Coronal X-ray emission from the short-period eclipsing binary XY UMa.

Abstract

XY UMa, a close binary showing photospheric and chromospheric activity, has been observed with Exosat through 1.26 orbital cycles, with supporting optical photometry. The G2-5 V primary star is found to be responsible for the observed soft X-ray flux, which shows a clear primary eclipse. The X-ray-emitting coronal plasma is modeled to have two components: an enhanced component occupying a region which faces the secondary star, and a quiescent component on the far side of the primary which possibly encircles the star. Supporting optical data show photospheric activity underlying the enhanced coronal component. The height of the X-ray corona is less than 0.75 solar radius, with no evidence for any very extended component. Spectral analysis of the integrated emission shows that T is less than 1.3 x 10 to the 7th K, EM = 1.1-3.3 x 10 to the 53rd/cu cm. The X-ray luminosity is calculated to be 3.4 x 10 to the 30th - 1.1 x 10 to the 31st erg/sec.