  Classical gamma-ray bursts (GRBs) have two distinct emission episodes: prompt
emission from ultra-relativistic ejecta and afterglow from shocked
circumstellar material. While both components are extremely luminous in known
GRBs, a variety of scenarios predict the existence of luminous afterglow
emission with little or no associated high-energy prompt emission. We present
AT 2019pim, the first secure example of this phenomenon to be identified.
Serendipitously discovered during follow-up observations of a
gravitational-wave trigger and located in a contemporaneous TESS sector, it is
hallmarked by a fast-rising (t ~ 2 hr), luminous (M_UV,peak ~ -24.4 mag)
optical transient with accompanying luminous X-ray and radio emission. No
gamma-ray emission consistent with the time and location of the transient was
detected by Fermi-GBM or by Konus, placing strong limits on an accompanying
GRB. We investigate several independent observational aspects of the afterglow
in the context of constraints on relativistic motion and find all of them are
consistent with an initial Lorentz factor of Gamma_0 ~ 30-50, significantly
lower than in any well-observed GRB and consistent with the
theoretically-predicted "dirty fireball" scenario in which the high-energy
prompt emission is stifled by pair production. However, we cannot rule out a
structured jet model in which only the line-of-sight material was ejected at
low-Gamma, off-axis from a classical high-Gamma jet core. This event represents
a milestone in orphan afterglow searches, demonstrating that luminous
afterglows with weak or no detectable gamma-ray radiation exist in nature and
can be discovered by high-cadence optical surveys.
