Detection of super-soft emission in nova V339 Del

ATel #5470; K. L. Page, J. P. Osborne (U. Leicester), N. P.M. Kuin (UCL-MSSL), C. E. Woodward (U. Minnesota), G. J. Schwarz (AAS), S. Starrfield (ASU), S. N. Shore (U. of Pisa, INFN-Pisa) & F. M. Walter (Stony Brook U.)
on 14 Oct 2013; 20:35 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Swift has been regularly observing the fourth LAT-detected classical nova (ATEL #5302) V339 Del (Nova Del 2013; PNV J20233073+2046041) since its outburst on 2013 August 14. Once optical loading had been correctly accounted for (ATel #5429), a real X-ray source was first detected on September 19. The XRT 0.3-10 keV X-ray count rate has been slowly rising since this time, from a value of (1.8 +0.7/-0.6)x10^-3 count s^-1 on September 19 to (7 +/- 2)x10^-3 count s^-1 on October 8 [these count rates are from grade 0 (single pixel) events only, to mitigate any residual optical loading effects]. On October 9 V339 Del showed a noticeably higher count rate of 0.012 +/- 0.004 count s^-1. The first detection of significant X-ray flux occurred at about the start of a phase of much faster V-band fading (around day ~ 40, see the AAVSO light curve).

Before October 9 almost all counts detected were in the hard (1-10 keV) band; however, after that time, soft (0.3-1 keV) counts began to be detected. The spectrum obtained on October 13 (day 60), when the XRT count rate was 0.019 +/- 0.004 count s^-1, showed a significant soft component: count rate of (8 +/- 2)x10^-3 count s^-1 over 0.3-1 keV, compared to (0.011 +/- 0.002) count s^-1 over 1-10 keV, possibly indicating the start of the super-soft source (SSS) phase in which the white dwarf photosphere becomes visible. The rise in the soft emission was further confirmed on October 14, when the soft and harder count rates were (6 +/- 2)x10^-3 and (7 +/- 2)x10^-3 count s^-1, respectively.

The optical/UV magnitude of V339 Del has, until recently, been too bright to measure accurately with the Swift-UVOT. However, using the new calibration of the image readout-streaks (Page et al. 2013), the uvm2 magnitude can now be estimated to be 8.33 +/- 0.13 (October 07), 8.76 +/- 0.41 (October 09) and 8.81 +/- 0.13 (October 11).

We are continuing to monitor the source in both X-rays and UV, and thank the Swift PI and mission operations team for their ongoing support.