X-ray and UV emission of the ultrashort-period, low-mass eclipsing binary system BX Trianguli

Abstract

Context.Close binary systems provide an excellent tool for determining stellar parameters such as radii and masses with a high degree of precision. Due to the high rotational velocities, most of these systems exhibit strong signs of magnetic activity, postulated to be the underlying reason for radius inflation in many of the components.Aims.We extend the sample of low-mass binary systems with well-known X-ray properties.Methods.We analyze data from a singularXMM-Newtonpointing of the close, low-mass eclipsing binary system BX Tri. The UV light curve was modeled with the eclipsing binary modeling toolPHOEBEand data acquired with the EPIC cameras was analyzed to search for hints of orbital modulation.Results.We find clear evidence of orbital modulation in the UV light curve and show thatPHOEBEis fully capable of modeling data within this wavelength range. Comparison to a theoretical flux prediction based onPHOENIXmodels shows that the majority of UV emission is of photospheric origin. While the X-ray light curve does exhibit strong variations, the signal-to-noise ratio of the observation is insufficient for a clear detection of signs of orbital modulation. There is evidence of a Neupert-like correlation between UV and X-ray data.