Three X-ray flares near primary eclipse of the RS CVn binary XY UMa

Abstract

We report on an archival X-ray observation of the eclipsing RS CVn binary XY UMa (P-orb approximate to 0.48 d). In two Chandra ACIS observations spanning 200 ks and almost five orbital periods, three flares occurred. We find no evidence for eclipses in the X-ray flux. The flares took place around times of primary eclipse, with one flare occurring shortly (< 0.125 P-orb) after a primary eclipse, and the other two happening shortly (< 0.05 P-orb) before a primary eclipse. Two flares occurred within roughly one orbital period (Delta phi approximate to 1.024 P-orb) of each other. We analyze the light curve and spectra of the system, and investigate coronal length scales during both quiescence and flares, as well as the timing of the flares. We explore the possibility that the flares are orbit-induced by introducing a small orbital eccentricity, which is quite challenging for this close binary.