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 0.86955$\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${}_{-0.03}^{+0.02}$ 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.

Accepted for publication in MNRAS; 9 pages, 8 figures, minor changes after referee suggestions

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• Properties of the redback millisecond pulsar binary 3FGL J0212.1+5320
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• Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610
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• Discovery of the variable optical counterpart of the redback pulsar PSR J2055+1545
• Periodicity search for Pulsar Binaries with TESS
• High Energy Radiation from Spider Pulsars