Late-Time Evolution and Modeling of the Off-Axis Gamma-ray Burst Candidate FIRST J141918.9+394036. (arXiv:2107.04703v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Mooley_K/0/1/0/all/0/1">K. P. Mooley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Margalit_B/0/1/0/all/0/1">B. Margalit</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Law_C/0/1/0/all/0/1">C. J. Law</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perley_D/0/1/0/all/0/1">D. A. Perley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Deller_A/0/1/0/all/0/1">A. T. Deller</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lazio_T/0/1/0/all/0/1">T. J. W. Lazio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bietenholz_M/0/1/0/all/0/1">M.F. Bietenholz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shimwell_T/0/1/0/all/0/1">T. Shimwell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Intema_H/0/1/0/all/0/1">H. T. Intema</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gaensler_B/0/1/0/all/0/1">B. M. Gaensler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Metzger_B/0/1/0/all/0/1">B. D. Metzger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dong_D/0/1/0/all/0/1">D.Z. Dong</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hallinan_G/0/1/0/all/0/1">G. Hallinan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ofek_E/0/1/0/all/0/1">E.O. Ofek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sironi_L/0/1/0/all/0/1">L. Sironi</a>

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

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

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