Nature of 4FGL J2249.4+6229: Evidence for a redback system with a cool companion and low X-ray and $\gamma$-ray luminosities

TL;DR

This study identifies 4FGL J2249.4+6229 as a promising redback binary system with a cool M-type companion, exhibiting low X-ray and gamma-ray luminosities, based on multi-wavelength observations and light curve analysis.

Contribution

The paper provides the first detailed multi-wavelength characterization of 4FGL J2249.4+6229, confirming it as a redback candidate with unique low luminosity features.

Findings

Confirmed orbital period of approximately 5.6 hours.

Optical and X-ray data suggest a redback system with a cool M-type companion.

The source has the lowest X-ray and gamma-ray luminosities among known redbacks.

Abstract

We report the identification of the likely X-ray and optical counterpart to the unassociated Fermi source 4FGL J2249.4+6229. To clarify its nature, we investigate the X-ray data from Swift/XRT and SRG/eROSITA as well as photometric data from optical catalogues and archival spectroscopic data from the Gemini-North telescope. Using Zwicky Transient Facility data spanning over 6.6 yr, we confirmed a period of $\approx$5.6 h likely associated with the orbital motion in a binary system. The folded light curves have a smooth sinusoidal shape with two peaks per period and the amplitude of $\approx$0.2 mag. The X-ray spectra of the source are well fitted by an absorbed power law with the photon index of $\approx$2.0 and unabsorbed flux of $\approx$1.4$\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$. All these together with the X-ray to optical flux ratio of $\sim$0.2 implies that 4FGL J2249.4+6229 is a promising redback candidate. Fitting the optical light curves with the direct heating model, we obtained the companion mass of $\approx$0.5 M$_\odot$ and temperature of $\approx$3600 K implying an M-type star. This places it among the coldest and most massive companions known in redback systems.Optical spectra confirms the M-type star and shows the broad asymmetric H$\alpha$ emission line. For the distance of 500--550 pc derived from the optical data, the source can be the redback with the lowest X-ray and $\gamma$-ray luminosities.