  We explore simple semi-analytic fits to the bolometric light curve of
Gaia17biu/SN 2017egm, the most nearby hydrogen-deficient superluminous
supernova (SLSN I) yet discovered. SN 2017egm has a quasi-bolometric light
curve that is uncharacteristic of other SLSN I by having a nearly linear rise
to maximum and decline from peak, with a very sharp transition. Magnetar models
have difficulty explaining the sharp peak and may tend to be too bright 20 d
after maximum. Light curves powered only by radioactive decay of nickel fail on
similar grounds and because they demand greater nickel mass than ejecta mass.
Simple models based on circumstellar interaction do have a sharp peak
corresponding to the epoch when the forward shock breaks out of the
optically-thick circumstellar medium or the reverse shock reaches the inside of
the ejecta. We find that models based on circumstellar interaction with a
constant-density shell provide an interesting fit to the bolometric light curve
from 15 d before to 15 d after peak light of SN 2017egm and that both magnetar
and radioactive decay models fail to fit the sharp peak. Future photometric
observations should easily discriminate basic CSI models from basic magnetar
models. The implications of a CSI model are briefly discussed.
