Emission Spectra of Fallback Disks around Young Neutron Stars

Abstract

The nature of the energy source powering anomalous X-ray pulsars (AXPs) is uncertain. Proposed scenarios involve either an ultramagnetized neutron star or accretion onto a neutron star. We consider the accretion model proposed recently by Chatterjee, Hernquist, & Narayan, in which a disk is fed by fallback material following a supernova. We compute the optical, infrared, and submillimeter emission expected from such a disk, including both viscous dissipation and reradiation of X-ray flux impinging on the disk from the pulsar. We find that it is possible with current instruments to put serious constraints on this and on other accretion models of AXPs. Fallback disks could also be found around isolated radio pulsars, and we compute the corresponding spectra. We show that the excess emission in the R and I bands observed for the pulsar PSR 0656+14 is broadly consistent with emission from a disk.