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 \geq 247 \pm 8 km s ^ { -1 } , and derive a mass function of f ( M _ { 1 } ) \geq 1.23 \pm 0.12 M _ { \odot } .
Estimates for the rotational broadening based on the emission components suggest a possible massive neutron star of \geq 1.6 M _ { \odot } ( 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 .