[Paper Review] Observations of SNR G328.4+0.2 at 19 GHz

S. Johnston, N. M. McClure‐Griffiths+1 more|CERN Bulletin|Dec 11, 2003

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study presents high-frequency polarimetric observations of supernova remnant G328.4+0.2 at 19 GHz using the upgraded Australia Telescope Compact Array, revealing a highly polarized, circular SNR with a central bar and toroidal magnetic fields at the rim. The authors propose it is a composite remnant with a pulsar wind nebula in the bar—more similar to G11.2–0.3 than the Crab Nebula—resolving prior energetics issues by attributing the bar to a pulsar-powered nebula with a spin-down luminosity of ~3×10³⁷ erg s⁻¹ and a pulsar period of ~50 ms.

ABSTRACT

We report on the first polarimetric observations at 19 GHz made with the upgraded Australia Telescope Compact Array. Observations were made of the Galactic supernova remnant (SNR) G328.8+0.2. We find the SNR has circular morphology with a strong central bar, similar to that seen at lower frequencies. The SNR has high linear polarization throughout, with fractional polarization in the bar up to 50 per cent. The orientation of the magnetic field lines follow the filamentary structure of the SNR. The magnetic field at the edge of the SNR is generally toroidal, interspersed with radial fingers, likely caused by Rayleigh-Taylor instabilities. Although the SNR has been identified as Crab-like, we prefer an interpretation in which the bar is a pulsar powered wind nebula with the rest of the SNR consisting of the shell. The proposed pulsar parameters make the SNR / pulsar system more like SNR G11.2--0.3 than the Crab Nebula.

Motivation & Objective

• To investigate the magnetic field structure of SNR G328.4+0.2 using high-frequency polarimetry to resolve its classification and energetics.
• To test whether the central bar is a pulsar wind nebula rather than a Crab-like nebula, addressing inconsistencies in prior energy estimates.
• To compare the observed magnetic field morphology with MHD simulations of pulsar wind nebula evolution.
• To determine the pulsar parameters and remnant age by modeling the radio and polarization properties.
• To assess the SNR's similarity to other known composite remnants like G11.2–0.3 and 3C58.

Proposed method

• Observations were conducted at 19 GHz using the upgraded Australia Telescope Compact Array (ATCA), operating in two array configurations (750C and EW367) with six 22-m antennas.
• Stokes parameters (I, Q, U, V) were recorded simultaneously at two frequency bands (18.752 GHz and 18.880 GHz), each 128 MHz wide with 32 channels of 4 MHz.
• Seven pointings in a hexagonal pattern were used to cover the 5 arcmin diameter SNR, given the 1.8 arcmin primary beam at 18.8 GHz.
• Calibration used flux and phase calibrators (PKS 1934–638, PKS 1613–586), with bandpass and pointing corrections applied.
• Data reduction was performed using the MIRIAD software package, and Faraday rotation effects were minimized due to high observing frequency.
• Magnetic field vectors were derived from Q and U Stokes parameters, and their orientation was compared with total intensity and filamentary structures.

Experimental results

Research questions

• RQ1 Is the central bar in G328.4+0.2 a pulsar wind nebula rather than a Crab-like nebula, resolving the energetics problem of prior models?
• RQ2 How do the observed magnetic field structures align with MHD simulations of pulsar wind nebula evolution?
• RQ3 What are the implied pulsar spin-down energy and age based on the radio and polarization properties?
• RQ4 Does the SNR's morphology and magnetic field structure support classification as a composite remnant, similar to G11.2–0.3?
• RQ5 Where is the pulsar likely located, and can its position be inferred from the offset of the bar from the geometric center?

Key findings

• The SNR exhibits a circular morphology with a prominent central bar, and the entire structure shows high linear polarization, reaching up to 50% in the bar.
• Magnetic field lines are predominantly toroidal at the SNR rim, especially in the north and west, with radial fingers in the southwest likely due to Rayleigh-Taylor instabilities.
• The magnetic field is aligned with the filamentary structures and runs parallel to the major axis of the central bar, supporting a pulsar wind nebula origin.
• The pulsar wind nebula model implies a spin-down luminosity of ~3×10³⁷ erg s⁻¹, a pulsar period of ~50 ms, and a remnant age of ~10 kyr, resolving prior energetics inconsistencies.
• The geometric center of the SNR is offset from the bar’s center, suggesting a pulsar velocity of ~300 km s⁻¹, consistent with known pulsar speeds.
• The SNR is best classified as a composite remnant, with the bar as a pulsar wind nebula and the outer shell as a conventional shock-heated shell, resembling G11.2–0.3 and 3C58 more than the Crab Nebula.