The rotation speed of the companion star in Aquila X-1

We have obtained medium resolution spectra of the neutron star X-ray transient Aql X-1 during quiescence. We determine the spectral type of the companion star to be K1 and also estimate its rotation speed to be 62(+30 -20) kms-1. By measuring the width of the Halpha emission line profile of the accretion disc, we estimate the binary inclination to about 50 degrees and also estimate the semi-amplitude of the companion star's radial velocity curve to be about 170 kms-1.