  We present a 5A-100um Spectral Energy Distribution (SED) of the ultracool
dwarf star TRAPPIST-1, obtained as part of the Mega-MUSCLES Treasury Survey.
The SED combines ultraviolet and blue-optical spectroscopy obtained with the
Hubble Space Telescope, X-ray spectroscopy obtained with XMM-Newton, and models
of the stellar photosphere, chromosphere, transition region and corona. A new
Differential Emission Measure model of the unobserved extreme-ultraviolet
spectrum is provided, improving on the Lyman alpha to EUV relations often used
to estimate the 100-911A flux from low-mass stars. We describe the observations
and models used, as well as the recipe for combining them into an SED. We also
provide a semi-empirical, noise-free model of the stellar ultraviolet spectrum
based on our observations for use in atmospheric modelling of the TRAPPIST-1
planets.
