SRGe J194401.8+284452 -- an X-ray Cataclysmic Variable in the Field of the Gamma-Ray Source 4FGL J1943.9+2841

Abstract

SRGe J194401.8+284452 is the brightest point-like X-ray object within the position uncertainty ellipse of an unidentified $\gamma$-ray source 4FGL J1943.9+2841. We performed multi-wavelength spectral and photometric studies to determine its nature and possible association with the $\gamma$-ray source. We firmly established its optical counterpart with the Gaia based distance of about 415 pc. Our data show that the object is a cataclysmic variable with an orbital period of about 1.5 hours. SRGe J194401.8+284452 exhibits fast spontaneous transitions between the high and low luminosity states simultaneously in the optical and X-rays, remaining relatively stable between the transitions on scales of several months/years. This can be caused by an order of magnitude changes in the accretion rate. The brightness of the source is about 17 mag and 20 mag in the optical range and $5\times 10^{-12}$ and $5\times 10^{-13}$ erg/cm${}^2$/s in the $0.3-10$ keV range in the high and low states, respectively. We constrained the mass of the white dwarf ($0.3-0.9$ $M_{\odot }$) and its temperature in the low state (14750 $\pm$ 1250 K), the mass of the donor star ($\le$ 0.08 $\pm$ 0.01 $M_{\odot }$). In the low state, we detected regular optical pulsations with an amplitude of 0.2 mag and a period of 8 min. They are likely associated with the spin of the white dwarf, rather than with its non-radial pulsations. In the high state, the object demonstrates only stochastic optical brightness variations on time scales of $1-15$~minutes with amplitudes of $0.2-0.6$~mag. We conclude, that SRGe J194401.8+284452 can be classified as an intermediate polar, and its association with the $\gamma$-ray source is very unlikely.

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