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

• Published in: The Astrophysical journal Vol. 881; no. 1; pp. 12 - 21
• Main Authors: Becerra, R. L., Dichiara, S., Watson, A. M., Troja, E., Fraija, N., Klotz, A., Butler, N. R., Lee, W. H., Veres, P., Turpin, D., Bloom, J. S., Boer, M., González, J. J., Kutyrev, A. S., Prochaska, J. X., Ramirez-Ruiz, E., Richer, M. G.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10.08.2019; IOP Publishing; American Astronomical Society; Institute of Physics (IOP)
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysics; Collimation; Galaxies; Gamma ray bursts; gamma-ray burst; gamma-ray burst: individual (GRB 180418A); Optical observations; Photometry; Red shift; Sciences of the Universe; Ultraviolet filters; Upper bounds; Astrophysics - High Energy Astrophysical Phenomena; gamma-ray burst: individual: GRB 180418A
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ab275b

We present observations of the possible short GRB 180418A in γ-rays, X-rays, and in the optical. Early optical photometry with the TAROT and RATIR instruments shows a bright peak ( 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 θj > 7°. 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 γ-ray isotropic equivalent energy Eγ,iso < 3 × 1051 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%.