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 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 19.7-1.0+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-disc radius Rin, where accreting gas is captured by the magnetosphere. Assuming this radius scales as Rin ∝ M˙ -2/7, where M˙ is themass accretion rate, we obtain Rin = 1.9-0.2+0.4 RWD and Rin = 7.4-1.2+2.1 RWD in outburst and quiescence, respectively, where RWD is the white-dwarf (WD) radius of this system. Utilizing the measured temperatures and fluxes, as well as the standard mass-radius relation ofWDs, we estimate the WD mass as MWD = 0.87 ± 0.08M⊙ including typical systematic uncertainties by 7 per cent. The surface magnetic field is also measured as B ~ 5 × 105 G. These results exemplify a new X-ray method of estimating MWD and B of WDs by using large changes in M.