Late-time Evolution and Modeling of the Off-axis Gamma-Ray Burst Candidate FIRST J141918.9+394036

We present new radio and optical data, including very long baseline interferometry, as well archival data analysis, for the luminous decades-long transient FIRST J141918.9+394036. The reveal a synchrotron self-absorption peak around 0.3 GHz radius of 1.3 mas (0.5 pc) 26 years post-discovery, indicating blastwave energy $\sim5 \times 10^{50}$ erg. spectrum shows broad [OIII]$\lambda$4959,5007 emission-line that may indicate collisional-excitation in host galaxy, but its association with cannot be ruled out. properties galaxy are suggestive massive stellar progenitor formed at low metallicity. Based on light curve, velocity, energetics, nature rates we find J1419+39 most consistent gamma-ray burst (LGRB) afterglows. Other classes (optically-discovered) explosions neutron star mergers disfavored, invoking any exotic scenario not necessary. It is therefore likely an off-axis LGRB afterglow (as suggested by Law et al. Marcote al.), under this premise inverse beaming fraction found to $f_b^{-1}\simeq280^{+700}_{-200}$, corresponding average jet half-opening angle $<\theta_j>\simeq5^{+4}_{-2}$ degrees (68% confidence), previous estimates. From volumetric rate predict surveys VLA, ASKAP MeerKAT will handful J1419+39-like events over coming years.