  The emission from shock breakouts (SBOs) represents the earliest
electromagnetic (EM) signal emitted by cataclysmic events involving the
formation or the merger of neutron stars (NSs). As such, SBOs carry unique
information on the structure of their progenitors and on the explosion energy.
The characteristic~SBO emission is expected in the UV range, and its detection
is one of the key targets of~the ULTRASAT satellite. Among SBO sources, we
focus on a specific class involving the formation of fast spinning magnetars in
the core-collapse (CC) of massive stars. Fast spinning magnetars are expected
to produce a specific signature in the early UV supernova light curve, powered
by the extra spin energy quickly released by the NS. Moreover, they are
considered as optimal candidates for the emission of long-transient
gravitational wave (GW) signals, the detection of which requires early EM
triggers to boost the sensitivity of dedicated GW search pipelines. We
calculate early supernova UV light curves in the presence of a magnetar central
engine, as a function of the explosion energy, ejecta mass and magnetar
parameters. We then estimate the ULTRASAT detection horizon (z < 0.15) as a
function of the same physical parameters, and the overall expected detection
rate finding that magnetar-powered SBOs may represent up to 1/5 of the total
events detected by ULTRASAT. Moreover, at the expected sensitivity of the
LIGO/Virgo/Kagra O5 science run, one such event occurring within 5 Mpc will
providean ideal trigger for a GW long transient search. Future GW detectors
like the Einstein Telescope will push the horizon for joint EM-GW detections to
35-40 Mpc.
