2017MNRAS.465.4602L

We present the discovery of a variable optical counterpart to the unidentified gamma-ray source 3FGL J0212.1+5320 and argue that this is a new compact binary millisecond pulsar (MSP) candidate. We show 3FGL J0212.1+5320 hosts a semidetached binary with a 0.869 55 ± 0.000 15 d orbital period and an F6-type companion star at an estimated distance of D = 1.1 ± 0.2 kpc, with a radial velocity curve semi-amplitude K₂ = 214.1 ± 5.0 km/s and a projected rotational velocity of V sin (i) = 73.2 ± 1.6 km/s. We find a hard X-ray source at the same location with a 0.5-10 keV luminosity L_X = 2.6 x 10³² (D/1.1 kpc)² erg s^–1, which strengthens the MSP identification. Our results imply a mass ratio q = M₂/M₁ = 0.26^+0.02_–0.03 if the companion star fills its Roche lobe, and q >= 0.26 in any case. This classifies 3FGL J0212.1+5320 as a 'redback' binary MSP; if its MSP nature is confirmed, this will be the brightest compact binary MSP in the optical band (r'' ≃ 14.3 mag) and will have the longest orbital period among Galactic field systems (nearly 21 h). Based on the light curve peak-to-peak amplitude (Δr = 0.19 mag), we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical (M₁ ≃ 1.3-1.6 M_☉). Finally, we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs.