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Swift detection of the fast turn-on of the recurrent nova M31N 1963-09c as a supersoft X-ray source

ATel #3038; W. Pietsch, M. Henze, F. Haberl, V. Burwitz (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 18 Nov 2010; 12:56 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Ultra-Violet, X-ray, Cataclysmic Variable, Nova

Referred to by ATel #: 8235, 8290

The recurrent nova (RN) M31N 1963-09c has been detected as a supersoft X-ray source (SSS) with the Swift X-ray telescope (XRT) only 17 days after its most recent outburst. Swift has been monitoring the nova in a target of opportunity program with a two day cadence since about 6 days after the first optical detection on 2010-10-30.70 UT (ATel #3001). In an 8.2 ks observation starting at 2010-11-17.09 UT (ObsID 00031861007) the RN was clearly detected with (3.2±0.8)e-3 ct/s (corrected for PSF, sampling dead time and vignetting). This count rate corresponds to an unabsorbed luminosity of (4.7±1.2)e37 erg/s, assuming a typical absorbed black body spectrum with temperature of 50 eV and Galactic foreground absorption of 6.7e20 cm^-2 and a distance of 780 kpc. All of the about 20 collected source counts had energies below 1 keV, allowing us to classify the X-ray source as SSS. Almost all source photons were recorded during the first three orbits of the five-orbit observation and half of those counts were measured in the first orbit. This indicates strong variability in the SSS flux on a time scale of hours. The 6.9 ks XRT observation starting at 2010-11-15.16 UT (day 15, ObsID 00031861006) showed the SSS already at 2 sigma confidence level with a count rate of (1.3±0.6)e-3 ct/s. All of these counts were supersoft and also in this observation the source flux might have been variable. Nothing was found at the position of the nova in an 7.9 ks XRT observation starting on 2010-11-13.09 UT (day 13, ObsID 00031861007) with a 3 sigma upper limit of 3.2e-3 ct/s.

The SSS turn-on time for M31N 1963-09c of about 15 days after the fist optical detection is only slightly longer than it was for the recent outburst of the Galactic RN U Scorpii (12 days, ATel #2430). With a recurrence time of about 5 years between the first two recorded outbursts, M31N 1963-09c is the fastest RN known (followed by U Sco: 8-12 years). Additional to their short recurrence times, both RNe also share a very high expansion velocity as derived from optical spectra via the H-alpha FWHM of about 8100 km/s for M31N 1963-09c (ATel #3006) and about 7600 km/s for U Sco (ATel #2411). Since U Sco started to fade as a SSS about 32 days after discovery (ATel #2477), a similar behavior might be expected for M31N 1963-09c. The X-ray light curve variability detected for M31N 1963-09c could be caused by an eclipse in the binary system, as observed for U Sco (ATel #2442). Continued Swift monitoring of this nova will determine its X-ray spectrum, follow the X-ray light curve and confine its SSS turn-off time.

The Swift Ultraviolet/Optical Telescope (UVOT) light curve of M31N 1963-09c using the UVW1 filter (181-321nm) shows that the ultra-violet magnitude of the nova is declining. Individual measurements are tabulated below. All UV magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and have not been corrected for extinction.

ObsID        MJD       Date            Day after  UVW1  
                       [UT]            outburst   [mag]  
00031861002  55506.20  2010-11-06.20    6         18.7±0.1  
00031861001  55507.06  2010-11-07.06    7         19.3±0.1  
00031861003  55509.53  2010-11-09.53   10         19.4±0.1  
00031861004  55511.01  2010-11-11.01   11         19.5±0.1  
00031861005  55513.09  2010-11-13.09   13         19.6±0.1  
00031861006  55515.16  2010-11-15.16   15         19.6±0.1  
00031861007  55517.09  2010-11-17.09   17         19.8±0.1  

We would like to thank the Swift team for their efforts, in particular N. Gehrels for approving the ToO program and the duty scientists as well as the science planners for making these observations possible.