  Accreting neutron stars and black holes in low-mass X-ray binaries (LMXBs)
radiate across the electromagnetic spectrum. Linking the emission produced at
different wavelengths can provide valuable information about the accretion
process and any associated outflows. In this work, we study simultaneous X-ray
and UV/optical observations of the neutron star LMXB \source, obtained with the
Neil Gehrels Swift Observatory during its 2013, 2014 and 2016 accretion
outbursts. We find that the UV/optical and X-ray emission are strongly
correlated during all three outbursts. For the 2013 and 2014 episodes, which
had the best Swift sampling, we find that the correlation between the
UV/optical and X-ray fluxes is significantly steeper during the decay (soft
state) of the outburst than during the rise (hard-to-soft state). We observe an
UV/optical hysteresis behaviour that is likely linked to the commonly known
X-ray spectral hysteresis pattern. For the decays of the three outbursts we
obtain a correlation index that cannot be directly explained by any single
model. We suspect that this is a result of multiple emission processes
contributing to the UV/optical emission, but we discuss alternative
explanations. Based on these correlations, we discuss which mechanisms are
likely dominating the UV/optical emission of Aql X-1.
