The outburst of Nova CSS 081007:030559+054715 (HV Ceti)

Abstract

Aims. The outburst of nova CSS 081007:030559+054715 (HV Cet) was extensively followed by Swift to study the evolution of its X-ray and UV emission.Methods. We obtained Swift X-ray and UV observations, at times with high cadence, over an interval of 266 days and analysed the temporal and spectral properties of the object. Optical polarisation measurements were also obtained, but no significant circular polarisation was identified. UV data from GALEX and AAVSO optical data were investigated in addition.Results. A variable supersoft source was discovered, with a 1.77 d periodic modulation in its X-ray, UV and optical emission, in phase across all three bands. High cadence observations revealed a declining trend of the X-ray modulation amplitude, while the UV oscillation became stronger over this interval; the UV data also show occasional secondary minima. The X-ray and UV flux declines sharply ~150 days after discovery (the date of optical outburst peak is uncertain). While bright, the UV and, to a lesser extent, the X-ray light curves show two cycles of a ~40–50 day modulation. We believe the 1.77 d modulation to be orbital, likely caused by structured disc-rim occultation. The X-ray spectra were soft at all times, with negligible flux above 1 keV. These spectra were fitted with an NLTE white dwarf model atmosphere of kT = 60–80 eV, with no spectral changes related to the 1.77 d or possible ~40-day periodicities seen. A Cloudy model demonstrated that it is possible to account for the UV flux through the reprocessing of X-rays. The UV flux is far in excess of the soft X-ray spectral component. We propose that the apparently sub-Eddington white dwarf luminosity is due to permanent obscuration by the accretion disc rim: we see only X-rays scattered by optically thin plasma interior to the disc rim, whereas the UV flux is mostly created by reprocessing in the disk rim interior surface. The in-phase 1.77 d UV and X-ray modulation is caused by the region in which the gas stream from the secondary raises the disc rim, the maximum X-ray obscuration coinciding with the minimum aspect of the UV reprocessing area. In summary, HV Cet had characteristics of both classical novae and persistent super-soft X-ray sources.