A Study Revealing Physical Attributes of Supernova Remnant in G321.3-3.9

TL;DR

The study analyzes the recently identified SNR G321.3-3.9 using archival radio data from 88–2304 MHz to determine its morphology, spectrum, polarization, and distance. The remnant appears as an elliptical shell with a flat spectral index of $α \approx -0.40$, and polarization suggests a broadly tangential magnetic field though interpretations are limited by projection and depolarization effects. Distance estimates come from the Σ–D relation ($1.6$–$2.9$ kpc) and HI kinematics near $v_{\rm LSR} \approx -50$ km s$^{-1}$, yielding a near-side solution of $\sim2.5$–$3.3$ kpc and a far-side solution of $\sim9.5$–$10.3$ kpc, with the association tentatively supported and some discrepancies with previous bounds. The results highlight systematic uncertainties in distance determinations for SNRs and motivate higher-resolution follow-up across CO/HI, X-ray, and polarization, as well as hydrodynamical modeling to better understand SNR evolution in non-uniform ISM.

Abstract

We present a radio analysis of the recently identified supernova remnant G321.3-3.9 using archival multi-wavelength data spanning 88-2304 MHz. The source exhibits an elliptical shell-like morphology (1.3 deg x 1.7 deg) and a relatively flat non-thermal spectral index of alpha = -0.40 +/- 0.03. The distance is estimated using both the Sigma-D relation (1.6-2.9 kpc) and tentative associations with HI structures, the latter suggesting a near-side solution of 2.5-3.3 kpc, though the physical connection remains uncertain.

Figures (5)

• Figure 1: Radio continuum observations of SNR G321.3-3.9 across six frequency bands, (a) 88 MHz, (b) 118 MHz, (c) 154 MHz, (d) 200 MHz, (e) 1.4 GHz and (f) 2.3 GHz. The colour scales on the right side have a unit of Jy beam$^{-1}$. Specifically, subtracted point sources are indicated by green crosses in panel (d).
• Figure 2: (a) 200 MHz intensity map after point source subtraction, convolved to a 14.4' beam and regridded to the 1.4 GHz pixel scale. The source region is shown as the central white ellipse, the background as the surrounding green annulus, while the northern gray area is excluded to avoid contamination. (b) Integrated spectrum of SNR G321.3-3.9. Blue points represent the mean flux densities with statistical uncertainties. The orange line shows the best-fit power-law model, with shaded bands for statistical (orange) and total (blue) uncertainties.
• Figure 3: Polarized intensity image of G321.3–3.9 at 2.3 GHz from S-PASS. White lines indicate the orientation of B-vectors, with lengths proportional to the local polarized intensity. Green contours show the continuum intensity at the same frequency, also derived from S-PASS.
• Figure 4: (A) HI brightness temperature maps around SNR G321.3–3.9, averaged over velocity channels of 2.5 km s$^{-1}$ from $-55$ to $-47$ km s$^{-1}$. Central LSR velocities are labeled in each panel. Green contours show the radio continuum intensity at 200 MHz, outlining the morphology of the remnant. (B) HI position–velocity diagrams toward the remnant, extracted as glon–velocity slices from the HI cube with a latitude width of 0.2$^{\circ}$. Central latitudes are indicated in each panel.
• Figure 5: Source region (white) and nine alternative background regions (R1–R9) adopted to assess systematic uncertainties in the spectral index. The region labeled R3 corresponds to the example background shown in Figure \ref{['Fig2']}, while the spectral analysis itself was performed using the full set of regions.