Where Are All the Fallback Disks? Constraints on Propeller Systems

K Y Ekşi,L Hernquist,R Narayan

doi:10.1086/429915

Abstract

Fallback disks are expected to form around new-born neutron stars following a supernova explosion. In almost all cases, the disk will pass through a propeller stage. If the neutron star is spinning rapidly (initial period $\sim 10$ ms) and has an ordinary magnetic moment ($\sim 10^{30}$ G cm$^3$), the rotational power transferred to the disk by the magnetic field of the neutron star will exceed the Eddington limit by many orders of magnitude, and the disk will be rapidly disrupted. Fallback disks can thus survive only around slow-born neutron stars and around black holes, assuming the latter do not torque their surrounding disks as strongly as do neutron stars. This might explain the apparent rarity of fallback disks around young compact objects.

Highlights

Fallback disks are expected to form around new-born neutron stars following a supernova explosion
Our work shows that for a neutron star with standard parameters, μ = 1030 G cm3, P = 10 ms, there is no way for the disk to penetrate the light cylinder and yet have its luminosity be less than the Eddington limit
We have shown in this paper that fallback disks around newborn neutron stars with conventional periods and magnetic moments would be disrupted by a combination of the accretion power and the rotational power of the neutron star being transferred to the disk

Summary

INTRODUCTION

Three, indicating that they are not spinning down purely by Following a supernova explosion, some of the ejected mat- emitting magnetic dipole radiation. Fallback disks (Chatterjee et al 2000; Alpar 2001; Marsden et al 2001a; Eksi & Alpar 2003) have been invoked to explain anomalous X-ray pulsars (AXPs; Woods & Thompson 2004) via an accretion model rather than the competing magnetar model (Duncan & Thompson 1992). Once a rapidly rotating neutron star progresses through the initial unstable accretion and propeller stages to an ejector/radio pulsar stage, it is the end of the road for the disk; the system cannot switch back to a propeller or accretion phase.

CONSTRAINTS ON A PROPELLERS

Findings

DISCUSSION