Chandra HETG line spectroscopy of the Non-magnetic Cataclysmic Variable SS Cyg

TL;DR

This paper analyzes Chandra HETG spectra of SS Cygni in different states, revealing plasma dynamics and temperature distribution in the accretion boundary layer through emission line profiles and ratios.

Contribution

It provides detailed spectral analysis of SS Cyg in quiescence and outburst, linking emission line characteristics to plasma motion and temperature in the accretion environment.

Findings

Line widths indicate plasma associated with the Keplerian disk during outburst.

Narrower lines in quiescence suggest emission from the boundary layer.

Temperature distribution is consistent with other dwarf novae.

Abstract

We present {\it Chandra} HETG observations of SS Cygni in quiescence and outburst. The spectra are characterized by He-like and H-like Ka emission lines from O to Fe, as well as L-shell emission lines from Fe. In quiescence, the spectra are dominated by the H-like Ka lines, whereas in outburst the He-like lines are as intense as the H-like lines. In outburst, the H-like Ka lines from O to Si are broad, with widths of 4--14 eV in Gaussian $\sigma$ (1800--2300$ {\rm km s^{-1}}$). The large line widths, together with line profiles, indicate that the line-emitting plasma is associated with the Keplerian disk and still retains the azimuthal bulk motion. In quiescence, the emission lines are narrower, with a Gaussian $\sigma$ of 1--3 eV (420--620$ {\rm km s^{-1}}$). A slightly larger velocity for lighter elements suggests that the lines in quiescence are emitted from an ionizing plasma at the entrance of the boundary layer, where the bulk motion of the optically thick accretion disk is converted into heat due to friction. Using the line intensity ratio of He-like and H-like Ka lines for each element, we have also investigated the temperature distribution in the boundary layer both in quiescence and outburst. The distribution of SS Cyg is found to be consistent with other dwarf novae investigated systematically with {\it ASCA} data.