Discovery of hard X-ray outbursts from the soft X-ray transient Aquila X-1

We report the BATSE discovery of hard X-ray outbursts from the soft X-ray transient Aquila X-1 (Aql X-1), Aql X-1 is the most prolific of the soft X-ray transient sources and it has been known to produce large outbursts near the Eddington limit in the 1-10 keV energy band. The typical recurrence time of outbursts is about 1 year. Aql X-1 shows type I X-ray bursts during the decay phase of the X-ray outbursts and is believed to contain a neutron star. These characteristics of Aql X-1 make it an ideal system to study time variable hard X-ray emission from accreting neutron stars. BATSE has monitored Aql X-1 continuously since the Compton Observatory mission began in April 1991. Several episodes of hard X-ray emission with durations of weeks to months have been detected in 1991-1994. These episodes are coincident with substantial brightening of the optical counterpart and to a lesser degree with observations of soft X-ray emission by ROSAT, EURECA/WATCH and ASCA. We find fluxes in the 20-100 mCrab range with hard spectra extending to above 100 keV and power law spectral fits yielding photon indices between -2 and -3.