  We show that a typical X-ray outburst light curve of Aql X-1 can be
reproduced by accretion onto the neutron star in the frame of the disk
instability model without invoking partial accretion or propeller effect. The
knee and the subsequent sharp decay in the X-ray light curve can be generated
naturally by taking into account the weak dependence of the disk aspect ratio,
$h/r$, on the disk mass-flow rate, $\dot{M}_\mathrm{in}$, in the X-ray
irradiation flux calculation. This $\dot{M}_\mathrm{in}$ dependence of $h/r$
only slightly modifies the irradiation temperature profile along the hot disk
in comparison to that obtained with constant $h/r$. Nevertheless, this small
difference has a significant cumulative effect on the hot disk radius leading
to a much faster decrease in the size of the hot disk, and thereby to a sharper
decay in the X-ray outburst light curve. The same model also produces the
long-term evolution of the source consistently with its observed outburst
recurrence times and typical light curves of Aql X-1. Our results imply that
the source accretes matter from the disk in the quiescent state as well. We
also estimate that the dipole moment of the source $\mu \lesssim 4 \times
10^{26}$ G cm$^3~$($B \lesssim 4 \times 10^{8}$ G at the surface).
