Nature versus Nurture: The Origin of Soft Gamma‐Ray Repeaters and Anomalous X‐Ray Pulsars

Soft gamma-ray repeaters (SGRs) and anomalous x-ray pulsars (AXPs) are youngand radio-quiet x-ray pulsars which have been rapidly spun-down to slow spinperiods clustered in the range 5-12 s. Most of these unusual pulsars alsoappear to be associated with supernova shell remnants (SNRs) with typical ages<30 kyr. By examining the sizes of these remnants versus their ages, wedemonstrate that the interstellar media which surrounded the SGR and AXPprogenitors and their SNRs were unusually dense compared to the environmentsaround most young radio pulsars and SNRs. We explore the implications of thisevidence on magnetar and propeller-based models for the rapid spin-down of SGRsand AXPs. We find that evidence of dense environments is not consistent withthe magnetar model unless a causal link can be shown between the development ofmagnetars and the external ISM. Propeller-driven spin-down by fossil accretiondisks for SGRs and AXPs appears to be consistent with dense environments sincethe environment can facilitate the formation of such a disk. This may occur intwo ways: 1) formation of a ``pushback'' disks from the innermost ejecta pushedback by prompt reverse shocks from supernova remnant interactions with massiveprogenitor wind material stalled in dense surrounding gas, or 2) acquisition ofdisks by a high velocity neutron stars, which may be able to capture asufficient amounts of co-moving outflowing ejecta slowed by the prompt reverseshocks in dense environments.Comment: Accepted for publication in the Astrophysical Journal. 35 pages with 4 figure