[Paper Review] Review of the Solar Array Telescopes

D. A. Smith|arXiv (Cornell University)|Aug 11, 2006

Solar and Space Plasma Dynamics1 references3 citations

TL;DR

This paper reviews solar array telescopes—ground-based Cherenkov detectors using large heliostat mirror arrays—to explore the 50–100 GeV gamma-ray window between EGRET and Cherenkov imaging telescopes. By sampling the Cherenkov wavefront with thousands of m² mirror areas, experiments like STACEE, CELESTE, and CACTUS achieved sub-100 GeV sensitivity, making the first differential flux measurements of the Crab, Mrk 421, and Mrk 501, and setting key upper limits on SUSY and blazar emissions.

ABSTRACT

For several years the only experiments sensitive to astrophysical gamma rays with energies beyond the reach of EGRET but below that of the Cherenkov imaging telescopes have been the "solar tower" detectors. They use >2000 m2 mirror areas to sample the Cherenkov wavefront generated by <100 GeV gamma rays, obtaining Crab sensitivities of more than 6$σ$ in one ON-source hour. I will review the history of the solar tower Cherenkov experiments from 1992 to the present and their key design features. I will describe some successful analysis strategies, then summarize the principal results obtained.

Motivation & Objective

• To explore the uncharted 50–100 GeV gamma-ray energy window between EGRET and Cherenkov imaging telescopes.
• To evaluate wavefront sampling using large solar tower mirror arrays as a viable alternative to imaging for low-energy gamma-ray detection.
• To achieve high sensitivity to faint astrophysical gamma-ray sources, such as the Crab nebula and blazars, in the 50–100 GeV range.
• To test theoretical models of dark matter, proton interactions, and inverse Compton scattering using upper limits from gamma-ray observations.
• To train a new generation of researchers in atmospheric Cherenkov techniques, contributing to future projects like HESS and VERITAS.

Proposed method

• Utilized large heliostat mirror arrays (over 2500 m²) at solar tower facilities to collect and focus Cherenkov light from extensive air showers.
• Employed wavefront sampling via photomultiplier tubes at the focal point of the heliostat array to measure intensity and arrival time of Cherenkov photons across the light pool.
• Applied Flash ADCs and programmable time delays to maintain sub-ns timing resolution despite changing optical path lengths due to Earth's rotation.
• Optimized signal-to-noise ratio by using one heliostat per channel with individual FADCs, minimizing night-sky background integration.
• Used event-by-event energy reconstruction to produce differential flux spectra, enabling precise spectral measurements of sources like Mrk 421.
• Implemented advanced analysis strategies, including pulse shape discrimination and background rejection, to achieve high significance detections.

Experimental results

Research questions

• RQ1 Can wavefront sampling with large heliostat arrays provide competitive sensitivity to gamma rays below 100 GeV, where imaging telescopes have limited performance?
• RQ2 What are the key design trade-offs between wavefront sampling and imaging in atmospheric Cherenkov detection at energies below 100 GeV?
• RQ3 Can solar tower experiments achieve sub-6σ sensitivity to the Crab nebula in one hour of observation, as demonstrated by STACEE and CELESTE?
• RQ4 What constraints do upper limits from solar tower experiments place on theoretical models of dark matter annihilation and proton-induced gamma-ray emission?
• RQ5 How do environmental and operational challenges—such as cloud cover and site-specific background—impact the performance and longevity of solar tower gamma-ray telescopes?

Key findings

• STACEE and CELESTE achieved more than 6σ sensitivity to the Crab nebula in a single ON-source hour, demonstrating high sensitivity in the 50–100 GeV range.
• The first differential flux measurements of the Crab nebula and Mrk 421 were made in the 50–100 GeV window, with energy reconstruction performed on an event-by-event basis.
• CELESTE set a stringent upper limit on gamma-ray emission from the Andromeda galaxy (M31), excluding certain SUSY neutralino annihilation models.
• GRAAL detected the Crab nebula with an energy threshold of 250 ± 110 GeV, confirming the feasibility of wavefront sampling despite a limited field of view.
• STACEE's upper limit on the Crab pulsar remains among the best ever achieved until the launch of GLAST.
• The CACTUS experiment demonstrated that Flash ADCs can enable efficient signal identification across multiple heliostats, reducing background integration and enabling scalable array designs.