We show that the X-ray outburst light curves of four transient anomalous
X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), namely XTE J1810-197,
SGR 0501+4516, SGR 1627-41 and CXOU J164710.2-455216, can be produced by the
fallback disk model that was also applied to the outburst light curves of
persistent AXPs and SGRs in our earlier work. The model solves the diffusion
equation for the relaxation of a disk which has been pushed back by a soft
gamma-ray burst. The sets of main disk parameters used for these transient
sources are very similar to each other and to those employed in our earlier
models of persistent AXPs and SGRs. There is a characteristic difference
between the X-ray outburst light curves of transient and persistent sources.
This can be explained by the differences in the disk surface-density profiles
of the transient and persistent sources in quiescence indicated by their
quiescent X-ray luminosities. Our results imply that a viscous disk instability
operating at a critical temperature in ~ 1300 - 2800 K range is a common
property of all fallback disks around AXPs and SGRs. The effect of the
instability is more pronounced and starts earlier for the sources with lower
quiescent luminosities, which leads to the observable differences in the X-ray
enhancement light curves of transient and persistent sources. A single active
disk model with the same basic disk parameters can account for the enhancement
phases of both transient and persistent AXPs and SGRs. We also present a
detailed parameter study to show the effects of disk parameters on the
evolution of the X-ray luminosity of AXPs and SGRs in the X-ray enhancement
phases.Comment: 41 pages, 9 figures. Appeared in the Astrophysical Journal, 758, 98
(2012
