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

Abstract

Abstract We carried out an international spectroscopic observation campaign of the dwarf nova GW Librae (GW Lib) during its 2007 superoutburst. Our observation period covered the rising phase of the superoutburst, maximum, slowly decaying phase (plateau), and long fading tail phase 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 the Balmer components and HeI. These emission lines significantly weakened during the latter part of the plateau phase. In the fading tail phase, all of 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. Compared with the observational results of WZ Sagittae during its 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 can be attributed to a photoionized region formed at the outer edge of the accretion disk, irradiated from the white dwarf and inner disk.