Typical X-Ray Outburst Light Curves of Aql X-1

Abstract

We show that the typical X-ray outburst light curve of Aql X-1 can be reproduced by accretion onto a neutron star in the frame of the disk instability model without invoking partial accretion or the 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, Ṁin , in the X-ray irradiation flux calculation. This Ṁin dependence of h/r only slightly modifies the irradiation temperature profile along the hot disk in comparison to that obtained with a 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 Aql X-1, consistent with its observed outburst recurrence time and typical light curve. 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 μ ≲ 4 × 1026 G cm3 (B ≲ 4 × 108 G at the surface).