Disentangling multiple high-energy emission components in the Vela X pulsar wind nebula with the Fermi Large Area Telescope

L Tibaldo,J A Hinton,R Zanin,J Ballet,M Lemoine-Goumard,M.-H Grondin,G Faggioli

doi:10.1051/0004-6361/201833356

Abstract

Context. Vela X is a pulsar wind nebula in which two relativistic particle populations with distinct spatial and spectral distributions dominate the emission at different wavelengths. An extended 2° × 3° nebula is seen in radio and GeV gamma rays. An elongated cocoon prevails in X-rays and TeV gamma rays. Aims. We use ~9.5 yr of data from the Fermi Large Area Telescope (LAT) to disentangle gamma-ray emission from the two components in the energy range from 10 GeV to 2 TeV, bridging the gap between previous measurements at GeV and TeV energies. Methods. We determine the morphology of emission associated to Vela X separately at energies <100 and >100 GeV, and compare it to the morphology seen at other wavelengths. Then, we derive the spectral energy distribution of the two gamma-ray components over the full energy range. Results. The best overall fit to the LAT data is provided by the combination of the two components derived at energies <100 and >100 GeV. The first component has a soft spectrum, spectral index 2.19 ± 0.16−0.22+0.05, and extends over a region of radius 1.°36±0.°04, consistent with the size of the radio nebula. The second component has a harder spectrum, spectral index 0.9 ± 0.3−0.1+0.3, and is concentrated over an area of radius 0.°63±0.°03, coincident with the X-ray cocoon that had already been established as accounting for the bulk of the emission at TeV energies. Conclusions. The spectrum measured for the low-energy component corroborates previous evidence for a roll-over of the electron spectrum in the extended radio nebula at energies of a few tens of GeV possibly due to diffusive escape. The high-energy component has a very hard spectrum: if the emission is produced by electrons with a power-law spectrum, the electrons must be uncooled, and there is a hint that their spectrum may be harder than predictions by standard models of Fermi acceleration at relativistic shocks.

Highlights

Pulsars generate powerful winds that form nebulae filled by magnetized relativistic plasma, which, in turn, produce nonthermal radiation from radio to gamma rays
The best overall fit to the Large Area Telescope (LAT) data is provided by the combination of the two components derived at energies
The high-energy component has a very hard spectrum: if the emission is produced by electrons with a power-law spectrum, the electrons must be uncooled, and there is a hint that their spectrum may be harder than predictions by standard models of Fermi acceleration at relativistic shocks

Summary

Introduction

Pulsars generate powerful winds that form nebulae filled by magnetized relativistic plasma, which, in turn, produce nonthermal radiation from radio to gamma rays. Pulsar wind nebulae (PWNe, e.g., Amato 2014) are a prime observational target to understand particle acceleration in relativistic plasmas and are plausibly an important source of high-energy cosmic ray electrons and positrons. Vela X appears as an extended 2◦ × 3◦ nebula with a butterfly-like morphology, consistent with GeV gamma-ray observations. The pulsar lies at the position of the butterfly head, and we observe a series of filaments departing from it along the butterfly body, forming an elongated 1◦ structure dubbed “the cocoon”, which dominates emission in X-rays and TeV gamma rays

Results

Discussion

Conclusion