An estimation of the white dwarf mass in the Dwarf Nova GK Persei with NuSTAR observations of two states

We report on X-ray observations of the Dwarf Nova GK Persei performed by NuSTAR in 2015. GK Persei, behaving also as an Intermediate Polar, exhibited a Dwarf Nova outburst in 2015 March-April. The object was observed with NuSTAR during the outburst state, and again in a quiescent state wherein the 15-50 keV flux was 33 times lower. Using a multitemperature plasma emission and reflection model, the highest plasma temperature in the accretion column was measured as 19.7^{+1.3}_{-1.0} keV in outburst and 36.2^{+3.5}_{-3.2} keV in quiescence. The significant change of the maximum temperature is considered to reflect an accretion-induced decrease of the inner-disc radius R_in, where accreting gas is captured by the magnetosphere. Assuming this radius scales as R_in ∝ \dot{M}^{-2/7}, where \dot{M} is the mass accretion rate, we obtain R_in = 1.9 ^{+0.4}_{-0.2} R_WD and R_in = 7.4^{+2.1}_{-1.2} R_WD in outburst and quiescence, respectively, where R_WD is the white-dwarf (WD) radius of this system. Utilizing the measured temperatures and fluxes, as well as the standard mass-radius relation of WDs, we estimate the WD mass as M_WD = 0.87 ± 0.08 M_⊙ including typical systematic uncertainties by 7 per cent. The surface magnetic field is also measured as B ∼ 5 × 10⁵ G. These results exemplify a new X-ray method of estimating M_WD and B of WDs by using large changes in \dot{M}.