Detection of Super-Soft X-ray emission in Nova LMC 2009

ATel #2025; 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 20 Apr 2009; 14:57 UT
Credential Certification: Michael Bode (mfb@astro.livjm.ac.uk)

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We report Swift satellite X-ray Telescope (XRT) observations of the probable recurrent Nova LMC 2009 (IAUC # 9019 , ATel #1930, #2001). We have detected a highly variable soft X-ray source, initially at a count rate of 0.010 +0.007/-0.005 c/s on day 70.74 after discovery of the outburst (2009 February 5.067 UT, IAUC # 9019 ), which reached 0.042+/-0.009 c/s on day 70.88, then returned to a level of ~0.01 c/s on days 71.55 and 71.94, before climbing to 0.130+/-0.017 c/s on day 73.43.

The spectrum from days 70.7 to 70.9 can be approximately modelled with an absorbed black body. With N_H = 1.13e21 cm^-2 in this direction (Kalberla et al. 2005, A&A, 440, 775), kT = 38.8 +10.3/-7.6 eV. The resulting unabsorbed luminosities for d = 48.5 kpc are 2.4e36 erg s^-1 (0.3-10 keV) and 5.3e37 erg s^-1 (bolometric).

The nova has been observed with the XRT every few days since mid-February but remained undetected with count rate upper limits (3 sigma) of ~2e-3 c/s over 0.3-10 keV. Although there may have been a marginal detection on day 63 after outburst, we note that the emergence of the Super-Soft Source in this object is at a significantly later time than that in either U Sco (<20 days, Kahabka et al. 1999, A&A 347, L43) or RS Oph (~30 days, Osborne et al., ATel #764) implying that in Nova LMC 2009 the mass loss rate was higher and/or the ejection velocities were lower than in either of these two recurrent novae.

The UV brightness, determined from Swift Ultraviolet/Optical Telescope (UVOT) observations, has continued the declining trend, reported in ATel #2001, to around mag(uvw2) = 14 on day 68. Similar variability to the 1.19 day modulation reported in ATEL #2001 appears to be continuing. We note that recent AAVSO photometric timeseries are not so far sufficient to decide between the periods listed there.

Swift observations are continuing and further ground-based monitoring (both photometry and spectroscopy) is strongly encouraged. We thank the Swift PI, Neil Gehrels, the Swift science team and the Swift mission operations team for their support of these observations.