We present a detailed spectroscopic study of the optical counterpart of the neutron star X-ray transient Aquila X-1 during its 2011 , 2013 and 2016 outbursts .
We use 65 intermediate resolution GTC-10.4m spectra with the aim of detecting irradiation-induced Bowen blend emission from the donor star .
While Gaussian fitting does not yield conclusive results , our full phase coverage allows us to exploit Doppler mapping techniques to independently constrain the donor star radial velocity .
By using the component N iii 4640.64/4641.84 Å we measure K _ { \mathrm { em } } = 102 \pm 6 km \mathrm { s ^ { -1 } } .
This highly significant detection ( \gtrsim 13 \sigma ) is fully compatible with the true companion star radial velocity obtained from near-infrared spectroscopy during quiescence .
Combining these two velocities we determine , for the first time , the accretion disc opening angle and its associated error from direct spectroscopic measurements and detailed modelling , obtaining \alpha = 15.5 ^ { +2.5 } _ { -5 } deg .
This value is consistent with theoretical work if significant X-ray irradiation is taken into account and is important in the light of recent observations of GX339-4 , where discrepant results were obtained between the donor ’ s intrinsic radial velocity and the Bowen-inferred value .
We also discuss the limitations of the Bowen technique when complete phase coverage is not available .