Discovery of the redback pulsar PSR J2053+6905 and multi-epoch optical observations of the system
Using the Green Bank Telescope, we discovered the 1.68-ms binary pulsar PSR J2053+6905. The discovery resulted from a targeted search for radio pulsations from a redback candidate, previously identified through optical and X-ray observations and spatially coincident with the $γ$-ray source 4FGL J2054.2+6904. We report the first results from the pulsar timing, including the spin period, dispersion measure, precisely measured Keplerian binary parameters, and the pulsar mass function. We also found potential evidence of scintillation and long radio eclipses. To constrain the system parameters, we performed multi-epoch time-series photometric observations using the 2.1-m telescope at the Observatorio Astronómico Nacional on the Sierra San Pedro Mártir (Mexico) and the 1.5-m telescope of the Maidanak Astronomical Observatory (Uzbekistan), complemented by time-series optical spectroscopy with the 10.4-m Gran Telescopio Canarias. The optical light curves revealed a pronounced asymmetry, as well as significant long-term variations. From the spectra, we derived a radial velocity curve semi-amplitude $K = 337(4)$ km s$^{-1}$. Combining it with the pulsar mass function, we constrained the ratio of pulsar and companion masses to $M_{\rm p}/M_{\rm c} = 7.05(8)$. By fitting the stable segments of the light curves with a symmetric direct heating model, we derived constraints on the component masses ($M_{\rm p} \approx 2.1\ {\rm M}_\odot$ and $M_{\rm c} \approx 0.3\ {\rm M}_\odot$), the Roche lobe filling factor, and the companion's day- and night-side temperatures. The long-term evolution of the light curve profiles is likely driven by the dissipation or migration of a cold stellar spot.