  We report the discovery and multi-wavelength data analysis of the peculiar
optical transient, ATLAS17aeu. This transient was identified in the skymap of
the LIGO gravitational wave event GW170104 by our ATLAS and Pan-STARRS
coverage. ATLAS17aeu was discovered 23.1hrs after GW170104 and rapidly faded
over the next 3 nights, with a spectrum revealing a blue featureless continuum.
The transient was also detected as a fading x-ray source by Swift and in the
radio at 6 and 15 GHz. A gamma ray burst GRB170105A was detected by 3
satellites 19.04hrs after GW170104 and 4.10hrs before our first optical
detection. We analyse the multi-wavelength fluxes in the context of the known
GRB population and discuss the observed sky rates of GRBs and their afterglows.
We find it statistically likely that ATLAS17aeu is an afterglow associated with
GRB170105A, with a chance coincidence ruled out at the 99\% confidence or
2.6$\sigma$. A long, soft GRB within a redshift range of $1 \lesssim z \lesssim
2.9$ would be consistent with all the observed multi-wavelength data. The
Poisson probability of a chance occurrence of GW170104 and ATLAS17aeu is
$p=0.04$. This is the probability of a chance coincidence in 2D sky location
and in time. These observations indicate that ATLAS17aeu is plausibly a normal
GRB afterglow at significantly higher redshift than the distance constraint for
GW170104 and therefore a chance coincidence. However if a redshift of the faint
host were to place it within the GW170104 distance range, then physical
association with GW170104 should be considered.
