tokens: of the accretion disk (White Swank 1982 ). The spectral evolution during dips depends on the ionization state of the absorbing material (Diaz Trigo Boirin 2016 ). In the case of XMM0042, we do not detect any significant spectral variation possibly due to the low statistics. Van Paradijs McClintock ( 1994 ) study the optical emission from LMXB disks; if X-rays from the central source are reprocessed by the accretion disk, this implies that the optical luminosity in the V-band M V scales with the X-ray luminosity and size of the accretion disk. We considered the relation from Van Paradijs McClintock ( 1994 ) between L X , the orbital period, and M V , taking into account erg s −1 (following Van Paradijs McClintock 1994 ), our orbital period of 4.01 hr, and our mean luminosity erg s −1 :obtaining (1 σ error) for the optical counterpart. Among the 17 positionally consistent HST candidate optical/UV counterparts reported in Yukita et al. ( 2017 ), all but one are very similar to the surrounding stellar populations. The remaining source shows an excess in the blue band, making it an interesting potential counterpart of the accretion disk of an LMXB. From the blue apparent magnitude of the counterpart m B = 24.78, standard extinction toward M31, and Equation (4) from Barnard et al. ( 2012 ), we obtain an absolute visual magnitude . This is consistent (within 2 σ ) with the one derived from the X-ray luminosity and period of XMM0042, thus making it the likely optical counterpart. The X-ray spectrum of XMM0042 (as seen by Yukita et al. 2017 and in this work) is quite hard, with and . This is consistent with what is seen in dipping LMXBs in our Galaxy, where a wide range of spectral properties is displayed, with photon indexes varying from 0.4 to 2 and cutoff energies from 3.5 to 80 keV (Diaz Trigo et al. 2006 ). The most luminous Galactic dipper, X 1624–490, has an X-ray luminosity in the 1–30 keV energy band of erg s −1 (in the 0.5–50 keV energy range—Galactic dippers reach persistent luminosities only ∼3 times lower than XMM0042; Balucinska-Church et al. 2000 ; Iaria et al. 2007 ) and a much softer spectrum. The period, optical counterpart, and the spectrum of XMM0042 are reminiscent of a dipping LMXB system. The mean luminosity we found through our XMM-Newton analysis, ranging from 0.8 to erg s −1 , makes XMM0042 the most luminous dipper to date. Trudolyubov et al. ( 2002 ) found the first dipping source of M31, located in Bo 158. The period of the dipping behavior was 2.78 hr with a flux modulation of ∼83%. A study of the other XMM-Newton , Chandra , and ROSAT observations (Trudolyubov Priedhorsky 2004 ) revealed the amplitude of the dips to be anti-correlated with the source luminosity ( erg s −1 ), disappearing at high luminosities. The source also showed hour-timescale and month-timescale luminosity variations. This source has a rather hard spectrum, but it has never been observed by NuSTAR , so we have no information about the cutoff energy or
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