NuSTAR observations of the Dwarf Nova GK Persei in 2015: comparison between outburst and quiescent phases

TL;DR

This study uses NuSTAR X-ray observations to compare the outburst and quiescent phases of the dwarf nova GK Persei, revealing temperature and flux changes that inform the white dwarf's magnetic and accretion properties.

Contribution

First detailed comparison of GK Persei's outburst and quiescent X-ray spectra, deriving the white dwarf's mass and magnetic field from temperature variations.

Findings

Flux during outburst is 26 times higher than in quiescence.

Post-shock temperature is lower during outburst (19.2 keV) than in quiescence (38.5 keV).

White dwarf mass estimated at 0.90 solar masses.

Abstract

We report on NuSTAR observations of the Intermediate Polar GK Persei which also behaves as a Dwarf Nova. It exhibited a Dwarf Nova outburst in 2015 March-April. The object was observed in 3-79 keV X-rays with NuSTAR, once at the outburst peak, and again in 2015 September during quiescence. The 5-50 keV flux during the outburst was 26 times higher than that during the quiescence. With a multi-temperature emission model and a reflection model, we derived the post-shock temperature as 19.2 +/- 0.7 keV in the outburst, and 38.5 +4.1/-3.6 keV in the quiescence. This temperature difference is considered to reflect changes in the radius at which the accreting matter, forming an accretion disk, is captured by the magnetosphere of the white dwarf (WD). Assuming that this radius scales as the power of -2/7 of the mass accretion rate, and utilizing the two temperature measurements, as well as the standard mass-radius relation of WDs, we determined the WD mass in GK Persei as 0.90 +/- 0.06 solar masses. The magnetic field is estimated as 4*10^5 G.