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

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 T (90) approximate to 2.56 and 1.90 s, respectively. Modeling the Fermi/GBM catalog of 1405 bursts (2008-2014) in the hardness-T (90) plane, we obtain a probability of approximate to 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 approximate to 38.5 days after the burst and exhibits a single power-law decline with F (X) proportional to t (-0.98). Late-time Gemini observations reveal a faint r approximate to 25.69 mag host galaxy at an angular offset of approximate to 0.'' 16. At the likely redshift range of z approximate to 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 E ( gamma,peak)-E ( gamma,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 theta (j) greater than or similar to 9 degrees-14 degrees. 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-T (90) and E ( gamma,peak)-E ( gamma,iso) planes.

Automated summary

The observations of GRB 180418A suggest that it may be a short-duration burst with a hard spectrum. The afterglow characteristics indicate that its luminosity is intermediate between short and long bursts, and it lies closer to the E (gamma,peak)-E (gamma,iso) correlation for short bursts.