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We present a detailed spectroscopic study of the optical counterpart of theneutron star X-ray transient Aquila X-1 during its 2011, 2013 and 2016outbursts. We use 65 intermediate resolution GTC-10.4m spectra with the aim ofdetecting irradiation-induced Bowen blend emission from the donor star. WhileGaussian fitting does not yield conclusive results, our full phase coverageallows us to exploit Doppler mapping techniques to independently constrain thedonor star radial velocity. By using the component N III 4640.64/4641.84 A wemeasure Kem = 102 +-6 km/s. This highly significant detection ( >13 sigma) isfully compatible with the true companion star radial velocity obtained fromnear-infrared spectroscopy during quiescence. Combining these two velocities wedetermine, for the first time, the accretion disc opening angle and itsassociated error from direct spectroscopic measurements and detailed modelling,obtaining alpha = 15.5 +2.5 -5 deg. This value is consistent with theoreticalwork if significant X-ray irradiation is taken into account and is important inthe light of recent observations of GX339-4, where discrepant results wereobtained between the donor's intrinsic radial velocity and the Bowen-inferredvalue. We also discuss the limitations of the Bowen technique when completephase coverage is not available.

Bowen emission from Aquila X-1: evidence for multiple components and constraint on the accretion disc vertical structure