  Phase-resolved high resolution optical spectroscopy has revealed narrow N III
and He II emission lines from the soft X-ray transient Aquila X-1 during its
2004 outburst that move as a function of the orbit consistent with the phasing
of the donor star. Under the assumption that these lines come from the
irradiated side of the donor star, we can constrain its K_2 velocity to
>247+/-8 km/s, and derive a mass function of f(M_1)>1.23+/-0.12M_sun. Estimates
for the rotational broadening based on the emission components suggest a
possible massive neutron star of >1.6M_sun (at 95% confidence). However, an
updated ephemeris and additional high resolution spectroscopy of Aql X-1 during
a future outburst are warranted in order to confirm that the narrow lines
indeed originate on the donor star surface, and reliably characterise the
system parameters of this important X-ray binary. Spectra taken during the end
of the outburst show that the morphology of the emission lines changed
dramatically. No donor star signature was present anymore, while the presence
of narrow low-velocity emission lines became clear, making Aql X-1 a member of
the slowly growing class of low-velocity emission line sources.
