Aquila X‐1 in Outburst and Quiescence

Abstract

We present photometry and spectroscopy of the soft X-ray transient Aql X-1 . Optical photometry during an active state shows a strong (0.6 mag peak-to-peak) modulation at a period of 19 hr. Infrared (K' band) photometry during a quiescent state limits any ellipsoidal variations to less than 0.07 mag (peak-to-peak), which implies an inclination i<31° (90% limit). Spectroscopy in a quiescent state shows at most very small radial velocity variations, which implies a very low inclination of i<12° (90% limit). The low inclination is rather unexpected given the large photometric modulation seen in the active state. The upper limit to the equivalent width of the anomalous Li 6707 A line is less than 0.3 A, which is similar to the measured strength of this line in several other X-ray transients.