Modeling the Heating and Cooling of WZ Sagittae Following the 2001 July Outburst

Abstract

Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope Space Telescope Imaging Spectograph spectra of the dwarf nova WZ Sge, obtained during and following the early superoutburst of 2001 July over a time span of 20 months, monitor changes in the components of the system during its different phases. The synthetic spectral fits to the data indicate a cooling in response to the outburst of about 12,000 K, from ≈28,000 down to ≈16,000 K. The cooling timescale τ (of the white dwarf temperature excess) is on the order of ≈100 days in the early phase of the cooling period, and increases to τ ≈ 850 days toward the end of the second year following the outburst. In the present work, we numerically model the accretional heating and subsequent cooling of the accreting white dwarf in WZ Sge. The best compressional heating model fit is obtained for a 1.2 M☉ white dwarf accreting at a rate of 9 × 10-9 M☉ yr-1 for 52 days. However, if one assumes a lower mass accretion rate or a lower white dwarf mass, then compressional heating alone cannot account for the observed temperature decline, and other sources of heating have to be included to increase the temperature of the model to the observed value. We quantitatively check the effect of boundary layer irradiation as such an additional source.