Prospects on detection of the Fermi Bubbles with CTAO

In 2010, the Fermi Gamma-ray Space Telescope observed two gamma-ray emitting structures, the Fermi Bubbles (FBs), that extend up to 55° above and below the Galactic plane and that seem to emanate from the Galactic center region. Although the spectrum at latitudes |b| > 10° has a softening or a cutoff around 100 GeV, the one at the base of the FBs, |b| <10°, extends up to about 1 TeV without a significant cutoff in the Fermi LAT data. The mechanism behind the FBs production is currently under debate. More observations of the FBs at different energies are required to improve our understanding of their origin. Recently, H.E.S.S. and HAWC observatory have set upper limits on the FBs. In this work, we assess the sensitivity of the Cherenkov Telescope Array Observatory (CTAO) using the “alpha configuration” in the South site to detect the FBs and investigate the optimal strategies for their detection at low latitudes. We simulate the observations using the official CTAO science tool gammapy, considering several benchmark models for the FBs and the interstellar emission and test different observational strategies taking advantage of the proposed CTAO consortium surveys. We use these simulations to estimate the CTAO sensitivity to the FBs.

💡 Research Summary

This paper evaluates the capability of the Cherenkov Telescope Array Observatory (CTAO), specifically the southern‑site “Alpha” configuration, to detect gamma‑ray emission from the low‑latitude portion of the Fermi Bubbles (FBs). The authors simulate observations using the official CTA science tool Gammapy (v1.2), adopting the latest instrument response functions (IRFs) “prod5v0.1”. The simulated campaign reproduces the planned Galactic‑Center (GC) survey: nine pointings at latitudes b = −1°, 0°, +1°, amounting to a total exposure of 525 hours, covering a 12° × 12° region of interest (ROI) centred on the GC. To minimise contamination from the bright Galactic plane, the inner |b| < 1.5° is masked.

Three emission components are modelled: (i) the FBs themselves, using the low‑latitude spectrum reported by Ackermann et al. (2017). The spectrum is represented by a power‑law with an exponential cutoff (PLEC): dN/dE = N₀ (E/1 GeV)⁻ᵞ exp(−E/E_cut), with N₀ = 9 × 10⁻¹¹ cm⁻² s⁻¹ TeV⁻¹, γ = 2.03, and a fiducial cutoff energy E_cut = 1 TeV. (ii) the interstellar emission (IE), for which four benchmark models from De La Torre et al. (2023) are considered; the “VariableMin” model is used as the baseline, capturing uncertainties in the cosmic‑ray diffusion coefficient. (iii) the instrumental cosmic‑ray background, taken from the CTA IRFs. The analysis employs 55 logarithmically spaced energy bins between 30 GeV and 100 TeV.

A template‑fitting likelihood approach based on Poisson statistics is applied. The predicted counts in each spatial‑energy bin are expressed as a linear combination of the three templates, each scaled by a normalization factor (A_FB, A_IE, A_CR). The test statistic (TS) is defined as 2 ln(L₁/L₀), where L₁ is the likelihood with a free A_FB and L₀ is the likelihood with A_FB = 0. With one degree of freedom, a TS > 4 corresponds to a ≈2σ detection. Systematic uncertainties are incorporated via differential acceptance parameters α_ij (energy‑ and spatial‑dependent) and overall scaling parameters β_i, both treated as Gaussian nuisance parameters with standard deviations σ_α and σ_β. In the presented results β_i is fixed to 1, while σ_α is varied (1 %, 3 %, 10 %).

Key findings: (1) In the ideal case (no systematics) a 525 h exposure enables detection of the FB signal from ~60 GeV up to ~5 TeV at the 2σ level. (2) Introducing a 10 % acceptance systematic reduces the TS by up to 77 %, shifting the detectable lower bound to ~200 GeV, but still allowing a statistically meaningful measurement of the flux. (3) Reducing the exposure to 50 h (a factor of ten) still yields a detection between ~200 GeV and ~1 TeV, albeit with ~20 % larger flux uncertainties. (4) The ability to constrain the exponential cutoff energy is robust: simulated data with cutoffs ranging from 1 TeV to 300 TeV can be distinguished at the 3σ level for most cases, provided the spectral index is not too close to a pure power law. When the index approaches 2.0, the PLEC and simple power‑law models become harder to separate. (5) Systematic levels of 1 % and 3 % cause modest TS degradations (≈4 % and ≈36 % respectively), indicating that realistic control of instrumental systematics will preserve most of CTA’s sensitivity advantage over existing H.E.S.S. and HAWC limits.

The authors conclude that CTA‑South, even in its early “Alpha” configuration, is well positioned to detect the low‑latitude FB emission and to place meaningful constraints on a possible high‑energy cutoff. This would provide critical insight into the underlying particle acceleration mechanisms (e.g., hadronic versus leptonic processes, past activity of the central supermassive black hole, or star‑formation‑driven winds). Future work will extend the analysis to early‑array configurations, to the high‑latitude bubbles, and will explore multi‑wavelength synergies to further refine the physical interpretation of the FBs.