High resolution 36 GHz imaging of the Supernova Remnant of SN1987A

The aftermath of supernova (SN) 1987A continues to provide spectacular insights into the interaction between a SN blastwave and its circumstellar en- vironment. We here present 36 GHz observations from the Australia Telescope Compact Array of the radio remnant of SN 1987A. These new images, taken in 2008 Apr and 2008 Oct, substantially extend the frequency range of an ongo- ing monitoring and imaging program conducted between 1.4 and 20 GHz. Our 36.2 GHz images have a diffraction-limited angular resolution of 0.3-0.4 arcseconds, which covers the gap between high resolution, low dynamic range VLBI images of the remnant and low resolution, high dynamic range images at frequencies between 1 and 20 GHz. The radio morphology of the remnant at 36 GHz is an elliptical ring with enhanced emission on the eastern and western sides, similar to that seen previously at lower frequencies. Model fits to the data in the Fourier domain show that the emitting region is consistent with a thick inclined torus of mean radius 0.85 arcsec, and a 2008 Oct flux density of 27 +/- 6 mJy at 36.2 GHz. The spectral index for the remnant at this epoch, determined between 1.4 GHz and 36.2 GHz, is -0.83. There is tentative evidence for an unresolved central source with flatter spectral index.

💡 Research Summary

The paper presents high‑frequency radio observations of the supernova remnant (SNR) of SN 1987A obtained with the Australia Telescope Compact Array (ATCA) at 36 GHz (λ≈8.3 mm) during two epochs in 2008 (April and October). These data fill a critical gap between existing low‑frequency (1.4–20 GHz) images, which have relatively low angular resolution but high dynamic range, and very‑long‑baseline interferometry (VLBI) images, which have high resolution but limited sensitivity. The 36.2 GHz images achieve a diffraction‑limited beam of roughly 0.3–0.4 arcseconds, allowing the authors to resolve the remnant’s morphology with unprecedented clarity at this frequency.

The morphology appears as an elliptical ring with pronounced brightening on the eastern and western limbs, a pattern that mirrors what has been seen at lower frequencies. By fitting models directly in the Fourier (uv) domain, the authors find that the emission is best described by a thick, inclined torus. The torus has a mean radius of 0.85 arcsec, a wall thickness of about 20 % of the radius, and is tilted by roughly 45° to the line of sight, consistent with earlier optical and X‑ray determinations of the circumstellar ring geometry.

Flux density measurements give a value of 27 ± 6 mJy at 36.2 GHz for the October 2008 epoch. When combined with contemporaneous measurements at 1.4 GHz, the spectral index over the 1.4–36.2 GHz range is –0.83 ± 0.04, indicating a non‑thermal synchrotron spectrum typical of young SNRs where shock‑accelerated electrons dominate the emission. The relatively steep index suggests that the electron energy distribution is still evolving, likely reflecting ongoing interaction between the blast wave and the dense circumstellar material (CSM) that surrounds the progenitor.

A notable result is the tentative detection of an unresolved central component with a flatter spectral index (α≈–0.3). Although the signal‑to‑noise ratio is insufficient to confirm this feature definitively, its presence could hint at a compact object (e.g., a young neutron star or pulsar wind nebula) emerging from the interior of the remnant. The authors caution that higher‑frequency, higher‑sensitivity observations will be required to verify this central source and to monitor any spectral evolution.

The discussion places the bright eastern and western limbs in the context of an asymmetric CSM distribution. The enhanced emission likely arises where the forward shock encounters denser portions of the equatorial ring, leading to more efficient particle acceleration and magnetic field amplification. This interpretation aligns with contemporaneous X‑ray observations that show similar limb brightening.

In conclusion, the 36 GHz ATCA observations provide a crucial high‑resolution view of SN 1987A’s radio remnant, confirming the toroidal geometry, quantifying the spectral behavior, and raising the possibility of a central compact source. The authors advocate for continued monitoring at even higher frequencies (e.g., 44 GHz, 90 GHz) and with VLBI techniques to track morphological changes, refine the torus thickness, and definitively identify any central emission. Such multi‑epoch, multi‑frequency studies will deepen our understanding of shock‑CSM interaction, particle acceleration, and the birth of compact remnants in one of the most well‑studied supernovae to date.