A Candidate Recurrent Nova in M31: A fast supersoft X-ray counterpart found with Swift XRT
ATel #5627; M. Henze, J.-U. Ness (both ESA/ESAC), M. F. Bode, M. J. Darnley, S. C. Williams (all LJMU)
on 4 Dec 2013; 10:43 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)
Subjects: Ultra-Violet, X-ray, Nova
We report Swift XRT and UVOT detections of the most recent outburst of a candidate recurrent nova (RN) in M31 (see ATel #5607). This nova has had previous recorded outbursts in 2008, 2011 and 2012 (ATels #3725,#4503). An optical spectrum taken after the 2012 outburst confirmed the object as a nova and provided evidence that it belongs to the He/N spectroscopic class (ATel #4503). A potential progenitor system, showing indications for having an evolved secondary and potentially high accretion rate, was discovered in archival HST data (ATel #5611). Recurrence times of only about a year are unprecedented in any RN and would be a test of current thermonuclear runaway models of nova outbursts.
An X-ray counterpart of the nova was discovered in a 6-ks Swift XRT observation starting at 2013-12-03.03 UT, only six days after the optical discovery (Nov 27.1 UT, ATel #5607). The source is clearly detected with a count rate of (1.3±0.2) × 10-2 ct/s at the position RA = 00:45:29.32, Dec = +41:54:05.5 (J2000; statistical 90% error of 4.4 arcsec). Due to the relatively large uncertainties, this position is still consistent with the coordinates of the optical nova as reported in ATel #5607. The count rate was corrected for exposure. We extracted an X-ray spectrum from the approximately 65 source counts, all of which are below 1 keV. This spectrum can be fitted with a blackbody model with best-fit temperature of (57+11-9) eV and NH = (1.9+1.0-0.8) × 1021 cm-2 (1σ confidence ranges for the statistical uncertainty). This object is clearly a supersoft X-ray source (SSS).
The nova displayed the fastest SSS phase turn on (i.e. the start of the SSS phase) ever observed in any nova, in particular within the M31 nova sample (Henze et al. 2011, A&A, 533, A52). The date of the optical discovery is accurate to within a day (ATel #5607). This constrains the SSS turn on to < 7d, which is even faster than for the Galactic RN U Sco (> 8d; see ATel #2430). The exceptionally fast evolution is consistent with the unusually short recurrence time scale. We note that following discovery of the previous outburst on 2012-10-18.68UT (see ATel #4503) Swift XRT observations yielded no detections at 4 epochs from 19 to 110 days after discovery. We will continue to monitor the evolution of this peculiar nova in X-rays.
The nova is also clearly detected as an ultraviolet (UV) source in the corresponding Swift UVOT data. We estimated a (Vega system) magnitude in the uvw1 filter (112-264 nm) of 19.8±0.1 mag. This magnitude is in the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and has not been corrected for extinction. Also, a crowded field might introduce additional photometrical uncertainties.
We wish to thank the Swift Team for making the ToO observations possible, in particular N. Gehrels, the duty scientists as well as the science planners.
