Comments on the radio spectrum of HB 3

TL;DR

This paper reviews the radio spectrum of supernova remnant HB 3, clarifying that previous reports of spectral flattening are due to contamination from nearby thermal emission, and confirms a consistent power-law spectrum from 22 to 2695 MHz.

Contribution

It provides a detailed analysis of HB 3's radio spectrum, demonstrating that the apparent spectral flattening is caused by contamination, and establishes a reliable power-law spectrum.

Findings

HB 3's spectrum follows a power-law with index 0.56

Contamination from thermal emission explains previous spectral flattening

Spectrum consistent from 22 to 2695 MHz

Abstract

It has recently been suggested that the radio spectrum of the Galactic supernova remnant HB 3 shows flattening at higher frequencies (above about 1 GHz). Here I review the radio spectrum of HB 3, noting the difficulties in deriving accurate flux densities for this remnant, particularly at high frequencies, due to the proximity of bright, thermal emission from W3 and its surroundings. A flux density for HB 3 at 2695 MHz is derived from Effelsberg survey data. The spectrum of HB 3 is well represented by a simple power-law spectrum from 22 to 2695 MHz, with a spectral index of 0.56 +/- 0.03. It is concluded that contamination with thermal emission from adjacent regions is the cause for the reported spectral flattening of HB 3.