The Outburst of the Young Star Gaia19bey
  We report photometry and spectroscopy of the outburst of the young stellar
object Gaia19bey. We have established the outburst light curve with archival
Gaia G, ATLAS Orange, ZTF r-band and Pan-STARRS rizy-filter photometry, showing
an outburst of approximately 4 years duration, longer than typical EXors but
shorter than FUors. Its pre-outburst SED shows a flat far-infrared spectrum,
confirming the early evolutionary state of Gaia19bey and its similarity to
other deeply embedded young stars experiencing outbursts. A lower limit to the
peak outburst luminosity is approximately 182 L_sun at an assumed distance of
1.4 kpc, the minimum plausible distance. Infrared and optical spectroscopy near
maximum light showed an emission line spectrum, including HI lines, strong red
CaII emission, other metal emission lines, infrared CO bandhead emission, and a
strong infrared continuum. Towards the end of the outburst, the emission lines
have all but disappeared and the spectrum has changed into an almost pure
continuum spectrum. This indicates a cessation of magnetospheric accretion
activity. The near-infrared colors have become redder as Gaia19bey has faded,
indicating a cooling of the continuum component. Near the end of the outburst,
the only remaining strong emission lines are forbidden shock-excited emission
lines. Adaptive optics integral field spectroscopy shows the H_2 1--0 S(1)
emission with the morphology of an outflow cavity and the extended emission in
the [FeII] line at 1644 nm with the morphology of an edge-on disk. However, we
do not detect any large-scale jet from Gaia19bey.
