A 1.19 day UV Photometric Period in Nova LMC 2009?

ATel #2001; M F Bode (Liverpool JMU), J P Osborne, K L Page, A P Beardmore (Leicester), J-U Ness (ESA), F M Walter (Stony Brook), M J Darnley (Liverpool JMU), A Evans (Keele), S P S Eyres (Central Lancashire), J Krautter (Heidelberg), T J O'Brien (Manchester), M Orio (INAF-Padova & Wisconsin), G Schwarz (West Chester), S Starrfield (ASU), J Truran (Chicago)
on 1 Apr 2009; 15:37 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Michael Bode (mfb@astro.livjm.ac.uk)

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We report Swift satellite Ultraviolet/Optical Telescope (UVOT) observations of the probable recurrent Nova LMC 2009 (IAUC # 9019 , ATEL #1930). UVOT image mode observations through the 3 UV filters (uvw1, uvw2 and uvm2, with central wavelengths of 2600, 2246 and 1928 Angstroms respectively) began on 2009 February 14 (approximately 9 days after outburst) and continued, initially at 2 day intervals (until day 15), and then at 4 day intervals until day 37. At each epoch, typically 3 or 4 ~300s exposures in each filter were obtained over an interval of around half a day. During this time, the magnitudes in each filter showed an underlying monotonic decline from around mag = 12 to around mag = 13.5 (magnitudes are on the UVOT photometric system; Poole et al. 2008, MNRAS, 383, 627).

Superimposed on this decline is an apparently day-timescale variation of peak-to-peak amplitude ~0.6 mag, visible in all 3 filters. Further observations in each filter were thus carried out at a higher cadence: in the same filters between days 43 to 47 in most of the 96 minute Swift orbits; and in uvw2 alone in event mode in three consecutive Swift orbits from days 48.97 to 49.13. Periodogram analysis strongly suggests a period of 1.19 +/- 0.01 days (90% confidence), and such a period is clearly visible in the 4 days of high cadence data. We note that this could potentially be a beat with the Swift orbital period, and the true periodicity might be 0.063 or 0.071 days. While simulations do not favour the shorter periods, use of the UVOT event mode data has not allowed us to decide between the 1.19 day and the shorter periods.

In order to break this degeneracy, ground-based optical observations are urgently required.

We note that if the 1.19 day periodicity implied by the UVOT observations is related to the orbital period of the system, it is most likely of either the U Sco or T Pyx sub-type as defined by Anupama (2008, in RS Ophiuchi (2006) and the Recurrent Nova Phenomenon, ASP Conference Series, Vol. 401, p.31).

The source remains undetected in the Swift X-ray telescope throughout the observing period with a 0.3-10 keV upper limit of 2.1E-3 counts/s (3 sigma Baysian) in a 5ks observation on 2009 March 26 (day 49).