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Swift observations of the early development of the 2016 eruption of the recurrent nova LMCN 1968-12a (OGLE-2016-NOVA-01)

ATel #8587; M. J. Darnley (LJMU), N. P. M. Kuin (MSSL/UCL), K. L. Page (U. Leicester), J. P. Osborne (U. Leicester), G. J. Schwarz (AAS), S. N. Shore (Universita di Pisa), S. Starrfield (ASU), S. C. Williams (U. Lancaster)
on 25 Jan 2016; 19:34 UT
Credential Certification: Matt Darnley (M.J.Darnley@ljmu.ac.uk)

Subjects: Optical, Ultra-Violet, X-ray, Nova, Transient

Referred to by ATel #: 8615, 8616, 8626

The 2016 eruption of the LMC recurrent nova (RN) LMCN 1968-12a was announced in ATel #8578. This eruption is the fifth recorded following 1968 (Sievers 1970), 1990 (see Shore et al. 1991), 2002 and 2010 (for both see Mróz et al. 2014), suggestive of a recurrence period of ~6 years. If confirmed, this system has the third shortest recurrence time after M31N 2008-12a (6 or 12 months; Henze et al. 2015) and M31N 1963-09c (5 year period; see Shafter et al. 2015, ATel #8233, #8242), and is the closest of these short recurrence interval novae.

Swift started observing the current nova eruption on 2016 January 23, 2.07 days after the detection by the OGLE team (2016 January 17.24 UT). A faint X-ray source, at a count rate of 0.024+0.007-0.005 count s-1 was detected in this first 500 s exposure. Further Swift observations were obtained 2.62, 3.07 and 3.20 days after the eruption. The X-ray source remained approximately constant during these observations, with a mean X-ray count rate of 0.014 ± 0.002 count s-1. All but two of the photons detected were between 1 and 4 keV. This early (pre-SSS) X-ray luminosity appears to be significantly greater than that seen at a similar phase following the 2010 eruption of U Sco (see, for example, ATel #2419).

The UVOT detected a bright UV source, with a magnitude of uvw1 ~ 12.2. During each of these four Swift visits the UVOT UV grism was deployed. These subsequent spectra show weak Balmer emission lines (with possibly a suggestion of P Cygni absorption profiles) that are consistent with the description of the H I lines from ground-based optical observations reported by ATel #8586.  These Swift spectra also show emission at C III] (1909Å) and He II (4686Å). As after the 1990 eruption (Shore et al. 1991), these early spectra indicate that the optical depth in the ejecta is already low.

Coupling the long-term quiescent brightness reported by Mróz et al. 2014 (I ~ 19.2) with UVOT quiescent photometry from Swift taken in 2013 (uvw1 ~ 18.4; uvw2 ~ 18.7; also, simultaneous XRT (0.3-10 keV) count rate < 0.008 count s-1), and typical distance and reddening towards the LMC, indicates a quiescent system whose optical/UV luminosity is dominated by an accretion disk that is brighter than that seen in U Sco (see, for example, Darnley et al. 2014). The low amplitude periodicity of 1.26d at quiescence (Mróz et al. 2014) is suggestive of an orbital period modulation, the quiescent photometry and the period suggest that this system is similar to U Sco (but probably non-eclipsing), which is consistent with the findings of Shore et al. 1991.

Based on the previous eruptions, this system is known to evolve quickly (t2 ~ 3.5 days) so rapid follow-up, particularly spectroscopic, is greatly encouraged. We also encourage searches of archival observations to find any missed eruptions focusing particularly on a recurrence period of approximately 6 years.