Radio Polarization Observations of the Snail: A Crushed Pulsar Wind Nebula in G327.1–1.1 With a Highly Ordered Magnetic Field

Abstract

Pulsar wind nebulae (PWNe) are suggested to be acceleration sites of cosmic rays in the Galaxy. While the magnetic field plays an important role in the acceleration process, previous observations of magnetic field configurations of PWNe are rare, particularly for evolved systems. We present a radio polarization study of the "Snail" PWN inside the supernova remnant G327.1−1.1 using the Australia Telescope Compact Array. This PWN is believed to have been recently crushed by the supernova (SN) reverse shock. The radio morphology is composed of a main circular body with a finger-like protrusion. We detected a strong linear polarization signal from the emission, which reflects a highly ordered magnetic field in the PWN and is in contrast to the turbulent environment with a tangled magnetic field generally expected from hydrodynamical simulations. This could suggest that the characteristic turbulence scale is larger than the radio beam size. We built a toy model to explore this possibility, and found that a simulated PWN with a turbulence scale of about one-eighth to one-sixth of the nebula radius and a pulsar wind filling factor of 50%-75% provides the best match to observations. This implies substantial mixing between the SN ejecta and pulsar wind material in this system.

Pulsar wind nebulae created by fast-moving pulsars

“…(2004), [47] – Acero (2011), [48] – Ma et al. (2016), [49] – Marelli et al. (2016 a ), [50] – Gaensler et al.…”

“…The G327.1–1.1 (Snail) PWNe contains an undetected pulsar whose tail radio brightness remains constant over its length until it connects to a spherical structure (most likely a reverse shock, as the PWN is located inside its SNR; see figure 7; Ma et al. 2016).…”

Polarization properties of turbulent synchrotron bubbles: an approach based on Chandrasekhar–Kendall functions

“…Recent observations (e.g. Ma et al (2016)), and a set of theoretical indications based either on numerical simulations (Jun 1998;Blondin, Chevalier & Frierson 2001;Porth, Komissarov & Keppens 2014) or on spectral modeling (Tang & Chevalier 2012;Olmi et al 2015Olmi et al , 2016Tanaka & Asano 2016) suggest that a non negligible level of turbulence should be present in these systems. Some simplified model for the turbulent field has already been presented attempting to explain some of the observed results (Ma et al 2016;Bucciantini et al 2017).…”

“…The study of radio emission and polarization enable us to infer information on the strength and structure of the magnetic field, that are relevant to understand the dynamics of these systems. For example, in pulsar wind nebulae, radio emission traces old particles, that fill the entire volume of the nebula, and it is detectable also in old systems, where the injection from the pulsar has faded away (Frail et al 1996;Giacani et al 1997;Roberts et al 1999;Ma et al 2016). Recent observations (e.g.…”

“…Ma et al (2016)), and a set of theoretical indications based either on numerical simulations (Jun 1998;Blondin, Chevalier & Frierson 2001;Porth, Komissarov & Keppens 2014) or on spectral modeling (Tang & Chevalier 2012;Olmi et al 2015Olmi et al , 2016Tanaka & Asano 2016) suggest that a non negligible level of turbulence should be present in these systems. Some simplified model for the turbulent field has already been presented attempting to explain some of the observed results (Ma et al 2016;Bucciantini et al 2017). The level of turbulence in wind bubbles could be important to understand how particles diffuse (Tang & Chevalier 2012), the level of mixing and penetration of the ambient medium (Blondin, Chevalier & Frierson 2001), the strength of the magnetic field (its ability to quench the cascade) and its role in the nebular dynamics (Bucciantini et al 2004).…”