The First Orbital Period of a Very Bright and Fast Nova in M31: M31N 2013-01b

We present the first X-ray and UV/optical observations of a very bright and fast nova in the disk of M31, M31N 2013-01b. The nova reached a peak magnitude R ∼ 15 mag and decayed by 2 mag in only 3 days, making it one of the brightest and fastest novae ever detected in Andromeda. From archival multiband data we have been able to trace its fast evolution down to U > 21 mag in less than two weeks and to uncover for the first time the super-soft X-ray phase, whose onset occurred 10-30 days from the optical maximum. The X-ray spectrum is consistent with a blackbody with a temperature of ∼50 eV and emitting radius of ∼4 × 10⁹ cm, larger than a white dwarf (WD) radius, indicating an expanded region. Its peak X-ray luminosity, 3.5 × 10³⁷ erg s^-1, places M31N 2013-01b among the most luminous novae in M31. We also unambiguously detect a short 1.28 ± 0.02 hr X-ray periodicity that we ascribe to the binary orbital period, possibly due to partial eclipses. This makes M31N 2013-01b the first nova in M31 with an orbital period determined. The short period also makes this nova one of the few known below the 2-3 hr orbital period gap. All of the observed characteristics strongly indicate that M31N 2013-01b harbors a massive WD and a very low mass companion, consistent with being a nova belonging to the disk population of the Andromeda galaxy.