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

The remarkable old nova and an intermediate polar (IP) - GK Per was observed with Swift, the Chandra HETG and NuSTAR during its recent dwarf nova (DN) outburst in March - April 2015. Monitoring the outburst, we noticed several processes occurring on different time scales, such as: the slow evolution of the very soft X-ray emission (below 0.6 keV) during the first two weeks of the outburst and the very fast saturation of the X-ray flux above 1 keV. The Swift UVOT lights curves also showed different behaviour, depending on the filter. The broad band X-ray spectra revealed the presence of at least three different emitting sources. The white dwarf (WD) spin was observed even in the very hard X-ray range of NuSTAR, indicating that the modulation is not due to absorption, in contrast to a typical IP. It is also supported by the similarity of the on-pulse and off-pulse X-ray spectra. We propose that the scenario when the inner accretion disk pushed towards the WD by the increased accretion obscures the lower WD pole can work also for GK Per.