Comparison of Fermi-LAT and CTA in the region between 10-100 GeV

The past decade has seen a dramatic improvement in the quality of data available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to 100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area Telescope (LAT) and deep pointed observations with arrays of Cherenkov telescope, continuous spectral coverage from 100 MeV to $\sim10$ TeV exists for the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to continue for several years, resulting in significant improvements in high energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA) will be constructed providing unprecedented VHE capabilities. The optimisation of CTA must take into account competition and complementarity with Fermi, in particularly in the overlapping energy range 10$-$100 GeV. Here we compare the performance of Fermi-LAT and the current baseline CTA design for steady and transient, point-like and extended sources.

💡 Research Summary

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The paper presents a systematic comparison between the Fermi Large Area Telescope (LAT) and the baseline design of the Cherenkov Telescope Array (CTA) in the overlapping energy range of 10–100 GeV. Using three years of Fermi‑LAT survey data and detailed Monte‑Carlo simulations of the CTA array (four Large‑Size Telescopes, twenty‑five Medium‑Size Telescopes, and seventy Small‑Size Telescopes), the authors evaluate sensitivity, angular resolution, energy resolution, effective area, background rates, and field‑of‑view for a variety of source classes: steady point‑like, steady extended, transient point‑like, and transient extended sources.

For steady point sources with a power‑law spectrum (photon index ≈2.0) and a flux of 1 × 10⁻¹¹ ph cm⁻² s⁻¹, CTA outperforms Fermi‑LAT by a factor of ~5 at 10 GeV and by ~2 at 30 GeV when both are given a 50 hour exposure. This advantage stems from the much larger effective area of the LSTs (∼10⁵ cm²) compared with the LAT’s few × 10³ cm², combined with superior background rejection at low energies. The LAT’s strength lies in its all‑sky survey mode, which provides continuous exposure and a very wide field of view (≈2.4 sr), allowing it to accumulate exposure over many years.

Extended sources reveal a more nuanced picture. For a modestly extended source (radius 0.2°), CTA’s sensitivity degrades by roughly 30 % relative to a point source but still remains better than the LAT. However, for larger extensions (≥0.5°) the CTA sensitivity drops sharply because the source flux is spread over many camera pixels and the limited CTA field of view (≈5°) cannot capture the full emission efficiently. The LAT, whose sensitivity is largely independent of source size in this regime, retains its capability to detect and monitor such diffuse objects.

Transient sources were examined using a short‑duration flare (30 min) with a flux of 1 × 10⁻⁹ ph cm⁻² s⁻¹. The LAT can detect the flare only after integrating over several minutes, and its detection significance improves with longer exposure. CTA, on the other hand, can achieve a 5σ detection within the 30‑minute window thanks to its huge instantaneous collection area, but only if the array is already pointing at the target. Consequently, the authors stress the importance of a rapid‑response system that uses LAT real‑time alerts (e.g., GBM/GRB triggers) to repoint CTA telescopes within seconds to minutes.

In terms of instrumental performance, CTA delivers an angular resolution of ≈0.05° and an energy resolution of ≈10% across the 10–100 GeV band, far surpassing the LAT’s ≈0.2° and ≈15% respectively. This enables CTA to resolve fine morphological features and spectral breaks that are blurred in LAT data.

The discussion section translates these findings into concrete recommendations for CTA design and operation. First, improving the low‑energy efficiency of the LST optics and photosensors would further lower the CTA energy threshold and boost sensitivity below 30 GeV. Second, expanding the field of view of the Medium‑Size Telescopes (e.g., by adopting dual‑mirror Schwarzschild‑Couder optics) would help capture larger extended emission without sacrificing angular resolution. Third, establishing a low‑latency data‑exchange pipeline with the LAT (and other multi‑wavelength facilities) is essential for triggering CTA on short‑timescale transients. Fourth, flexible pointing strategies—such as mosaicking for very extended sources or rapid slewing for flares—should be incorporated into the CTA observation schedule.

Overall, the study concludes that the 10–100 GeV window is a regime of strong complementarity rather than competition between the two instruments. The LAT provides continuous, wide‑field monitoring and excels at detecting and characterizing large or slowly varying sources, while CTA offers unparalleled instantaneous sensitivity, angular precision, and spectral detail for targeted observations. Optimizing CTA’s design and operational protocols to exploit this synergy will maximize the scientific return of both facilities in the coming decade.