Radio Continuum Study of One of the Brightest Large Magellanic Cloud Supernova Remnants N 132D

The gist The radio continuum study reveals a strong spatial correlation between radio and X-ray emission in N 132D, indicating a close link between thermal and non-thermal plasma components, while also providing new constraints on its magnetic field structure and energetic properties.

Morphology and Spatial Correlation

The radio continuum morphology of N 132D shows a "well-defined shell-like morphology in the radio continuum at 2100, 5500, and 9000 MHz (Figure 1, left panels), with bright emission concentrated along the outer boundary and significant internal structure. The study investigated the spatial correlation between radio, optical, and X-ray emission by extracting azimuthally resolved radial intensity profiles from the geometric centre of the SNR out to 90 arcsec. The radio and X-ray profiles show a "consistently strong correlation in all four regions (see Figure 2), with Pearson coefficients of (r = 0.88), 0.92, 0.94, and 0.92 for the north, east, south, and west regions respectively. In contrast, the radio-optical correlation varies substantially from (r = 0.92) in the north to (r = 0.36) in the south. The strong spatial correlation between the radio and X-ray emission may suggest a contribution from nonthermal X-ray emission. The [O iii] optical emission extends further into the cooling zones beyond the shock, and this outer diffuse optical emission likely arises from ambient material which has been photoionised by the fast SNR shock.

Polarization and Magnetic Field Structure

Polarisation measurements reveal predominantly radial magnetic fields, with mean fractional polarisations of 1.2 per cent, 3.0 per cent, and 4.0 per cent at 2100, 5500, and 9000 MHz respectively. The Faraday Depth spectrum shows a double-peaked profile that indicates regions with differing magnetisation properties or varying magnetic field geometry. The rotation measure derived from ATCA data has an average value of ∼ 2 rad m−2, with a maximum of ∼ 338 rad m−2. This complex Faraday structure likely arises from different magnetised regions with partially varying magnetic field geometry. The rotation measure map shows an average value of 2±27 rad m−2 and reaches a maximum of 338±70 rad m−2 located on the western side of the remnant".

Energetics and Magnetic Field Strength

Using a constant partition model, the study estimates a magnetic field strength of ∼90 µG and a minimum energy of Emin = 3 × 10 50 erg. This result is higher than the value previously reported by Dickel and Milne (1995). The equipartition model suggests a mean constant partition field over the entire remnant of ∼90 µG with an estimated minimum energy of Emin = 3.3×10 50 erg. The steep radio spectral index of −0.74 ± 0.01 is consistent with synchrotron emission from shock-accelerated electrons in young SNRs.

Spectral Index and Particle Acceleration

The average spectral index across the remnant is ∼ -0.5. The spectrum in N 132D appears to exhibit a "concave-up (Figure 9, blue dashed curve), which is characteristic of nonlinear effects in young SNRs where particle acceleration is highly efficient and produces a significant population of CRs. The accelerated CRs can modify the shock structure, resulting in a concave-up radio spectrum".

Conclusion

Together, these results provide a comprehensive picture of the magnetic field properties, energetics, and particle acceleration processes operating in N 132D. The remnant shows significant polarisation, with average fractional polarisation of 1.2 ± 0.9 per cent, 3 ± 1 per cent, and 4 ± 1 per cent at 2100, 5500, and 9000 MHz. The Faraday depth structure suggests the presence of multiple magnetised components along the line of sight. The rotation measure, obtained from ATCA observations, has an average value of 2 ± 27 rad m−2 and reaches a maximum of 338 ± 70 rad m−2. From a constant partition model, the study estimates a magnetic field strength of ∼90 µG and a minimum energy of Emin = 3.3×10 50 erg. Finally, the steep radio spectral index of −0.74 ± 0.01 is consistent with synchrotron emission from shock-accelerated electrons in young SNRs.

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TABLE 2

Flux density measurements of N 132D used in this work

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Fig. 5.— Results of rotation measure synthesis performed on N 132D using MeerKAT radio polarisation data over the Faraday Depth range-500 to +500 rad m−2.

