4FGL J1318.2+6754: A Long Orbital-period Redback Candidate

By combining the Fermi catalog and the single-lined spectroscopic binaries of Gaia Data Release 3 (DR3), we report a new candidate redback millisecond pulsar binary, which is recommended as a target for radio telescopes to detect pulsed signals. Using TESS data in the T band spanning over 4 yr, we find a period of similar to 4.13 days, which is consistent with the radial velocity solution of the Gaia DR3 and optical spectroscopy from the LAMOST telescope. The light curve in the T band reveals periodic modulation of low amplitude that is a characteristic feature of the heating of the photosphere of a nondegenerate low-mass stellar companion by the pulsar wind. The nondegenerate companion is a bright (G = 11.96), slightly metal-poor ([Fe/H] = - 0.55), G-type main-sequence star (T eff = 5891 K; logg=4.07; R = 1.356R circle dot). This orbital period of 4.13 days would be the longest one for known redback candidates, which could hardly be understood for its formation in the standard recycling scenario due to its large Roche-lobe radius (>> the radius of the G-type main-sequence star). It is proposed here that this candidate redback millisecond pulsar binary could be created via accretion-induced collapse of an oxygen-neon-magnesium white dwarf.