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

Understand

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.