What the study actually tests
Centaurus A is the nearest radio galaxy, and its gamma-ray emission is well established, but the physical mechanism behind that emission is still unresolved. This paper does not report new telescope data. It asks a forward-looking question: if the Cherenkov Telescope Array Observatory (CTAO) observed Cen A, could it tell competing particle-acceleration models apart?
To answer that, the authors generated simulated observations using the CTAO Instrument Response Functions for the Alpha Configuration, which pairs 14 Medium-Sized and 37 Small-Sized Telescopes at the southern site with 4 Large-Sized and 9 Medium-Sized Telescopes in the north. Because Cen A's declination makes it barely observable from the northern site, the analysis concentrates on CTAO-South at zenith angles of 20, 40, and 60 degrees.
The study separates two targets: the kiloparsec-scale jet, treated first as a point source and then as an extended source, and the Southern Giant Lobe, evaluated through diffuse and filamentary emission templates for the S1 region previously detected by Fermi-LAT.
Why 50 hours can already resolve the jet
The H.E.S.S. Collaboration established that the TeV signal from Cen A's jet is spatially extended and better described by a two-dimensional Gaussian than by a point source. The paper tests whether CTAO could recover that distinction.
In the morphological simulations, CTAO discriminates between point-like and Gaussian spatial models within 50 h of observation, assuming a known background model and a zenith angle of 20 degrees. That is a statement about spatial discrimination, not about identifying which particles produce the emission. The same 50 h dataset is not sufficient to separate the competing spectral models of very-high-energy origin.
This distinction matters because resolving an extended source and identifying its particle content are different measurements. A 50 h campaign answers the morphology question; it does not answer the acceleration-mechanism question on its own.
200 hours separates leptonic from hadronic scenarios
Extending the simulated exposure to 200 h changes what becomes visible. At that depth, the paper reports that CTAO can distinguish purely leptonic scenarios, such as the H.E.S.S., AX1C, and BX models, from scenarios containing a hadronic component, such as B+20, Z&M, and O&S.
The separation rests on spectral shape. A leptonic signal based on the BX model would appear as a smooth decline in flux above 10 TeV. A hadronic signature would instead show a peak in the tens-of-TeV range for the Z&M and O&S models. The O&S model produces a slight spectral hardening around 10 TeV, though detecting emission at even higher energies remains difficult.
The authors also caution that purely leptonic and hadronic models can be spectrally degenerate in general, meaning energy-spectrum measurements alone may not always separate them. The 200 h discrimination reported here applies to the specific model set and assumptions of this analysis.
Morphology breaks a degeneracy the spectrum cannot
Two of the high-energy scenarios, Z&M and O&S, are hard to separate using energy spectra alone because large uncertainties blur their distinction. The paper points to angular information as the additional handle.
A gamma-ray signal produced during extragalactic propagation of ultra-high-energy cosmic rays is expected to be broadened into a wide angular halo. An in-situ acceleration scenario should instead yield diffuse emission more aligned with the jet. CTAO's angular resolution at these extreme energies is what makes this spatial test possible.
The angular distribution of photons from ultra-high-energy cosmic-ray propagation depends strongly on the extragalactic magnetic field, and the paper states that a detailed treatment of that dependence is beyond its scope. So morphology offers a discriminating channel, but one whose interpretation inherits magnetic-field assumptions the study does not resolve.
The Giant Lobe threshold sits near 10 TeV
For the Southern Giant Lobe, the paper evaluates the S1 region detected by Fermi-LAT using two emission templates: a diffuse Gaussian with sigma of 0.5 degrees and a filamentary, Vertex-like morphology. It also tests two maximum proton energies, with cutoff energies of 10 and 100 TeV.
CTAO-South is sensitive to hadronic scenarios with proton energies extending beyond 10 TeV for the Alpha Configuration at a zenith angle of 20 degrees. In all cases considered, the detection significance exceeds the square root of TS equal to 5 for 50 h at the optimum angle. A filamentary, Vertex-like emission is more likely to be detected than the diffuse Gaussian.
The cutoff energy is not reconstructed exactly. For a Gaussian profile with a true cutoff of 100 TeV, the reconstructed value is 58 with a range of plus 16 and minus 14 TeV, with a mean fit error of 19 TeV. For a true cutoff of 10 TeV, the reconstructed value is 7 with a range of plus 2 and minus 2 TeV, with a mean fit error of 2.3 TeV. The authors conclude that the proton cutoff can be constrained but not precisely recovered.
The lower maximum proton energy, around 10 TeV, begins to challenge detectability for the current CTAO-South array configuration. The paper expects that even more extended emission could still be detected, provided the cutoff energy is not significantly below roughly 10 TeV.
Where these projections stop being optimistic
The authors describe their results as an optimistic upper baseline. Two assumptions drive that framing. First, additional astrophysical background emission was not modeled; the instrumental background comes directly from the CTAO IRFs and consists only of hadronic and electronic air-shower events. Second, the zenith angle was fixed at 20 degrees, whereas realistic 50 to 200 h observations of Cen A span a broader range with angles greater than 20 degrees.
Because detection significance drops significantly above 40 degrees, the fixed 20-degree assumption inflates the projected sensitivity. Unmodeled field sources, extended structures, and other components of Cen A within the field of view could further degrade CTAO's ability to detect and characterize the signal.
A further omitted variance is the intrinsic uncertainty within each considered model. The authors expect these model uncertainties to degrade the classification performance once they exceed the statistical precision achievable with 200 h. The Z&M and O&S scenarios are a special case: because their flux in the 1 to 10 TeV range is relatively low, they were combined with the H.E.S.S. baseline for comparison. If the intrinsic flux in that band were higher, as the BX model predicts, it could dominate over their high-energy components and make discrimination substantially harder.
The priority is 200 hours, not 50
The paper's own comparison sets the priority. A 50 h CTAO observation is expected to surpass the significance level that H.E.S.S. reached with roughly 200 h, and a 50 h core-region observation is already planned by the CTAO Consortium. That exposure is enough to constrain existing models and to resolve the jet's extended morphology.
But the decisive step toward identifying the acceleration mechanism requires the longer campaign. Only at 200 h does the simulated data separate leptonic from hadronic scenarios and probe hard spectral components extending to tens of TeV. The 200 h exposure is comparable both to the total exposure H.E.S.S. has currently achieved and to the 150 h allocated in the CTAO Collaboration's initial observation program, which makes it a plausible extension rather than an arbitrary target.
The practical reading is therefore a staged one. Fifty hours answers whether the jet emission is extended. Two hundred hours, under the optimistic assumptions of this study, is what would begin to answer what accelerates the particles. The Giant Lobe adds a separate threshold: hadronic scenarios remain detectable only while the proton cutoff stays at or above roughly 10 TeV.
Sources used in this answer
Centaurus A: Opportunities to unveil particle acceleration with the CTAO
Simulated CTAO Alpha Configuration observations show that 50 h can resolve the extended gamma-ray emission along Cen A's kiloparsec-scale jet and discriminate point-like from Gaussian spatial models, while 200 h can separate leptonic from hadronic scenarios and probe hard spectral components to tens of TeV; CTAO-South is sensitive to Southern Giant Lobe hadronic scenarios with proton energies beyond 10 TeV.