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

Abstract

Abstract 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.