AN OUTSIDE-IN OUTBURST OF AQL X-1

We present optical spectroscopy and optical and infrared photometry of the neutron star soft Xray transient Aq1 X-1 during its X-ray outburs t of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV-optical delay seen in dwarf novae outbursts and black hole X-r ay transients. We interpret this delay as the signature of an 'outside -in' outburst, in which a thermal instability in the outer disc propag ates inward. This outburst is the first of this type definitively iden tified in a neutron star X-ray transient.