Optical and Physical Characteristics of the FL Psc Dwarf Nova before and after the 2023 Superoutburst

The paper studies the characteristics of optical emission formation and determines the fundamental parameters of the FL Psc dwarf nova for the epochs before and after its superoutburst in 2023. Moderate-resolution spectral sets were obtained in 2021, 2023 and 2024 with the BTA-6 telescope of the SAO RAS. Observations in all epochs are dominated by the continuous spectrum of the white dwarf (WD) with H I, He I, He II, C II, Mg I, Ca I, Ca II, Fe I emissions and Fe II from the accretion disk and a bright spot at its outer radius. Changes in emission intensities and their Doppler half-widths showed that during the transition from the relaxation stage to quiescence, the zone of maximum radiation shifted to the periphery of the disk, and the temperature in it decreased to $$T_{e}<8000$$ K. An analysis of the observed spectra for three epochs was performed using model flux (i.e. in flux units) and normalized spectra of the WD and a modified method for determining the parameters of their atmospheres. As a result, the following estimates of the accretor’s temperature were obtained for a single surface gravity value $$\log g=8.26\pm 0.13$$ : $$T_{\textrm{eff}}=17\,700\pm 1400$$ K, $$26\,700\pm 1900$$ K and $$19\,600\pm 1300$$ K in 2021, 2023 and 2024, respectively. It is shown that the observed systematic variations in the brightness of FL Psc after the end of the outburst are correctly explained by changes in the WD temperature, taking into account the additional contribution of emission lines. Based on the parameters found for the accretor’s atmosphere, the complete set of fundamental parameters of the system is refined. Its Doppler tomography showed that 1.3 years after the outburst, the accretion disk remained optically thick in the H I lines, but became optically thin in the lines of other elements. Analysis of the FL Psc maps suggests that the H I, He I and He II emissions are formed predominantly under thermalization conditions, while the Ca II and Fe II emissions are formed under the influence of the fluorescence effects of soft ultraviolet radiation from the WD in the accretion disk and the surrounding gaseous envelope.