Hard X-Ray Flares Preceding Soft X-Ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions

We have analyzed {\it Rossi X-ray Timing Explorer} (RXTE) data of the neutronstar transient Aquila X-1 obtained during its outbursts in May/June 1999 andSeptember/October 2000. We find that in the early rise of these outbursts, ahard flare in the energy range above 15 keV preceded the soft X-ray peak. Thehard X-ray flux of the hard flares at maximum was more than a factor of threestronger than at any other point in the outbursts. The rise of the hard X-rayflare to this maximum, was consistent with a monotonically brightening low/hardstate spectrum. After the peak of the hard flare, a sharp spectral transitionoccurred with spectral pivoting in the range 8--12 keV. Our timing analysisshows that during the hard flare the power spectra were mainly composed ofband-limited noise and a $\sim$ 1--20 Hz QPO, which correlate in frequency.Immediately after the hard flare, the power spectra turned into power lawnoise. The spectral and timing properties during and after the hard flares arevery similar to those in black hole transients during the early rise of anoutburst. We suggest that these hard flares and spectral transitions in Aql X-1are of the same origin as those observed in black hole transients. This leadsto the association of the 1--20 Hz QPOs and band-limited noise in Aql X-1 withthose in black hole transients. We discuss the impact of this discovery on ourunderstanding of soft X-ray transient outbursts, state transitions andvariability in X-ray binaries.