The nature of Aquila X-1

THE unusual X-ray source Aquila X-1 (=4U1908+00; =2S1908+005) has been observed for almost a decade, and represents a singular link between the 'steady' galactic X-ray stars and the flaring and transient sources. Although this object normally has an X-ray flux of the order 1-10% of the Crab Nebula^1-3, approximately once per year it undergoes an intense outburst to a level comparable to the Crab^2-4. A 1.3-d X-ray periodicity during the outburst has also been suggested⁵. A recent very accurate X-ray position for Aql X-1⁶ has permitted the optical identification of the source⁷ using primarily the extensive plate material described by Davidsen et al.². We report here photometry and spectroscopy of the counterpart and the stars in the immediately surrounding field, and use these data to arrive at some inferences regarding the nature of this unusual X-ray system.