A double degenerate ultra-compact binary in the direction of the LMC

Abstract

During four all-sky surveys (eRASS1--4) the soft X-ray instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray source in the direction of the Large Magellanic Cloud (LMC). We arranged follow-up observations in the X-ray and optical wavelengths and further searched in archival observations to reveal the nature of the object. Using X-ray observations with we investigated the temporal and spectral behaviour of the source. We discover pulsations at 374\,s with a pulse profile consistent with 100<!PCT!> modulation. We identify two other periodicities in the data, which we establish as aliases due to the sampling of the light curve. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The timing and spectral characteristics of the source are consistent with a double degenerate ultra-compact binary system in the foreground of the LMC. belongs to a rare class of AM CVns, which are important to study in the context of progenitors of SN\,Ia and for persistent gravitational wave detection. We identify as a new double degenerate ultra-compact binary located in the foreground of the LMC.