GRB 180418A: A possibly-short GRB with a wide-angle outflow in a faint host galaxy

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}\approx$2.56s and $\approx$1.90s, respectively. Modeling the $Fermi$/GBM catalog of 1405 bursts (2008-2014) in the Hardness-$T_{90}$ plane, we obtain a probability of $\approx$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 $\approx$38.5 days after the burst, and exhibits a single power-law decline with $F_{\mathrm{X}}\propto t^{-0.98}$. Late-time Gemini observations reveal a faint r$\approx$25.69 mag host galaxy at an angular offset of $\approx$0.16''. At the likely redshift range of z$\approx$1-2.25, 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 $E_{\gamma ,\mathrm{peak}}-E_{\gamma ,\mathrm{iso}}$ 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 ${\theta }_{\mathrm{j}}\gtrsim 9-14^{\circ }$. 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 $E_{\gamma ,\mathrm{peak}}-E_{\gamma ,\mathrm{iso}}$ planes.

26 pages, 9 figures, 3 tables (Accepted, ApJ)

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