We present the discovery of the radio afterglow and near-infrared (NIR)
counterpart of the Swift short GRB 200522A, located at a small projected offset
of ≈1 kpc from the center of a young, star-forming host galaxy at
z=0.5536. The radio and X-ray luminosities of the afterglow are consistent
with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by
our HST observations at a rest-frame time of ≈2.3 days, has a
luminosity of ≈(1.3−1.7)×1042 erg s−1. This is
substantially lower than on-axis short GRB afterglow detections, but is a
factor of ≈8-17 more luminous than the kilonova of GW170817, and
significantly more luminous than any kilonova candidate for which comparable
observations exist. The combination of the counterpart's color (i−y=−0.08±0.21; rest-frame) and luminosity cannot be explained by standard radioactive
heating alone. We present two scenarios to interpret the broad-band behavior of
GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially
boosted by magnetar energy deposition), or forward and reverse shocks from a
≈14∘, relativistic (Γ0≳80) jet. Models which
include a combination of enhanced radioactive heating rates, low-lanthanide
mass fractions, or additional sources of heating from late-time central engine
activity may provide viable alternate explanations. If a stable magnetar was
indeed produced in GRB 200522A, we predict that late-time radio emission will
be detectable starting ≈0.3-6 years after the burst for a deposited
energy of ≈1053 erg. Counterparts of similar luminosity to GRB
200522A associated with gravitational wave events will be detectable with
current optical searches to ≈250 Mpc.Comment: 33 pages, 13 figures, 5 tables. Submitted to AAS Journal