Detection of X-rays from ASASSN15-po
ATel #8056; Thomas J. Maccarone(Texas Tech University)
on 14 Sep 2015; 16:48 UT
Credential Certification: Tom Maccarone (thomas.maccarone@ttu.edu)
Subjects: Cataclysmic Variable, Transient
Through director's discretionary time graciously approved by the Swift team, Swift observed ASASSN15-po (ATel #8042) on 2015-09-12 at UT 23:43:44 for 1805 seconds of live time in photon counting mode. It detected 18 X-ray counts within an annulus of 47". If one assumes a Gamma=1.7 power law for conversion from counts to flux, then the flux from the source is about 4.4e-14 erg/sec/cm^2.
The UVOT data show a very bright source in the UVM2 filter with a magnitude of 12.0 according to the standard pipeline analysis. A more careful analysis should be done before this value is used in refereed publications, since this magnitude is near the limit of 10.3 in UVM2 where the standard tools have major problems with coincidence losses.
In ATel #8042, it was suggested that the source is a likely WZ Sge cataclysmic variable, on the basis of its large amplitude of variation. They reported a V=13.7 detection. Our detection at approximately 12th magnitude suggests that the source has continued to brighten, or is extremely blue, or both, providing additional support for the hypothesis that it is an extreme cataclysmic variable outburst.
Given the Warner (1987, MNRAS, 227, 23) relation between CV peak outburst magnitude and orbital period, it is reasonable to expect that this system has a peak absolute magnitude of about 5-6, giving it a distance modulus of about 6-8, or a distance of roughly 300 pc, with an uncertainty of about a factor of 2. At 300 pc, the X-ray luminosity would be only 4.5e29 erg/sec. This is a factor of about 100 below, e.g. even the faintest observations seen from SS Cygni (e.g. Wheatley et al. 2003, MNRAS, 345, 49), indicating that the X-ray to optical flux ratio may be substantially smaller for this object than for SS Cyg, even when it was relatively X-ray faint due to its boundary layer becoming optically thick near the peak of its outburst. This may be indicative of a lower mass white dwarf in ASASSN15-po, or of details in the X-ray production mechanism which are not yet well understood.
An additional 9 observations are planned with Swift, over the next 18 days, so monitoring at other wavelengths, and, particularly, spectroscopic monitoring, is encouraged.
