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Supersoft X-ray emission detected from Nova Mon 2012

ATel #4590; Thomas Nelson (Minnesota), Koji Mukai (UMBC and NASA/GSFC), Jennifer Sokoloski (Columbia), Laura Chomiuk (Michigan State), Michael Rupen (NRAO), Amy Mioduszewski (NRAO), Kim Page (Leicester) and Julian Osborne (Leicester)
on 20 Nov 2012; 17:11 UT
Credential Certification: Thomas Nelson (tnelson@physics.umn.edu)

Subjects: X-ray, Nova

Referred to by ATel #: 4614, 4633, 4709, 4727, 4737

Nova Mon 2012 is one of only three novae identified as a transient gamma-ray source with the Fermi satellite (ATel #4224, #4310). The nova was discovered in the optical on 2012 August 9 (CBET #3202), although the Fermi detection indicates that the outburst likely started in June. Nova Mon is a bright radio source with spatially resolved substructure (ATel #4352, #4408), and has been detected in X-rays at energies up to 10 keV by the Swift and Chandra satellites (ATel #4321, #4569). Infrared spectra obtained on 2012 November 1 and 2 revealed that Nova Mon has transitioned to the coronal phase, thought to indicate that the ejecta have become optically thin (ATel #4542).

We have been monitoring Nova Mon with the Swift satellite since August 2012 (see ATel #4321). Our most recent observation was carried out on 2012 November 18, with a total X-ray Telescope (XRT) exposure time of 1048 s. The source count rate in the 0.3-10 keV energy range was 0.43 counts/s, an increase of 60% from the previous Swift observation on 2012 November 11. In addition to the hard X-ray emission detected in previous observations, a new soft spectral component was observed for the first time.

We fit the overall spectrum with an absorbed blackbody plus thermal plasma model (wabs*(bb+apec) in Xspec). The best fit column density of the absorber was N(H) = 8.5 (+0.2,-0.3) x 10^21 cm^-2. The blackbody component had a temperature kT = 0.021 +/- 0.006 keV (T ~ 250,000 +/- 70,000 K), and an observed 0.3-10 keV flux of 5.8 x 10^-12 erg/s/cm^2. The normalization of this component was poorly constrained, making an estimate of the intrinsic luminosity difficult. The thermal plasma component had kT = 0.95 +/- 0.23 keV (T ~ (1.1 +/- 0.8) x 10^7 K) and an observed 0.3-10 keV flux of 5.4 x 10^-12 erg/s/cm^2.

Further X-ray monitoring observations are planned, and follow-up observations at other wavelengths are encouraged. We thank Neil Gehrels and the Swift mission operations team for making these observations possible.