# Multi-mission observations of the old nova GK Per during the 2015 outburst

## TL;DR

This study presents multi-mission X-ray and UV observations of GK Per during its 2015 outburst, revealing changes in plasma temperature, accretion dynamics, and emission features, advancing understanding of magnetic accretion in old novae.

## Contribution

It provides the first detailed multi-wavelength analysis of GK Per during an outburst, showing how plasma temperature and accretion processes evolve in an intermediate polar system.

## Key findings

- Plasma temperature decreased from 26 to 16.2 keV during outburst.
- Detected high-amplitude WD spin modulation of hard X-rays.
- Observed emission lines indicating lower temperature plasma at the magnetospheric boundary.

## Abstract

GK Per, a classical nova of 1901, is thought to undergo variable mass accretion on to a magnetized white dwarf (WD) in an intermediate polar system (IP). We organized a multi-mission observational campaign in the X-ray and ultraviolet (UV) energy ranges during its dwarf nova (DN) outburst in 2015 March-April. Comparing data from quiescence and near outburst, we have found that the maximum plasma temperature decreased from about 26 to 16.2+/-0.4 keV. This is consistent with the previously proposed scenario of increase in mass accretion rate while the inner radius of the magnetically disrupted accretion disc shrinks, thereby lowering the shock temperature. A NuSTAR observation also revealed a high-amplitude WD spin modulation of the very hard X-rays with a single-peaked profile, suggesting an obscuration of the lower accretion pole and an extended shock region on the WD surface. The X-ray spectrum of GK Per measured with the Swift X-Ray Telescope varied on time-scales of days and also showed a gradual increase of the soft X-ray flux below 2 keV, accompanied by a decrease of the hard flux above 2 keV. In the Chandra observation with the High Energy Transmission Gratings, we detected prominent emission lines, especially of Ne, Mg and Si, where the ratios of H-like to He-like transition for each element indicate a much lower temperature than the underlying continuum. We suggest that the X-ray emission in the 0.8-2 keV range originates from the magnetospheric boundary.