Spectroscopic Observations of the WZ Sge-Type Dwarf Nova GW Librae during the 2007 Superoutburst

TL;DR

This study presents detailed spectroscopic observations of the dwarf nova GW Librae during its 2007 superoutburst, revealing dramatic spectral changes and insights into the accretion disk's behavior and system inclination.

Contribution

First comprehensive spectroscopic monitoring of GW Lib during superoutburst, highlighting spectral evolution and the role of photoionized regions in the accretion disk.

Findings

Spectral features changed dramatically during the outburst phases.

Hα emission remained stable over orbital phases, indicating low inclination.

Comparison with WZ Sge suggests similar accretion disk phenomena.

Abstract

We carried out an international spectroscopic observation campaign of the dwarf nova GW Librae (GW Lib) during the 2007 superoutburst. Our observation period covered the rising phase of the superoutburst, maximum, slowly decaying phase (plateau), and long fading tail after the rapid decline from the plateau. The spectral features dramatically changed during the observations. In the rising phase, only absorption lines of H$\alpha$, H$\beta$, and H$\gamma$ were present. Around the maximum, the spectrum showed singly-peaked emission lines of H$\alpha$, He I 5876, He I 6678, He II 4686, and C III/N III as well as absorption lines of Balmer components and He I. These emission lines significantly weakened in the latter part of the plateau phase. In the fading tail, all the Balmer lines and He I 6678 were in emission, as observed in quiescence. We find that the center of the H$\alpha$ emission component was mostly stable over the whole orbital phase, being consistent with the low inclination of the system. Comparing with the observational results of WZ Sge during the 2001 superoutburst, the same type of stars as GW Lib seen with a high inclination angle, we interpret that the change of the H$\alpha$ profile before the fading tail phase is attributed to a photoionized region formed at the outer edge of the accretion disk, irradiated from the white dwarf and inner disk.