased χ2 (Δχ2 = +13) for one less d.o.f.
Comparing this with archival spectra in the low hard state (e.g. Jun 2013) reveals that the flux in the softest band (0.6-2 keV) has increased dramatically, but the contribution from 2-10 keV has dropped to only ~10% of the total flux. Fitting the same model to the Jun 2013 Swift-XRT spectra shows that the total flux then was dominated by contributions in the 2-10 keV band. We therefore believe that Swift J1753.5-0127 is now entering a much softer state.
The source was also observed by Swift UVOT with the B and V filters. The magnitude of the optical counterpart is B=17.27+/-0.04 and V=16.93+/-0.06. It is interesting to note that these magnitudes have not significantly changed since the last published optical photometry in the low-hard state (Neustroev et al. 2014, MNRAS, 445, 2424), and the source has only declined ~1 mag since its initial outburst, indicating that activity in the disc and/or hot flow has been continual since 2005.
Further ToO observations with Swift have been scheduled and multiwavelength observations are encouraged.
We would like to thank the Swift team for promptly scheduling these observations.
