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Nova SMC 2012: a probable 20.4 hour period detected by Swift in the UV and X-ray

ATel #4920; K. L. Page, J. P. Osborne, A. P. Beardmore (U. Leicester) and G. J. Schwarz (AAS)
on 27 Mar 2013; 09:08 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: Nova

Following the detection of Nova SMC 2012 (OGLE-2012-NOVA-002) as a super-soft X-ray source on 2013 March 2, 270 days after optical detection (ATel #4853), Swift has monitored the source, generally daily, and recently also at higher cadences. The X-ray spectrum of this nova has not changed significantly over this time. Periodogram analysis of de-trended UVOT data (collected using the uvw2 filter, central wavelength 1928 Angstrom) since the detection of the super-soft emission (i.e. days 270-294) reveals a strongly-detected sine-like modulation with a period of 20.4 +/- 0.3 hours and an amplitude of 0.11 +/- 0.01 magnitudes. During this interval the mean uvw2 brightness fell from magnitude ~16.1 to ~16.3, while the X-ray count-rate shows no decline over this interval, having a mean of ~0.1 count s-1. The highest peak in the X-ray periodogram is at the same frequency as detected in the UV. Folded at the UV period, the X-ray count-rate shows a modulation with an amplitude of 0.020 +/- 0.005 count s-1, in phase with the UV brightness variation.

This pattern of in-phase X-ray & UV modulation is similar to that seen in Nova Mon 2012 and HV Cet (ATel #4727; Beardmore et al., 2012, A&A, 545, A116). In these objects the modulation was ascribed to a raised portion of an accretion disk rim passing through the line of sight, reducing the observed size of a scattering region around the hot white dwarf, and likely reprocessing the illuminating flux from its inner face. The detected period in this scenario would be the orbital period of the binary system. This result suggests that Nova SMC 2012 has a moderately high orbital inclination, however no strong eclipse is evident in the UV folded light-curve.

Although most novae have orbital periods of a few hours, novae with Porb = 20-50 hours are known (including the well-known recurrent nova U Sco at Porb=29.5 hours). A nova with an orbital period in this range requires an evolved secondary star.

Further Swift observations are planned, and follow-up observations at other wavelengths are encouraged. We thank the Swift PI and operations team for their support.