  The old nova and intermediate polar (IP) GK Persei underwent one of its
recurrent dwarf nova (DN) outbursts in 2018. We proposed monitoring it in UV
and X-rays with the Neil Gehrels Swift Observatory, starting less than six days
after the eruption, until 16 days after the eruption ended. For the first time
we could follow the decay to minimum light UV and X-rays. We present the timing
and spectral analysis, comparing the results with the previous outbursts and
with the quiescent status. We confirm the spin modulation in X-rays with a
period 351.325(9) s, only in the 2-10 keV range. The period was not detected in
the 0.3-2 keV range and in the UV band, suggesting that the soft portion of the
X-ray spectrum in GK Per does not originate near the poles, but in a wind or
circumstellar material. The amplitude of the modulation was less prominent than
in 2015, a fact that seems correlated with a lower average mass accretion rate.
The spectral fits are consistent with a mass accretion rate increasing by a
factor of 2 from rise to maximum and decreasing during the return to minimum,
following the trend of the modulation amplitude. The maximum plasma temperature
is higher than the Swift XRT energy range of 0.3-10 keV, thus it is not well
constrained, but our spectral fits indicate that it may have varied irregularly
during the outburst.
