Swift detection of UV and super-soft X-ray emission in Nova LMC 2012

ATel #4043; K. L. Page (U. Leicester), F. M. Walter (Stony Brook U.), G. J. Schwarz (AAS), J. P. Osborne (U. Leicester), M. J. Darnley (LJMU), J. J. Drake (CfA) and A. W. Shafter (SDSU)
on 15 Apr 2012; 12:00 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Swift started observing Nova LMC 2012-03a (TCP J04550000-7027150) on 2012 March 27, 1.3 days after its discovery (CBET #3071; ATel #4002). Between March 27 and April 03 (days 1.3 and 8.2), there was no X-ray detection, with an upper limit of 1.2x10^-3 count s^-1 (encompassing 6.2 ks of exposure time) on the 0.3-10 keV count rate. During this time, a bright, though rapidly fading, UV source was seen, starting with a uvm2 magnitude (central wavelength of 2246 A) of 11.3 and declining to 14.1 by April 03.

Between April 03 and 12, Nova LMC 2012-03a could not be monitored by Swift due to an observing constraint; however, SMARTS data (photometry and spectra) continued to be regularly obtained during this interval. The optical spectrum is reminiscent of U Sco and any early P Cygni absorption had disappeared by March 28/29. A spectrum taken on April 08 (day 13) revealed a strengthening He II 4686 A line compared to earlier data; this suggested that the nova might have become a super-soft X-ray source. The optical nova declined by 5 mag in the 7 days running up until 02 April; the rate then slowed, with SMARTS photometry showing the source to have plateaued at about V = 17.

A Swift observation on April 13 (day 18.2) confirmed this suggestion, with a strong X-ray source detected at a mean rate of 3.68 +/- 0.06 count s^-1. The source is soft, with almost all the counts below 1 keV. The spectrum can be fitted by an absorbed blackbody, with BB kT = 49 +4/-3 eV and N_H = (3.7 +/- 0.7) x 10²¹ cm^-2. There is some evidence for excess emission between ~ 0.55 and 0.65 keV, possibly indicating the presence of nitrogen and oxygen emission lines. A further observation on April 15 (day 19.6) showed the count rate had increased to 5.72 +/- 0.07 count s^-1. The UV magnitude on April 13 was m2 = 15.0, and on April 15, m2 = 15.7, showing that the rate of decline of the UV source has also slowed, in agreement with the SMARTS optical monitoring.

The rapid decline seen in the optical/UV and the swift onset of the super-soft phase hints that this object may contain a high mass white dwarf and, therefore, be a recurrent nova candidate.

Swift observations of Nova LMC 2012 are continuing, as well as nightly SMARTS photometry (BVRIJHK; together with spectra on most nights). Monitoring of this source during the super-soft phase at other wavelengths is encouraged. We thank the Swift PI and operations team for their support.