Year: 2026
Type: preprint
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.
Source: arXiv (Cornell University)
Institutions: National Tsing Hua University, Jagiellonian University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, California Institute of Technology, European Space Agency, Commonwealth Scientific and Industrial Research Organisation, National Tsing Hua University, National Science and Technology Council, Jet Propulsion Laboratory, University of California, Los Angeles, Smithsonian Astrophysical Observatory
Awards: G1390571956, G3561758408, G3815726687
OpenAlex ID: W7222565164