ChandraLow Energy Transmission Grating Spectrum of SS Cygni in Outburst

Abstract

We have fitted the Chandra Low Energy Transmission Grating spectrum of SS Cygni in outburst with a single temperature blackbody suffering the photoelectric opacity of a neutral column density and the scattering opacity of an outflowing wind. We find that this simple model is capable of reproducing the essential features of the observed spectrum with the blackbody temperature T_bl = 250+/-50 kK, hydrogen column density N_H = 5.0^{+2.9}_{-1.5} x 10^{19} cm^{-2}, fractional emitting area f = 5.6^{+60}_{-4.5} x 10^{-3}, boundary layer luminosity L_bl = 5^{+18}_{-3} x 10^{33} erg/s, wind velocity v ~ 2500 km/s, wind mass-loss rate Mdot_w ~ 1.1 x 10^{16} g/s, and arbitrary values of the wind ionization fractions of 20 ions of O, Ne, Mg, Si, S, and Fe. Given that in outburst the accretion disk luminosity L_disk ~ 1 x 10^{35} erg/s, L_bl/L_disk ~ 0.05^{+0.18}_{-0.03}, which can be explained if the white dwarf (or an equatorial belt thereon) is rotating with an angular velocity Omega_wd ~ 0.7^{+0.1}_{-0.2} Hz, hence V_rot sin i ~ 2300 km/s.