Study of Fe Kα lines in nonmagnetic cataclysmic variables using Chandra HEG data

Abstract

Results from a study of Fe Kα emission lines for a sample of six nonmagnetic cataclysmic variables (CVs) using high-resolution X-ray data from the Chandra High Energy Transmission Grating (HETG) are presented. Two of the sources, SS Cyg and U Gem, are observed in both quiescent and outburst states, whereas V603 Aql, V426 Oph, WX Hyi, and SU UMa are observed only in quiescence. The fluorescent Fe line is prominent in V603 Aql, V426 Oph, and SS Cyg during quiescence, indicating the presence of a conspicuous reflection component in these sources. The observed equivalent width of the fluorescent Fe line is consistent with reflection from a white dwarf surface that subtends a 2π solid angle at the X-ray source. During the outburst in SS Cyg, the fluorescent line is redshifted by about 2300 km s⁻¹. The Fe XXV triplet at 6.7 keV is found to be dominant in all sources. The value of the G ratio derived from the Fe XXV triplet indicates that the plasma is in collisional ionization equilibrium during the quiescent state. The Fe XXV line is significantly broadened in U Gem and SS Cyg during the outbursts compared to quiescence, indicating the presence of high-velocity material near the white dwarf during the outburst. The ratio of Fe XXVI/XXV indicates a higher ionization temperature during quiescence than in outburst in U Gem and SS Cyg.