Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A. (arXiv:1904.05987v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Becerra_R/0/1/0/all/0/1">Rosa L. Becerra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dichiara_S/0/1/0/all/0/1">Simone Dichiara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Watson_A/0/1/0/all/0/1">Alan M. Watson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Troja_E/0/1/0/all/0/1">Eleonora Troja</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fraija_N/0/1/0/all/0/1">Nissim I. Fraija</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Klotz_A/0/1/0/all/0/1">Alain Klotz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Butler_N/0/1/0/all/0/1">Nathaniel R. Butler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_W/0/1/0/all/0/1">William H. Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Veres_P/0/1/0/all/0/1">Péter Veres</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bloom_J/0/1/0/all/0/1">Joshua S. Bloom</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boer_M/0/1/0/all/0/1">Michel L. Boer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gonzalez_J/0/1/0/all/0/1">J. Jesús González</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kutyrev_A/0/1/0/all/0/1">Alexander Kutyrev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Prochaska_J/0/1/0/all/0/1">Jason X. Prochaska</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ramirez_Ruiz_E/0/1/0/all/0/1">Enrico Ramirez-Ruiz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Richer_M/0/1/0/all/0/1">Michael G. Richer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Turpin_D/0/1/0/all/0/1">Damien Turpin</a>
We present observations of the possible short GRB 180418A in $gamma$-rays,
X-rays, and in the optical. Early optical photometry with the TAROT and RATIR
instruments show a bright peak ($approx$ 14.2 AB mag) between $T+28$ and
$T+90$ seconds that we interpret as the signature of a reversal shock. Later
observations can be modeled by a standard forward shock model and show no
evidence of jet break, allowing us to constrain the jet collimation to
$theta_j> 7^circ$. Using deep late-time optical observations we place an
upper limit of $r>24$ AB mag on any underlying host galaxy. The detection of
the afterglow in the textit{Swift} UV filters constrains the GRB redshift to
$z<1.3$ and places an upper bound on the $gamma$-ray isotropic equivalent
energy $E_{rm{gamma,iso}} < 3 times 10^{51}$ erg.
The properties of this GRB (e.g. duration, hardness ratio, energetic, and
environment) lie at the intersection between short and long bursts, and we can
not conclusively identify its type. We estimate that the probability that it is
drawn from the population of short GRBs is 10%-30%.
We present observations of the possible short GRB 180418A in $gamma$-rays,
X-rays, and in the optical. Early optical photometry with the TAROT and RATIR
instruments show a bright peak ($approx$ 14.2 AB mag) between $T+28$ and
$T+90$ seconds that we interpret as the signature of a reversal shock. Later
observations can be modeled by a standard forward shock model and show no
evidence of jet break, allowing us to constrain the jet collimation to
$theta_j> 7^circ$. Using deep late-time optical observations we place an
upper limit of $r>24$ AB mag on any underlying host galaxy. The detection of
the afterglow in the textit{Swift} UV filters constrains the GRB redshift to
$z<1.3$ and places an upper bound on the $gamma$-ray isotropic equivalent
energy $E_{rm{gamma,iso}} < 3 times 10^{51}$ erg.
The properties of this GRB (e.g. duration, hardness ratio, energetic, and
environment) lie at the intersection between short and long bursts, and we can
not conclusively identify its type. We estimate that the probability that it is
drawn from the population of short GRBs is 10%-30%.
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