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 ^ { +0.5 } _ { -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 .