The orbital light curve of Aquila X-1

The R-band light curve of the X-ray nova/neutron-star binary Aql X-1 is dominated by ellipsoidal variations when it is at quiescence, although the ellipsoidal variations are severely distorted and have unequal maxima. The peak-to-peak amplitude of the variations is approximate to 0.25 mag. The orbital period measured from the ellipsoidal variations is consistent with the 18.95 hr period measured by Chevalier & Ilovaisky (1998); and the orbital inclination must be greater than 36, and probably lies between 36degrees and 55degrees. During outbursts the light curve of Aql X-1 is dominated by the "reflection effect," that is, by heating of the side of the companion star facing the neutron star.