The Blue Supergiant Progenitor of the Supernova Imposter AT~2019krl
  Extensive archival \textit{Hubble Space Telescope}, \textit{Spitzer Space
Telescope}, and Large Binocular Telescope imaging of the recent
intermediate-luminosity transient, AT~2019krl in M74, reveal a bright optical
and mid-infrared progenitor star. While the optical peak of the event was
missed, a peak was detected in the infrared with an absolute magnitude of
$M_{4.5\,\mu {\rm m}} = -18.4$ mag, leading us to infer a visual-wavelength
peak absolute magnitude of $-$13.5 to $-$14.5. The pre-discovery light curve
indicated no outbursts over the previous 16\,yr. The colors, magnitudes, and
inferred temperatures of the progenitor best match a 13--14 M$_{\sun}$ yellow
or blue supergiant (BSG), if only foreground extinction is taken into account,
or a hotter and more massive star, if any additional local extinction is
included. A pre-eruption spectrum of the star reveals strong H$\alpha$ and
[N~{\sc ii}] emission with wings extending to $\pm 2000$\,km\,s$^{-1}$. The
post-eruption spectrum is fairly flat and featureless with only H$\alpha$,
\ion{Na}{1}~D, [\ion{Ca}{2}], and the \ion{Ca}{2} triplet in emission. As in
many previous intermediate-luminosity transients, AT~2019krl shows remarkable
observational similarities to luminous blue variable (LBV) giant eruptions,
SN~2008S-like events, and massive-star mergers. However, the information about
the pre-eruption star favors either a relatively unobscured BSG or a more
extinguished LBV with $M > 20$\,M$_{\sun}$ likely viewed pole-on.
