Diprotodon on the sky. The Large Galactic Supernova Remnant (SNR) G278.94+1.35

TL;DR

This paper re-identifies the supernova remnant G278.94+1.35, named Diprotodon, as possibly the largest known Galactic SNR, with new radio and gamma-ray observations suggesting it is closer and in a radiative phase, raising questions about its high-energy emission.

Contribution

The study provides new radio and gamma-ray data that revise the size, distance, and evolutionary phase of Diprotodon, highlighting its unique properties among Galactic SNRs.

Findings

Diprotodon is likely closer at ~1 kpc, not 2.7 kpc.

Its size is approximately 58x56 pc, making it potentially the largest known SNR.

Gamma-ray emission shows a hard spectrum with no breaks, posing new puzzles.

Abstract

We present a re-discovery of G278.94+1.35 as possibly one of the largest known Galactic supernova remnants (SNR) - that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new EMU and GLEAM radio continuum images at an angular size of 3.33x3.23 deg, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a diameter of 157x152 pc. This size would qualify Diprotodon as the largest known SNR and pushes our estimates of SNR sizes to the upper limits. We investigate the environment in which the SNR is located and examine various scenarios that might explain such a large and relatively bright SNR appearance. We find that Diprotodon is most likely at a much closer distance of $\sim$1 kpc, implying its diameter is 58x56 pc and it is in the radiative evolutionary phase. We also present a new Fermi-LAT data analysis that confirms the angular extent of the SNR in gamma-rays. The origin of the high-energy emission remains somewhat puzzling, and the scenarios we explore reveal new puzzles, given this unexpected and unique observation of a seemingly evolved SNR having a hard GeV spectrum with no breaks. We explore both leptonic and hadronic scenarios, as well as the possibility that the high-energy emission arises from the leftover particle population of a historic pulsar wind nebula.