The orbital light curve of aquila X-1

We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron-star binary Aq1 X-1 in 1998 June while it was at quiescence. We find that its light curve is dominated by ellipsoidal variations, although the ellipsoidal variations are severely distorted and have unequal maxima. After we correct for the contaminating flux from a field star located only 0″.46 away, the peak-to-peak amplitude of the modulation is ≈0.25 mag in the R band, which requires the orbital inclination to be greater than 36°. The orbital period we measure is consistent with the 18.95 hr period measured by Chevalier & Ilovaisky. During its outbursts the light curve of Aq1 X-1 becomes single-humped. The outburst light curve observed by Garcia et al. agrees in phase with our quiescent light curve. We show that the single-humped variation is caused by a "reflection effect," that is, by heating of the side of the secondary star facing toward the neutron star.