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

Abstract

We have carried out multiwavelength spectroscopic and photometric studies of the object SRGe J194401.8+284452 (2RXS J194401.4+284456, XMMSL2 J194402.0+284451, 2SXPS J194401.7+284450), the brightest X-ray source in the position uncertainty ellipse of the unidentified gamma-ray source 4FGL J1943.9+2841, with the goal of determining its nature and the possible association with the gamma-ray source. The object is shown to be a cataclysmic variable with an orbital period of about 1.5 h and clear evidence for the presence of an accretion disk around the white dwarf. It can be classified by its properties as an intermediate polar whose association with the gamma-ray source is unlikely. SRGe J194401.8+284452 exhibits abrupt transitions between its high and low luminosity states simultaneously in the optical and X-ray bands, which remain relatively stable on time scales of several months/years. This may be related to the change in the accretion rate by an order of magnitude. We have obtained constraints on the mass (0.3–0.9 $$M_{\odot}$$) and temperature ($$14\,750\pm 1250$$ K) of the accreting white dwarf in the low state, the mass of the donor star ($${\leq}0.08\pm 0.01\;M_{\odot}$$), and the orbital inclination of the binary system ($$40^{\circ}{-}75^{\circ}$$). In the low state we have detected an 8-min brightness variability in the optical band that is most likely related to the white-dwarf spin and not to the nonradial pulsations. In the high state we have revealed stochastic brightness variations on time scales of 1–15 min with amplitudes of 0.2–$$0.6^{m}$$. SRGe J194401.8+284452 replenishes the small group of intermediate polars with the shortest orbital periods lying below the gap in the period distribution of these systems and exhibiting transitions between the states with high and low accretion rates. The brightness of the source at a level of $$17{-}20^{m}$$ in the 2000–8000 Å range and $$(5{-}50)\times 10^{-13}$$ erg s$${}^{-1}$$ cm$${}^{-2}$$ in the range 0.3–10 keV makes it an interesting object for a detailed study of the physics of such systems.