Swift UV and X-rays from TCP J05002724+1334199 / CSS J050027.2+133420
ATel #8387; E. Kuulkers (ESA/ESAC, Spain), K. L. Page & J. P. Osborne (U. Leicester, UK)
on 8 Dec 2015; 20:41 UT
Credential Certification: Erik Kuulkers (Erik.Kuulkers@sciops.esa.int)
Subjects: Ultra-Violet, X-ray, Cataclysmic Variable, Transient, Variables
TCP J05002724+1334199 (or CSS J050027.2+133420) was discovered on UT 28 Nov 2015 13:22, at an optical magnitude of ~12.3. This source is coincident with a known ROSAT X-ray source, 1RXS J050027.7+133415 (as well as SDSS J050027.24+133419.9, GALEX J050027.2+133420). It is also present in the 1st Swift XRT Point Source Catalogue as 1SXPS J050027.1+133418. Optical spectral features suggest that the variable is likely a dwarf nova in outburst (CBET #4207). On UT 30 Nov 2015 11:20 it was at V~13.8, so fading over a couple of days (see http://www.cbat.eps.harvard.edu/unconf/followups/J05002724+1334199.html ). T. Kato (vsnet-outburst #18949), however, noted it as an unusual outbursting object: the SDSS colours seemed too red for a dwarf nova.
We followed up the outburst with Swift observations on UT 2 Dec 2015 16:54-17:07, 18:29-18:43, and 19:57-20:10 for a total of 2 ksec. We find a mean XRT PC mode 0.3-10 keV count rate of (6.3 +1.8/-1.5)e-3 count s^-1, i.e., a total of 10 counts which all fall below ~1.2 keV. The UVOT uvm2 magnitude is 14.77 +/- 0.03.
Swift observations of the same field were performed on UT 25 and 31 July 2008, for a total of 5.3 ksec. The X-ray source was less soft at that time, with a XRT count rate of about 0.05 count s^-1, and a uvm2 magnitude of 17.27 +/- 0.04.
Fitting the 2008 X-ray spectrum with the optically thin plasma emission model APEC (Astrophysical Plasma Emission Code), typically used to characterise dwarf novae, we find kT = 5.1 (+5.4, -1.7) keV and N_H = 8 (+9, -7) x 10^20 cm^-2, with observed 0.3-10 keV X-ray fluxes of 2.0e-12 (absorbed) and 2.2e-12 (unabsorbed) erg cm^-2 s^-1. Fixing the parameters at the fit values from the 2008 data, we find 0.3-10 keV fluxes of 5.3e-13 (absorbed) and 5.9e-13 (unabsorbed) erg cm^-2 s^-1 on 2 Dec 2015, i.e., about a factor of 4 lower than in 2008.
The behaviour seen by Swift, i.e., the anti-correlation of X-ray and optical/UV flux between outburst and quiescence, is typical for a dwarf nova, and confirms the earlier suggested identification.
We thank the Swift team for the observations of source.
