  Soft gamma-ray repeaters (SGRs) and anomalous x-ray pulsars (AXPs) are young
and radio-quiet x-ray pulsars which have been rapidly spun-down to slow spin
periods clustered in the range 5-12 s. Most of these unusual pulsars also
appear to be associated with supernova shell remnants (SNRs) with typical ages
<30 kyr. By examining the sizes of these remnants versus their ages, we
demonstrate that the interstellar media which surrounded the SGR and AXP
progenitors and their SNRs were unusually dense compared to the environments
around most young radio pulsars and SNRs. We explore the implications of this
evidence on magnetar and propeller-based models for the rapid spin-down of SGRs
and AXPs. We find that evidence of dense environments is not consistent with
the magnetar model unless a causal link can be shown between the development of
magnetars and the external ISM. Propeller-driven spin-down by fossil accretion
disks for SGRs and AXPs appears to be consistent with dense environments since
the environment can facilitate the formation of such a disk. This may occur in
two ways: 1) formation of a ``pushback'' disks from the innermost ejecta pushed
back by prompt reverse shocks from supernova remnant interactions with massive
progenitor wind material stalled in dense surrounding gas, or 2) acquisition of
disks by a high velocity neutron stars, which may be able to capture a
sufficient amounts of co-moving outflowing ejecta slowed by the prompt reverse
shocks in dense environments.

Astrophysics