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

Abstract

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 years, we find a period of $\approx$7.5 h. The folded light curve has a smooth sinusoidal shape with the peak-to-peak amplitude of $\approx$0.4 mag. The spectral fit to the optical spectral energy distribution of the counterpart candidate gives the star radius of 0.5$\pm$0.1$R_{\odot }$ and temperature of 5500$\pm$300 K implying a G2--G9-type star. Its X-ray spectrum is well fitted by an absorbed power law with the photon index of 1.0$\pm$0.3 and unabsorbed flux of $\approx 2\times 10^{-13}$ 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.

Accepted for publication in MNRAS, 6 pages, 6 figures

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Cited by in corpus (3)

• Nature of 4FGL J1838.2+3223: a flaring `spider' pulsar candidate
• 4FGL J1544.2$-$2554: a new spider pulsar candidate
• Nature of 4FGL J2249.4+6229: Evidence for a redback system with a cool companion and low X-ray and $\gamma$-ray luminosities