GK Per, a classical nova of 1901, is thought to undergo variable mass accretion onto 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 26 to 16 keV. This is consistent with the previously proposed scenario of
increase in mass accretion rate while the inner radius of the magnetically disrupted
accretion disk shrinks, thereby lowering the shock temperature.
A {\sl NuSTAR} observation also revealed a high-amplitude WD spin modulation of the very
hard X-rays, suggesting an obscuration of the lower accretion pole and an extended shock
region on the WD surface.
In the {\sl Chandra} observation with the High Energy Transmission Gratings (HETG), we
detected prominent emission lines, 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 spectrum in the 0.8--2 keV range results from emission from
different regions of collisionally ionized plasma with a possible contribution from photoionization processes.