The Vela Pulsar With an Active Fallback Disk

TL;DR

This study re-analyzes Vela pulsar data to suggest the presence of an active fallback disk influencing its emission and spin-down behavior, indicating such disks may be common around young neutron stars.

Contribution

It provides evidence for an active fallback disk around Vela pulsar using multi-wavelength data and models its properties, a novel approach in pulsar studies.

Findings

Detection of a 0.57 keV absorption feature indicating over-abundance of oxygen.

Inner disk radius inside the light cylinder, supporting the fallback disk hypothesis.

Possible contribution of the disk to the pulsar's braking index and nebula features.

Abstract

Fallback disks are expected to form around young neutron stars. The presence of these disks can be revealed by their blackbody spectrum in the infrared, optical and UV bands. We present a re-reduction of the archival optical and infrared data of the Vela pulsar, together with the existing infrared and UV spectrum of Vela, and model their unpulsed components with the blackbody spectrum of a supernova debris disk. We invoke the quiescent disk solution of Sunyaev and Shakura for the description of the disk in the propeller stage and find the inner radius of the disk to be inside the light cylinder radius. We perform a high resolution X-ray analysis with XMM-Newton and find a narrow absorption feature at 0.57 keV which can be interpreted as K$_{\alpha}$ line of He-like oxygen (OVII). The strength of the line indicates an element over-abundance in our line of sight exceeding the amounts that would be expected from ISM. The spectral feature may originate from the pulsar wind nebula and may be partly caused by the reprocessed X-ray radiation by the fallback disk. We discuss the lower-than-3 braking index of Vela as partially due to the contribution of the propeller torques. Our results suggest that the pulsar mechanism can work simultaneously with the propeller processes and that the debris disks can survive the radiation pressure for at least $\sim 10^4$ years. As Vela is a relatively close object, and a prototypical pulsar, the presence of a disk, if confirmed, may indicate the ubiquity of debris disks around young neutron stars.