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

Becerra, R. L. and Dichiara, S. and Watson, A. M. and Troja, E. and Fraija, N. and Veres, Péter (2019) Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A. ASTROPHYSICAL JOURNAL, 881 (1). ISSN 1538-4357

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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 shows a bright peak (approximate to 14.2 AB mag) between T + 28 and T + 90 s that we interpret as the signature of a reverse shock. Later observations can be modeled by a standard forward shock model and show no evidence of a jet break, allowing us to constrain the jet collimation to theta(j) > 7 degrees. 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 Swift UV filters constrains the GRB redshift to z < 1.3 and places an upper bound on the gamma-ray isotropic equivalent energy E-gamma,E- iso < 3 x 10(51) erg. The properties of this GRB (e.g., duration, hardness ratio, energy, and environment) lie at the intersection between short and long bursts, and we cannot 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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