No X-ray increase from MASTER OT J053123.94+120051.5
ATel #6713; Arash Bahramian, Craig O. Heinke (Alberta)
on 17 Nov 2014; 20:48 UT
Credential Certification: Arash Bahramian (bahramia@ualberta.ca)
Subjects: Optical, Ultra-Violet, X-ray, Cataclysmic Variable, Transient, Young Stellar Object, Pre-Main-Sequence Star
MASTER OT J053123.94+120051.5 is a new optical transient discovered by MASTER (ATel #6702). The source is located in the direction of the star-forming region Orion OB association. This transient also has an X-ray quiescent counterpart (3XMM J053123.9+120051 or 1SXPS J053124.1+120048). We investigated available archival XMM/EPIC data from 2008 (Obs.ID: 0405210101). Fitting the spectrum in Xspec with absorbed powerlaw (TBABS*PEGPWRLW, abundances from Wilms et al. 2000) shows a moderately absorbed spectrum (N_H ~ 2e21 cm-2, photon-index = 2±1) with unabsorbed flux of ~1e-13 erg/s/cm2 in 0.5-10 keV band. Investigating the lightcurve from this observation suggests variation at the 95% confidence level, according to a K-S test.
Following the report of activity, we observed this transient with Swift/XRT in PC mode on Nov. 16, 2014 (Obs.ID: 33531001). The source is not clearly detected in this 2ks observation, with 3 photons in a 9" radius circle (Swift/XRT PSF HPD at 1.5 keV). We expect 0.07 photon from background in this 2ks observation in a 9" radius circle, so this suggests that X-rays are detected. The implied flux is consistent with the XMM spectrum; an increase by a factor of 3 can be ruled out.
Analysis of the Swift/UVOT (UVW2 filter) image from this observation shows a source detected at the 2sigma level with coordinates RA = 05:31:24.032 and Dec = +12:00:51.49 (with radial error of 0.2" in position). This agrees with the position of the optical transient. We perform photometry using the
''uvotsource'' task, and find a magnitude of 20.1±0.2 (Vega mag) in the UVW2 band.
The nature of this unusual transient is unclear. The colors quoted in ATel #
6702 are red, and the XMM spectrum is absorbed, so we would not expect a UV detection, but it is clearly detected, suggesting an increased UV flux. The apparently increased UV flux suggests the possibility of a UV-bright source, such as a cataclysmic variable, but other possibilities (a protostar, a background AGN, an X-ray binary) cannot be excluded yet.
We thank the Swift team for rapidly scheduling our observation.
