Constraints on particle acceleration sites in the Crab nebula from relativistic magnetohydrodynamic simulations

Abstract

The Crab Nebula is one of the most efficient accelerators in the Galaxy and the only galactic source showing direct evidence of PeV particles. In spite of this, the physical process behind such effective acceleration is still a deep mystery. While particle acceleration, at least at the highest energies, is commonly thought to occur at the pulsar wind termination shock, the properties of the upstream flow are thought to be non-uniform along the shock surface, and important constraints on the mechanism at work come from exact knowledge of where along this surface particles are being accelerated. Here we use axisymmetric relativistic MHD simulations to obtain constraints on the acceleration site(s) of particles of different energies in the Crab Nebula. Various scenarios are considered for the injection of particles responsible for synchrotron radiation in the different frequency bands, radio, optical and X-rays. The resulting emission properties are compared with available data on the multi wavelength time variability of the inner nebula. Our main result is that the X-ray emitting particles are accelerated in the equatorial region of the pulsar wind. Possible implications on the nature of the acceleration mechanism are discussed.

Modeling the effect of small-scale magnetic turbulence on the X-ray properties of Pulsar Wind Nebulae

“…2 we show how the simulated synchrotron map changes due to the inclusion of a small scale fluctuating magnetic field. The model is the one described in Olmi et al (2015), and targeted to the Crab nebula. For reference to the fully ordered case (σ = 0) one should take Olmi et al (2015), but for σ raising from 0.3 at r = 0.6 ly to 1 at r = 1.25 ly.…”

“…The model is the one described in Olmi et al (2015), and targeted to the Crab nebula. For reference to the fully ordered case (σ = 0) one should take Olmi et al (2015), but for σ raising from 0.3 at r = 0.6 ly to 1 at r = 1.25 ly. Color-scale normalized to the maximum.…”

“…In general, two dimensional models are built on the axisymmetric assumption of a purely toroidal magnetic field, while three dimensional models have usually much lower resolution, and can only ⋆ E-mail: niccolo@arcetri.astro.it investigate large scale deviations from axisymmetry. However, recently, several arguments have been put forward, advocating the presence of small scale turbulence in PWNe: the presence of a large diffuse X-ray halo at distances in excess of the naive expectation for synchrotron cooling and advection (Tang & Chevalier 2012;Bühler & Blandford 2014;Zrake & Arons 2016); the suggestion that radio emitting particles could be continuously reaccelerated in the body of the nebula (Olmi et al 2015;Tanaka & Asano 2016); the observation or recurrent γ-ray flares, requiring localized strong current sheets (Uzdensky, Cerutti & Begelman 2011).…”

Pulsar-Wind Nebulae

“…This supports the Schweizer et al (2013) findings (based on earlier optical and X-ray monitoring) who concluded that the X-ray and optical emission must be produced by different populations of particles. Furthermore, according to Schweizer et al (2013), the fits with the Doppler-boosted tilted ring model require noticeably different (higher) flow velocities for the optical wisps (≈ 0.9c) compared to the X-ray wisps, which made Schweizer et al (2013) question the simple "boostedring" model (see, however, Olmi et al 2015). K+15 also found that for most individual features of the PWN (e.g., wisps) the NIR-optical-FUV spectra are harder than the contemporaneously measured X-ray spectra suggesting either an additional narrow spectral component or other kind of complex behavior between optical and soft X-rays.…”

“…In the MHD simulations these features are formed due to the combination of flow dynamics (i.e. fluid vortices produced near the TS due to magnetosonic oscillations; see e.g., (Olmi et al 2015) and references therein) and Doppler boosting. The emission from the oblique termination shock that has been also associated with the Crab's Inner Knot (Komissarov and Lyubarsky 2003;Yuan and Blandford 2015;see, however Melatos et al 2005 and references therein for alternative interpretations).…”