http://arxiv.org/abs/1711.01727
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 $\dot{M}$.
Y. Wada, T. Yuasa, K. Nakazawa, et. al.
Tue, 7 Nov 17
42/118
Comments: 8 pages, 4 figures, accepted for publication in MNRAS