The Cocoon from a Massive Star's Death: VLA Radio Polarization Study of Possible Historical Supernova Remnant G7.7$-$3.7

TL;DR

This study uses VLA radio polarization observations to analyze the morphology, magnetic fields, and environment of the possible historical supernova remnant G7.7−3.7, revealing insights into its origin and progenitor star.

Contribution

It provides high-resolution polarization data that link the remnant's cocoon-like structure to interactions with circumstellar shells and progenitor winds.

Findings

Revealed a cocoon-like morphology with multiple shells and blowouts.

Detected high polarization fractions indicating shock-aligned magnetic fields.

Identified RM variations suggesting magnetized progenitor winds.

Abstract

G7.7$-$3.7 is a possible historical SNR, with the origin of its cocoon-like morphology and its supernova type remaining unclear. We performed L-band radio polarization observations of G7.7$-$3.7 using the Very Large Array in C and B-configurations. The high-resolution 1.4 GHz continuum image reveals a cocoon-like morphology with multiple shells and faint blowout structures. The total flux density is 9.6$\pm$0.5 Jy and the spectral index map shows predominantly nonthermal emission, with an integrated spectral index of $-$0.38$\pm$0.04. Polarization images of G7.7$-$3.7 show high linear polarization fraction (30%-40%) in the northwestern filaments and moderate polarization (10%-20%) in the northeast and south. The magnetic fields aligned with the filamentary structures, consistent with shock compression. Large rotation measure (RM) variations across the SNR likely originate from magnetized massive progenitor winds. We suggest that the cocoon-like morphology results from the interaction between the SNR and pre-existing circumstellar shells, demonstrating that the radio polarization provides useful constraints on the environments and even the progenitor mass-loss.