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 {\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 $19.7_{-1.0}^{+1.3}$~keV in outburst and $36.2_{-3.2}^{+3.5}$~keV in quiescence. The significant change of the maximum temperature is considered to reflect an accretion-induced decrease of the inner-disk radius $R_{\mathrm{i}\mathrm{n}}$, where accreting gas is captured by the magnetosphere. Assuming this radius scales as $R_{\rm in} \propto \dot{M}^{-2/7}$where$\dot{M}$isthemassaccretionrate,weobtain$R_{\rm in} = 1.9 ^{+0.4}{-0.2}~R{\rm WD}$and$R_{\rm in} = 7.4^{+2.1}{-1.2}~R{\rm WD}$inoutburstandquiescencerespectively,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 $M_{\mathrm{W}\mathrm{D}}=0.87\pm 0.08M_{\odot }$ 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 $M_{\mathrm{W}\mathrm{D}}$ and $B$ of white dwarfs by using large changes in $\dot{M}$.