# Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A

## TL;DR

This paper reports early optical observations of a candidate short GRB, revealing a reverse shock signature, constraining jet collimation, and analyzing its properties to understand its classification between short and long GRBs.

## Contribution

First detection of reverse shock emission in early optical photometry of a short GRB candidate, with detailed modeling and environmental constraints.

## Key findings

- Bright optical peak indicating reverse shock
- Jet opening angle constrained to >7 degrees
- GRB redshift estimated to be less than 1.3

## Abstract

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\%.