An Estimation of the White Dwarf Mass in the Dwarf Nova GK Persei with NuSTAR Observations of Two States

Abstract

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 $R_{\mathrm{in}}$, where accreting gas is captured by the magnetosphere. Assuming this radius scales as $R_{\mathrm{in}}\propto {\dot{M}}^{-2/7}$ where $\dot{M}$ is the mass accretion rate, we obtain $R_{\mathrm{in}}=1.9_{-0.2}^{+0.4}{R}_{\mathrm{WD}}$ and $R_{\mathrm{in}}=7.4_{-1.2}^{+2.1}{R}_{\mathrm{WD}}$ in outburst and quiescence respectively, where $R_{\mathrm{WD}}$ 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 $M_{\mathrm{WD}}=0.87\pm 0.08M_{\odot }$ 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 $M_{\mathrm{WD}}$ and $B$ of white dwarfs by using large changes in $\dot{M}$.

8 pages, 4 figures, accepted for publication in MNRAS