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Recurrent nova M31N 2008-12a: the 2014 outburst detected in X-rays with Swift XRT

ATel #6558; M. Henze (ESA/ESAC), J.-U. Ness (ESA/ESAC), M. J. Darnley (LJMU), S. C. Williams (LJMU), M. F. Bode (LJMU), A. W. Shafter (SDSU), R. A. Hounsell (STSCI)
on 8 Oct 2014; 22:04 UT
Credential Certification: Martin Henze (martin.henze@sciops.esa.int)

Subjects: Ultra-Violet, X-ray, Nova, Transient

Referred to by ATel #: 6565, 6604, 9872, 11130

The sixth outburst in seven years of the remarkable M 31 nova M31N 2008-12a (see Darnley et al. 2014, Henze et al. 2014 and Tang et al. 2014) was recently discovered on 2014-10-02.90 UT (ATel #6527). After the previous outburst in late November 2013 (ATel #5607), we had detected X-ray emission connected to the optical nova for the first time (ATels #5627,#5633).

Following the current outburst we immediately initiated a daily Swift/XRT monitoring which managed to detect the supersoft X-ray source (SSS) counterpart of the nova on 2014-10-08.29 UT, only six days after the optical outburst. A faint source at the position of the optical nova was detected with a count rate of (6.1±1.2) × 10-3 ct/s in a 6.5 ks observation. Nothing was seen at this position in the preceding 7.3 ks XRT observation on 2014-10-07.35 UT with a 3σ upper limit of 2.1 × 10-3 ct/s. The position of the X-ray source is in good agreement with the 2013 detections.

All of the approximately 25 source photons have energies below 1 keV. This object is clearly a SSS. The early onset of the SSS phase is consistent with the evolution of the 2013 outburst (Henze et al. 2014). The X-ray source brightened noticeably during the observation which indicates the gradual emergence of the SSS emission. M31N 2008-12a remains the nova with the fastest SSS turn on in any galaxy.

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 uvw2 filter (112-264 nm) of 17.2±0.1 mag in the first observation of the Swift monitoring on 2014-10-03.63 UT. This magnitude is in the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and has not been corrected for extinction. A crowded field might introduce additional photometric uncertainties.

We will continue to monitor this remarkable nova. A comprehensive analysis of all observations is in preparation.

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.