Swift/XRT detection and optical binary period of SSS130101:122222-311525

ATel #4716; E. Kuulkers (ESA/ESAC), K. L. Page (U. Leicester), C. Knigge (U. Southampton), T. R. Marsh (U. Warwick), J. P. Osborne (U. Leixester), G. R. Sivakoff (U. Alberta)
on 8 Jan 2013; 19:57 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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Two ~2 ks Swift observations on UT Jan 6, 2013 (starting at 20:57 and 22:26) of the bright CRTS optical transient SSS130101:122222-311525 (ATels #4699, #4700) show an X-ray source at RA, Dec (J2000) = 185.59053 deg, -31.25711 deg (12:22:21.73, -31:15:25.6), with a 90% confidence error radius of 2.2". This is consistent with that of the optical transient. The XRT 0.3-10 keV count rate is 0.120+/-0.006 cts/s. There is neither evidence for variability between the two snapshots, nor strong periodicity on any timescale between 10 and 10000 s.

The average UVOT magnitude is m2 = 14.82+/-0.02. Comparing this with the GALEX measurements (ATel #4699), it shows the source to be still in outburst.

The X-ray spectrum can be best fit (C-stat = 208.7/256) with a power-law subjected to a column density consistent with the Galactic value of N_H = 5.45e20 atoms/cm^2 and a power-law index of Gamma = 1.65+/-0.12. Using this model we derive a 0.3-10 keV flux of 5.2e-12 erg/cm^2/s (observed) or 5.7e-12 erg/cm^2/s (unabsorbed). An optically thin thermal component model (APEC) yields a poorer fit (C-stat = 221.5/256). With a Galactic N_H, this fit gives a kT of ~7.6 keV. An APEC model obtained allowing N_H to vary yields a better fit (C-stat = 214/255), with N_H being consistent with zero and kT > 7 keV.

The positionally coincident source 1RXS J122221.5-311545 (ATel #4699) had a count rate of 0.05 cts/s in the ROSAT All-Sky Survey Faint Source Catalog (Voges et al. 2000, IAUC # 7432 ). Using PIMMS and the above best-fit model this converts to ~0.04 XRT cts/s. The ROSAT data likely indicates a lower X-ray luminosity in quiescence by more than an order of magnitude, but we can not rule out the possibility that ROSAT caught the source during another outburst, where we might expect a lower X-ray luminosity (see below).

At the current X-ray flux, the source would need to be more than 40 kpc away to have an X-ray luminosity of greater than ~1e36 erg/s, which would be consistent with a neutron star or black hole X-ray binary at the peak of its outburst. Combined with the low X-ray/optical flux ratio, this rules out the compact object being a neutron star or black hole (see ATel #4704). The object is indeed most likely a CV (ATels #4699, #4704). If we scale from WZ Sge, another, well-known, large amplitude outbursting CV that reached an optical magnitude of about V~8 near the peak, we can obtain a rough distance to SSS130101:122222-311525. It reached about V~12 mag (ATels #4699, #4700); the distance to WZ Sge is 43 pc (Thorstensen 2003, AJ 126, 3017). This leads to a distance estimate of a few hundred pc. Therefore, the X-ray luminosity observed by Swift/XRT is likely in the range of 1e31-1e33 erg/s.

Further optical spectroscopy obtained on UT Jan 6, 2013, from 05:48 to 07:20, with the 4.2m William Herschel Telescope on La Palma, revealed an "S-wave" sinusoidal component in the emission lines with a period of about 80 to 95 min (see http://www.astro.soton.ac.uk/~christian/transient.html ). This we believe to be related to the orbital period, and confirms the analogue with WZ Sge stars, having orbital periods near the CV period minimum.

There are no signs yet of the object being a magnetic CV. Also, we cannot distinguish between different non-magnetic CV types on the basis of the X-ray spectrum. Non-magnetic CVs in outburst generally tend to become fainter in hard (typically >2 keV) X-rays during the outburst (e.g., Collins & Wheatley 2010, MNRAS 402, 1816). We note that two other similar large-amplitude outbursting CVs with comparable orbital periods showed enhanced (hard) X-ray fluxes during their outbursts, namely, V455 And (Senziani et al. 2008, ATel #1372) and GW Lib (Byckling et al. 2009, MNRAS 399, 1576). They both reached a luminosity of about 10^32 erg/s, similar to what we see for SSS130101:122222-311525. We encourage further observations to study the source when it evolves into quiescence.