GRB 180418A: A possibly-short GRB with a wide-angle outflow in a faint host galaxy
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Date
2020-12-17
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A. Rouco Escorial et al. GRB 180418A: A Possibly Short Gamma-Ray Burst with a Wide-angle Outflow in a Faint Host Galaxy. The Astrophysical Journal, vol. 912, no. 95 (2021). https://iopscience.iop.org/article/10.3847/1538-4357/abee85
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law
Public Domain Mark 1.0
This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law
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Abstract
We present X-ray and multi-band 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_90~2.56s and ~1.90s, respectively. Modeling the Fermi/GBM catalog of 1405 bursts (2008-2014) in the Hardness-T_90 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 F_X proportional to t^-0.98. Late-time Gemini observations reveal a faint r ~24.95 mag host galaxy at an angular offset of ~0.16''. At the likely redshift range of z ~1-1.5, we find that the X-ray afterglow luminosity of GRB 180418A is intermediate between short and long GRBs at all epochs during which there is contemporaneous data, and that GRB 180418A lies closer to the E_({\gamma},peak)-E_({\gamma},iso) correlation for short GRBs. Modeling the multi-wavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of {\theta}_j =>9-14 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.