Swift detection of Super Soft X-ray emission in nova CSS081007:030559+054715

ATel #1847; G. J. Schwarz (West Chester), J.-U. Ness (Arizona State University), J. P. Osborne and K. Page (University of Leicester), R. M. Wagner (LBT/ Ohio State University), S. Starrfield (Arizona State University), J. Prieto, O. Pejcha, and K. Denney (Ohio State University)
on 17 Nov 2008; 14:30 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

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Swift obtained simultaneous X-ray and UV observations of the recent ONe nova CSS081007:030559+054715 (ATEL #1825, ATEL #1835) on November 10 and 14, 2008. The ~ 4 ks observation revealed a strong and variable source in both the XRT and UVOT detectors. The average X-ray count rate was 0.183+/-0.009 counts/s while the UVW2 band (lambda_c ~ 1928 Angstroms) was 15.1+/-0.1 mag. The source is very soft in the X-ray spectrum with most photons below 0.6 keV. The X-ray spectrum can be modelled with an absorbed blackbody with kT ~ (38 +5/-7) eV and NH ~ (1.6 +0.9/-0.5)x10^21 cm^-2. The 0.3-1 keV X-ray flux is 6.3x10^-12 erg cm^-2 s^-1 (1.1x10^-10 unabsorbed).

The unabsorbed bolometric flux of the model is 2.8x10^-9 erg cm^-2 s^-1. To convert this to a bolometric luminosity we estimate the distance through indirect methods since the initial outburst was not observed. Converting the SDSS ugriz photometry of the quiescent source to UBVRI photometry using the stellar transformations of Jester et al. (2005, AJ, 130, 873) gives V = 18.5 mag. The extinction maps of Schlegel et al. (1998 ApJ, 500, 525) show E(B-V) = 0.15 in the direction of CSS081007:030559+054715.

The first method assumes an absolute visual magnitude of M_V(max) ~ -9 mag and an outburst amplitude between 12 and 15 magnitudes (e.g. Table 5.4 in Warner's "Cataclysmic Variable Stars" 1995), both are typical of very fast novae and CSS081007:030559+054715 is likely to be a member of this speed class based on the similarity of its optical nebular spectrum with the very fast novae V838 Her and V4160 Sgr (see Schwarz et al. 2007, ApJ, 657, 453). These assumptions suggest a distance range of 2.5 to 10 kpc. A second method assumes that the absolute visual magnitude at minimum for post-novae and some cataclysmic variables is 7 > M_V(min) > 4 mag (e.g. Fig. 4.20 in Warner 1995) for a wide range of orbital periods and inclination angles. The distance based on this second method is between 1.6 and 6.4 kpc.

Therefore the bolometric luminosity ranges from 9x10^35 to 1.4x10^37 erg s^-1 for distances of 2.5 and 6.4 kpc. Note that luminosities based on blackbodies generally overestimate the true luminosity while underestimating the effective temperature. Interestingly, our distance estimate and the galactic coordinates (l,b = 172.6,-43.7) imply that the z-distance from the Galactic plane exceeds 1.7 kpc which is well above where most classical novae, particularly the ONe type, are found. Additional observations at other wavelengths of this interesting nova are strongly encouraged.

We thank the Swift PI, Neil Gehrels, the Swift science team, and the Swift mission operations team for their support of these observations.