The integrated radio spectrum of G2.4$+$1.4

TL;DR

This paper analyzes the radio spectrum of G2.4+1.4, showing it has a flat spectrum at gigahertz frequencies, clarifying its emission nature and correcting previous assumptions of non-thermal characteristics.

Contribution

It provides a comprehensive analysis of G2.4+1.4's radio spectrum, demonstrating its flat, thermal spectrum at high frequencies, which revises earlier non-thermal interpretations.

Findings

G2.4+1.4 has a flat radio spectrum at gigahertz frequencies.

The spectral index is approximately 0.02 ± 0.08.

The source is a thermal, photoionized nebula, not a supernova remnant.

Abstract

The Galactic source G2.4$+$1.4 is an optical and radio nebula containing an extreme Wolf--Rayet star. At one time this source was regarded as a supernova remnant, because of its apparent non-thermal radio spectrum, although this was based on limited observations. Subsequent observations instead supported a flat, optically thin thermal radio spectrum for G2.4$+$1.4, and it was identified as a photoionized, mass-loss bubble, not a supernova remnant. Recently, however, it has been claimed that this source has a non-thermal integrated radio spectrum. I discuss the integrated radio flux densities available for G2.4$+$1.4 from a variety of surveys, and show that it has a flat spectrum at gigahertz frequencies (with a spectral index $\alpha$ of $0.02 \pm 0.08$, where flux density $S$ scales with frequency $\nu$ as $S \propto \nu^{-\alpha}$).