lDISCOVERY OF A REDBACK MILLISECOND PULSAR CANDIDATE: 3FGL J0212.1+5320

We present a multiwavelength study of the unidentified Fermi object, 3FGL J0212.1+5320. Within the 95% error ellipse, Chandra detects a bright X-ray source (i.e., F0.5-7 (keV) = 1.4 x 10(-12) erg cm(-2) s(-1)) that has a low-mass optical counterpart (M less than or similar to 0.4M circle dot and T similar to 6000 K). A clear ellipsoidal modulation is shown in optical/infrared at 20.87 hr. The gamma-ray properties of 3FGL J0212.1+5320 are all consistent with that of a millisecond pulsar (MSP), suggesting that it is gamma-ray redback (RB) MSP binary with a low-mass companion filling greater than or similar to 64% of the Roche lobe. If confirmed, it will be an RB binary with one of the longest orbital periods known. Spectroscopic data taken in 2015 from the Lijiang observatory show no evidence of strong emission lines, revealing that the accretion is currently inactive (the rotation-powered pulsar state). This is consistent with the low X-ray luminosities (L-X approximate to 10(32) erg s(-1)) and the possible X-ray modulation seen by Chandra and Swift. Considering that the X-ray luminosity and the high X-ray-to-gamma-ray flux ratio (8%) are both comparable to those of the two known gamma-ray transitional MSPs, we suspect that 3FGL J0212.1+5320 could be a potential target to search for future transition to the accretion active state.