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

We present X-ray and multi-band optical observations of the afterglow and host galaxy of GRB 180418A, discovered by ${it Swift}$/BAT and ${it Fermi}$/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 ${it Fermi}$/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 ${it Swift}$/XRT and ${it Chandra}$ observations, the X-ray afterglow is detected to $approx$38.5 days after the burst, and exhibits a single power-law decline with $F_{rm X} propto t^{-0.98}$. 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 $E_{gamma,{rm peak}}-E_{gamma,{rm iso}}$ 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 $theta_{rm j} gtrsim 9-14^{circ}$. 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 $E_{gamma,{rm peak}}-E_{gamma,{rm iso}}$ planes.