We report on X-ray observations of the Dwarf Nova GK Persei performed by { \it 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 { \sl NuSTAR} during the outburst state, and again in a quiescent state wherein the 15--50 keV flux was 33 times lower. Using a multi-temperature 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-disk radius $R_{ \rm in}$, where accreting gas is captured by the magnetosphere. Assuming this radius scales as $R_{ \rm in} \propto \dot{ M}^{ -2/7}$ where $\dot{ M}$ is the mass accretion rate, we obtain $R_{ \rm in} = 1.9 ^{ +0.4}_{ -0.2}~R_{ \rm WD}$ and $R_{ \rm in} = 7.4^{ +2.1}_{ -1.2}~R_{ \rm WD}$ in outburst and quiescence respectively, where $R_{ \rm WD}$ is the white-dwarf radius of this system. Utilising the measured temperatures and fluxes, as well as the standard mass-radius relation of white dwarfs, we estimate the white-dwarf mass as $M_{ \rm WD} = 0.87~\pm~0.08~M_{ \rm \odot}$ including typical systematic uncertainties by 7%. The surface magnetic field is also measured as $B \sim 5 \times 10^{ 5}$~G. These results exemplify a new X-ray method of estimating $M_{ \rm WD}$ and $B$ of white dwarfs by using large changes in $\dot{ M}$.
Categories: astro-ph.SR astro-ph.HE
