The 1978 X-ray and optical outburst of Aquila X-1 /4U 1908+00/

During the summer of 1978 the recurrent transient X-ray source, Aquila X-1, underwent its first major outburst in two years. This paper presents the results of extensive X-ray and optical observations of this event, which lasted for about two months. The peak X-ray luminosity was about 1.3 times that of the Crab and exhibited spectrum-dependent flickering on time scales of about 5 minutes. In addition, one very large flare was observed about one month after maximum that was also correlated with spectral changes. During this flare the previously identified optical counterpart brightened from V = 19 to a peak of V = 14.8, where it was distinctly blue (U - B = 0.4), and then reddened during the decay. These observations are interpreted in terms of a standard accretion disk model with particular emphasis on the similarities to Sco -1 and other dwarf X-ray systems.