  Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as
a radio source sharing similar properties and host galaxy type to the compact,
persistent radio source associated with the first known repeating fast radio
burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a
transient source decaying in brightness over the last few decades. One possible
interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and
that the radio emission represents a young magnetar nebula (as several
scenarios assume for FRB 121102). Another interpretation is that FIRST
J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The
environment is similar to where most such events are produced. To distinguish
between these hypotheses, we conducted radio observations using the European
VLBI Network at 1.6 GHz to spatially resolve the emission and to search for
millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact
radio source with a flux density of $620 \pm 20\ \mathrm{\mu Jy}$ (on 2018
September 18) and a source size of $3.9 \pm 0.7\ \mathrm{mas}$ (i.e. $1.6 \pm
0.3\ \mathrm{pc}$ given the angular diameter distance of $83\ \mathrm{Mpc}$).
These results confirm that the radio emission is non-thermal and imply an
average expansion velocity of $(0.10 \pm 0.02)c$. Contemporaneous
high-time-resolution observations using the 100-m Effelsberg telescope detected
no millisecond-duration bursts of astrophysical origin. The source properties
and lack of short-duration bursts are consistent with a GRB jet expansion,
whereas they disfavor a magnetar birth nebula.
