  We performed an intensive accretion disk reverberation mapping campaign on
the high accretion rate active galactic nucleus Mrk 142 in early 2019. Mrk 142
was monitored with the Neil Gehrels Swift Observatory for 4 months in X-rays
and 6 UV/optical filters. Ground-based photometric monitoring was obtained from
the Las Cumbres Observatory, Liverpool Telescope and Dan Zowada Memorial
Observatory in ugriz filters and the Yunnan Astronomical Observatory in V. Mrk
142 was highly variable throughout, displaying correlated variability across
all wavelengths. We measure significant time lags between the different
wavelength light curves, finding that through the UV and optical the
wavelength-dependent lags, $\tau(\lambda)$, generally follow the relation
$\tau(\lambda) \propto \lambda^{4/3}$, as expected for the $T\propto R^{-3/4}$
profile of a steady-state optically-thick, geometrically-thin accretion disk,
though can also be fit by $\tau(\lambda) \propto \lambda^{2}$, as expected for
a slim disk. The exceptions are the u and U band, where an excess lag is
observed, as has been observed in other AGN and attributed to continuum
emission arising in the broad-line region. Furthermore, we perform a flux-flux
analysis to separate the constant and variable components of the spectral
energy distribution, finding that the flux-dependence of the variable component
is consistent with the $f_\nu\propto\nu^{1/3}$ spectrum expected for a
geometrically-thin accretion disk. Moreover, the X-ray to UV lag is
significantly offset from an extrapolation of the UV/optical trend, with the
X-rays showing a poorer correlation with the UV than the UV does with the
optical. The magnitude of the UV/optical lags is consistent with a highly
super-Eddington accretion rate.
