4FGL J2054.2+6904: a binary “redback” pulsar

The $\gamma$-ray source 4FGL J2054.2+6904 discovered with Fermi was recently classified as a pulsar candidate. Its possible X-ray and optical counterpart was identified with Swift. Using Zwicky survey data we show that the counterpart is variable in the optical with the period of about 7.5 h. The shape of the light curve is almost sinusoidal with the amplitude of $\approx$0.5$^m$. The spectral energy distribution corresponds to the stellar spectrum with the effective temperature of 5820 $\pm$ 410 K. We also firmly detected the source in X-rays using the SRG/eROSITA all sky survey data. Its X-ray spectrum can be described by a power law with the photon index of about 1.0 and unabsorbed flux $\approx$2$\cdot$10$^{-13}$ erg/(s $\cdot$ cm$^2$) in the 0.5–10 keV range. These results show that 4FGL J2054.2+6904 is a promising candidate to a millisecond pulsar in a close binary system of the “redback” type.