An Exploration of In-band Spectral Index Techniques for Extended Radio Sources with EMU
10 affiliations
- 1University of Alberta
- 2National Research Council Canada
- 3University of Tasmania
- 4Australia Telescope National Facility
- 5Western Sydney University
- 6Macquarie University
- 7Astronomical Observatory
- 8University of Calgary
- 9Lahore University of Management Sciences
- 10Ruhr University Bochum
Abstract
Spectral indices are an effective way of differentiating between thermal and nonthermal sources of radio emission. Most often, spectral indices are determined using flux measurements taken over multiple observations with different instruments. Since these data are not always available, determining spectral indices from a single telescope observation is desirable. The reliability of these in-band spectral indices has not been well studied, particularly for extended sources. Here, we explore three different techniques for finding in-band spectral indices with the Evolutionary Map of the Universe (EMU), a radio southern sky survey being conducted with the Australian Square Kilometre Array Pathfinder (ASKAP). These methods include: (1) integrating fluxes in sub-bands, (2) temperature–temperature (T–T) plots, and (3) using the ratio of the Taylor terms produced in the imaging process. We apply these techniques to a selection of extended sources of radio emission, primarily supernova remnants (SNRs) and H ii regions, and assess if the indices produced through these methods agree with the expected values. For the SNRs, we also compare the in-band results to broadband spectral indices determined using flux values obtained from published works. We examine whether and when the in-band techniques can differentiate between thermal and nonthermal sources. In general, we find the T–T plot method to be the most reliable across the range of source sizes and brightnesses studied for both SNRs and H ii regions. The sub-band and Taylor term methods can be useful under certain conditions but cannot be considered as broadly reliable for extended sources.
Versions
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BibTeX
@article{ball2026exploration,
title = {{An Exploration of In-band Spectral Index Techniques for Extended Radio Sources with EMU}},
author = {B. D. Ball and R. Kothes and E. Rosolowsky and J. L. West and W. Raja and M. D. Filipović and A. M. Hopkins and S. Lazarević and D. Leahy and S.F. Rahman and Z. J. Smeaton and S. Taziaux},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aeacde},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeacde},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - An Exploration of In-band Spectral Index Techniques for Extended Radio Sources with EMU
AU - B. D. Ball
AU - R. Kothes
AU - E. Rosolowsky
AU - J. L. West
AU - W. Raja
AU - M. D. Filipović
AU - A. M. Hopkins
AU - S. Lazarević
AU - D. Leahy
AU - S.F. Rahman
AU - Z. J. Smeaton
AU - S. Taziaux
PY - 2026
DA - 2026/10/30
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeacde
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeacde
AB - Spectral indices are an effective way of differentiating between thermal and nonthermal sources of radio emission. Most often, spectral indices are determined using flux measurements taken over multiple observations with different instruments. Since these data are not always available, determining spectral indices from a single telescope observation is desirable. The reliability of these in-band spectral indices has not been well studied, particularly for extended sources. Here, we explore three different techniques for finding in-band spectral indices with the Evolutionary Map of the Universe (EMU), a radio southern sky survey being conducted with the Australian Square Kilometre Array Pathfinder (ASKAP). These methods include: (1) integrating fluxes in sub-bands, (2) temperature–temperature (T–T) plots, and (3) using the ratio of the Taylor terms produced in the imaging process. We apply these techniques to a selection of extended sources of radio emission, primarily supernova remnants (SNRs) and H ii regions, and assess if the indices produced through these methods agree with the expected values. For the SNRs, we also compare the in-band results to broadband spectral indices determined using flux values obtained from published works. We examine whether and when the in-band techniques can differentiate between thermal and nonthermal sources. In general, we find the T–T plot method to be the most reliable across the range of source sizes and brightnesses studied for both SNRs and H ii regions. The sub-band and Taylor term methods can be useful under certain conditions but cannot be considered as broadly reliable for extended sources.
SN - 1538-4357
SN - 0004-637X
ER -