  Aquila X-1 is a prototypical neutron star low mass X-ray binary and one of
the most studied X-ray transients. We present optical spectroscopy obtained
with the Gran Telescopio Canarias (10.4 m) during the 2016 outburst, the
brightest in recent times, which showed a standard evolution with hard and soft
accretion states. Our data set includes a dense coverage of the brightest
phases of the event, as well as the decay towards quiescence. We searched for
optical winds by studying the profiles and evolution of the main emission lines
and found no indisputable wind signatures, such as P-Cyg profiles. Nonetheless,
our detailed analysis of the particularly strong and broad Halpha emission
line, detected at the end of the outburst, is consistent with the presence of a
nebular phase produced by optically thin ejecta at ~800 km/s or, alternatively,
an extended disc atmosphere. We discuss these possibilities as well as the
similarities with the phenomenology observed in other black hole and neutron
star systems. Our study suggests that optical nebular phases might be a
relatively common observational feature during the late stages of low mass
X-ray binaries outbursts, enabling to probe the presence of outflows at
low-to-intermediate orbital inclinations.