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Fig. 4.— Left: Fractional polarisation images of N 132D at 2100 MHz (top), 5500 MHz (middle), and 9000 MHz (bottom).

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Fig. 8.— Spectral index map of SNR N 132D between MeerKAT (1283 MHz) and ATCA (2100, 5500, and 9000 MHz) images.

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Fig. 3.— Hardness ratio map between the 0.3–1.3 and 1.3–3.0 keV bands obtained from Chandra data.

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Fig. 2.— Normalised radial intensity profiles of the SNR derived from the optical ([O iii]), radio (2100 MHz), and X-ray (broadband) images for the north, east, south, and west regions.

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Fig. 1.— Images of N 132D with 2100 MHz contours overlaid (top), 5500 MHz contours overlaid (middle), and 9000 MHz contours overlaid (bottom).

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TABLE 1

Summary of our ATCA observations for N 132D used in this study

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Fig. 7.— Magnetic field direction vectors of N 132D overlaid on ATCA intensity image at 5500 MHz.

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Fig. 8.— Spectral index map of SNR N 132D between MeerKAT (1283 MHz) and ATCA (2100, 5500, and 9000 MHz) images.

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Fig. 9.— Radio continuum spectrum of N 132D.

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Fig. 9.— Radio continuum spectrum of N 132D.

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Fig.

1. textbf Stronger Spatial Correlation Modeling for Thermal/Non-Thermal Components: The improved AI can perform spatially resolved correlation analysis, specifically quantifying the Pearson correlation coefficient was used to quantify their radial correlation between radio and X-ray profiles, leading to more robust identification of regions where the radio and X-ray profiles show a consistently strong correlation in all four regions.

2. textbf Complex Magneto-Ionic Environment Characterization: The improved AI can interpret complex Faraday Depth spectra by analyzing moment maps, allowing it to characterize the double-peaked structure and distinguish between components, such as the south-west rim (blue dot), interior (red dot), and south-east outer region (green dot).

3. textbf Enhanced Magnetic Field Strength Estimation: The AI can perform rigorous magnetic field calculations using both observational constraints and theoretical models, specifically estimating the mean constant partition field over the entire remnant is ∼90 µG based on the Urošević et al. (2018) model, which accounts for magnetic field amplification produced by the non-linear particle acceleration effects.

4. textbf Nonlinear Particle Acceleration Spectrum Mapping: The system can map spectral index variations using a spectral index map derived from multi-frequency data, identifying regions where the spectrum in N 132D appears to exhibit a concave-up (Figure 9, blue dashed curve), which is characteristic of nonlinear effects in young SNRs where particle acceleration is highly efficient.

We present a new radio continuum study of the Large Magellanic Cloud supernova remnant N 132D using observations from the Australia Telescope Compact Array (ATCA) and MeerKAT. The radio morphology shows a clear spatial correlation with the X-ray emission in the central and left regions of the remnant, as well as in the right portion of the shell, indicating a close connection between the thermal and non-thermal plasma components. Polarisation measurements reveal predominantly radial magnetic fields, with mean fractional polarisations of 1.2 per cent, 3.0 per cent, and 4.0 per cent at 2100, 5500, and 9000 MHz, respectively. There is also a complex structure in the Faraday Depth spectrum, showing a double-peaked profile that indicates regions with differing magnetisation properties or varying magnetic field geometry. The rotation measure derived from ATCA data has an average value of 2 rad m-2, with a maximum of 338 rad m-2. We estimate a constant partition magnetic field of 90 microG and a minimum energy of 3 x 10 50 erg, which is in a constant partition between the energy density of electrons and the energy density of the magnetic field. The integrated radio spectrum follows a power law with a spectral index of-0.74 +- 0.01. These results provide new constraints on the magnetic field structure and energetic properties of N 132D and offer insight into particle acceleration and shock evolution in young supernova remnants.