The low mass X-ray binary Aquila X-1 is one of the most active neutron star X-ray transients .
Despite its relatively bright quiescent optical counterpart , the detection of its companion has been hampered by the presence of a nearby interloper star .
Using the infrared integral field spectrograph SINFONI on the VLT-8.2m telescope , we unambiguously single out Aquila X-1 from the interloper .
Phase-resolved near infrared spectroscopy reveals absorption features from a K 4 \pm 2 companion star moving at a projected velocity of K _ { 2 } = 136 \pm 4 { km s ^ { -1 } } .
We here present the first dynamical solution and associated fundamental parameters of Aquila X-1 , imposing new constraints on the orbital inclination ( 36 ^ { \circ } < i < 47 ^ { \circ } ) and the distance ( d = 6 \pm 2 kpc ) to this prototypical neutron star transient .