A millisecond pulsar candidate in a 21-hr orbit: 3FGL J0212.1+5320
Abstract
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 0.869550.00015 d orbital period and a F6-type companion star at an estimated distance of D=1.10.2 kpc, with a radial velocity curve semi-amplitude K2=214.15.0 km s-1 and a projected rotational velocity of Vsin(i)=73.21.6 km s-1. We find a hard X-ray source at the same location with a 0.5-10 keV luminosity LX=2.6×1032 (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 q0.26 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 hr). Based on the light curve peak-to-peak amplitude (=0.19 mag), we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical (M11.3-1.6 M). Finally, we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs.