A new redback pulsar candidate 4FGL J2054.2+6904

The Fermi catalogue contains about 2000 unassociated gamma-ray sources. Some of them were recently identified as pulsars, including so-called redbacks and black widows, which are millisecond pulsars in tight binary systems with non- and partially-degenerate low-mass stellar companions irradiated by the pulsar wind. We study a likely optical and X-ray counterpart of the Fermi source 4FGL J2054.2+6904 proposed earlier as a pulsar candidate. We use archival optical data as well as Swift/XRT and SRG/eROSITA X-ray data to clarify its nature. Using Zwicky Transient Facility data in g and r bands spanning over 4.7 yr, we find a period of approximate to 7.5 h. The folded light curve has a smooth sinusoidal shape with the peak-to-peak amplitude of approximate to 0.4 mag. The spectral fit to the optical spectral energy distribution of the counterpart candidate gives the star radius of 0.5 +/- 0.1 R-circle dot and temperature of 5500 +/- 300 K implying a G2-G9-type star. Its X-ray spectrum is well fitted by an absorbed power law with the photon index of 1.0 +/- 0.3 and unabsorbed flux of approximate to 2 x10(-13) erg s(-1) cm(-2). All the properties of 4FGL J2054.2+6904 and its presumed counterpart suggest that it is a member of the redback family.

Automated summary

By studying the data, we find that the Fermi source 4FGL J2054.2+6904 is likely a member of the redback family, which consists of millisecond pulsars in tight binary systems with low-mass stellar companions. The optical spectral energy distribution suggests that it is a G2-G9-type star and the X-ray spectrum is well fitted by an absorbed power law.