GRB 180418A: A Possibly Short Gamma-Ray Burst with a Wide-angle Outflow in a Faint Host Galaxy


Escorial AR, Fong W, Veres P, Laskar T, Lien A, Paterson K, Lally M, Blanchard PK, Nugent AE, Tanvir NR, et al. GRB 180418A: A Possibly Short Gamma-Ray Burst with a Wide-angle Outflow in a Faint Host Galaxy. The Astrophysical Journal. 2021;912 (2) :95.


We present X-ray and multiband optical observations of the afterglow and host galaxy of GRB 180418A, discovered by Swift/BAT and Fermi/GBM. We present a reanalysis of the GBM and BAT data deriving durations of the prompt emission of T90 ≍ 2.56 and 1.90 s, respectively. Modeling the Fermi/GBM catalog of 1405 bursts (2008-2014) in the hardness-T90 plane, we obtain a probability of ≍60% that GRB 180418A is a short-hard burst. From a combination of Swift/XRT and Chandra observations, the X-ray afterglow is detected to ≍38.5 days after the burst and exhibits a single power-law decline with FX ∝ t-0.98. Late-time Gemini observations reveal a faint r ≍ 25.69 mag host galaxy at an angular offset of ≍0"16. At the likely redshift range of z ≍ 1-2.25, we find that the X-ray afterglow luminosity of GRB 180418A is intermediate between short and long gamma-ray bursts (GRBs) at all epochs during which there are contemporaneous data and that GRB 180418A lies closer to the Eγ,peak-Eγ,iso correlation for short GRBs. Modeling the multiwavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of θj ≳ 9°-14°. If GRB 180418A is a short GRB that originated from a neutron star merger, it has one of the brightest and longest-lived afterglows along with an extremely faint host galaxy. If, instead, the event is a long GRB that originated from a massive star collapse, it has among the lowest-luminosity afterglows and lies in a peculiar space in terms of the hardness-T90 and Eγ,peak-Eγ,iso planes.

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