Abstract
M31 UCXB-1 is one of the brightest X-ray point sources in the bulge of M31, with a peak X-ray luminosity L0.5-10 keV = 2.9-0.2+0.2 × 1038 erg s-1 . Both XMM-Newton and Chandra observations have detected an eclipsing signal with a period of about 465 s from this source, and we note that the periodic signal is detected exclusively during the source's high-luminosity states. This signal probably originates from its orbital motion; therefore, it is an ultra-compact X-ray binary (UCXB) candidate with the highest X-ray luminosity. Our theoretical analyses show that M31 UCXB-1 is in good agreement with the luminosity-orbital period relation (L (2-10 keV)-P (orb)) of the black hole/neutron star-white dwarf (BH/NS-WD) UCXB system. Moreover, our spectral analyses indicate that the primary in M31 UCXB-1 is more likely to be a BH than an NS. The results show that M31 UCXB-1 is a BH-WD system, with the shortest orbital period, the possibly strongest gravitational wave emission, and the most massive WD among the known UCXBs.