X-ray point sources towards the giant radio lobes of Centaurus A
Abstract
Centaurus A (NGC 5128) is the nearest radio galaxy (d = 3.8 Mpc), so individual X-ray point sources can be resolved against its giant radio lobes. We test whether a compact X-ray source population over the lobes exceeds the expected field population, once the selection function, foreground stars and the cosmic X-ray background (CXB) are accounted for. From five Chandra/ACIS-I lobe pointings we build a 0.5-7 keV catalogue of 185 sources, with Bayesian hardness ratios, Gaia DR3 and AllWISE counterparts, and spectral fits to the eight brightest. The hardness-ratio distribution (median -0.17; 80% of sources below N_H = 10^22 cm^-2) is that of background AGN plus a few foreground stars. Above the flux at which half the footprint is covered, the good sample holds 83 sources, 9 of them foreground stars, against 101 expected background sources: 27% below the expectation (-1.5 sigma including a 15% normalisation uncertainty), with no excess. The 90% upper limit on a lobe-associated excess above 6.9x10^36 erg/s is N_lobe < 18, or < 38 for a conservative 30% normalisation uncertainty. With the usual resolved-source masking and CXB subtraction, no additional lobe-associated correction to diffuse-emission or inverse-Compton measurements is indicated.
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BibTeX
@misc{balasubramaniam2026xray,
title = {{X-ray point sources towards the giant radio lobes of Centaurus A}},
author = {Karthik Balasubramaniam and Hsiang-Kuang Chang and Łukasz Stawarz and Volodymyr Marchenko},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22168},
eprint = {2610.22168},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22168},
}
RIS
TY - UNPB
TI - X-ray point sources towards the giant radio lobes of Centaurus A
AU - Karthik Balasubramaniam
AU - Hsiang-Kuang Chang
AU - Łukasz Stawarz
AU - Volodymyr Marchenko
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22168
UR - https://arxiv.org/abs/2610.22168
AB - Centaurus A (NGC 5128) is the nearest radio galaxy (d = 3.8 Mpc), so individual X-ray point sources can be resolved against its giant radio lobes. We test whether a compact X-ray source population over the lobes exceeds the expected field population, once the selection function, foreground stars and the cosmic X-ray background (CXB) are accounted for. From five Chandra/ACIS-I lobe pointings we build a 0.5-7 keV catalogue of 185 sources, with Bayesian hardness ratios, Gaia DR3 and AllWISE counterparts, and spectral fits to the eight brightest. The hardness-ratio distribution (median -0.17; 80% of sources below N_H = 10^22 cm^-2) is that of background AGN plus a few foreground stars. Above the flux at which half the footprint is covered, the good sample holds 83 sources, 9 of them foreground stars, against 101 expected background sources: 27% below the expectation (-1.5 sigma including a 15% normalisation uncertainty), with no excess. The 90% upper limit on a lobe-associated excess above 6.9x10^36 erg/s is N_lobe < 18, or < 38 for a conservative 30% normalisation uncertainty. With the usual resolved-source masking and CXB subtraction, no additional lobe-associated correction to diffuse-emission or inverse-Compton measurements is indicated.
ER -