2018MNRAS.474.1564W

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 x 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.