We present the discovery of a variable optical counterpart to the unidentified gamma-ray source 3FGL J0212.1+5320 and argue this is a new compact binary millisecond pulsar ( MSP ) candidate .
We show 3FGL J0212.1+5320 hosts a semi-detached binary with a 0.86955 \pm 0.00015 d orbital period and a F6-type companion star at an estimated distance of D=1.1 \pm 0.2 kpc , with a radial velocity curve semi-amplitude K _ { 2 } =214.1 \pm 5.0 km s ^ { -1 } and a projected rotational velocity of Vsin ( i ) =73.2 \pm 1.6 km s ^ { -1 } .
We find a hard X-ray source at the same location with a 0.5–10 keV luminosity L _ { \mathrm { X } } =2.6 \times 10 ^ { 32 } ( D/1.1 kpc ) ^ { 2 } erg s ^ { -1 } , which strengthens the MSP identification .
Our results imply a mass ratio q=M _ { 2 } /M _ { 1 } =0.26 ^ { +0.02 } _ { -0.03 } if the companion star fills its Roche lobe , and q \gtrsim 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 ’ \simeq 14.3 mag ) and will have the longest orbital period among Galactic field systems ( nearly 21 hr ) .
Based on the light curve peak-to-peak amplitude ( \Delta r=0.19 mag ) , we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical ( M _ { 1 } \simeq 1.3–1.6 M _ { \odot } ) .
Finally , we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs .