Ba. Harmon et al., DISCOVERY OF HARD X-RAY OUTBURSTS FROM THE SOFT-X-RAY TRANSIENT AQUILA X-1, Astronomy & Astrophysics. Supplement series, 120(4), 1996, pp. 197-200
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 th
e 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 ty
pical 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 b
elieved to contain a neutron star. These characteristics of Aql X-1 ma
ke it an ideal system to study time variable hard X-ray emission from
accreting neutron stars. BATSE has monitored Aql X-1 continuously sinc
e the Compton Observatory mission began in April 1991. Several episode
s of hard X-ray emission with durations of weeks to months have been d
etected in 1991-1994. These episodes are coincident with substantial b
rightening of the optical counterpart and to a lesser degree with obse
rvations of soft X-ray emission by ROSAT, EURECA/WATCH and ASCA. We fi
nd fluxes in the 20-100 mCrab range with hard spectra extending to abo
ve 100 keV and power law spectral fits yielding photon indices between
-2 and -3.