A millisecond pulsar candidate in a 21-h 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 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−1 and a projected rotational velocity of V sin (i) = 73.2 ± 1.6 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 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.