Search for Short Bursts of Gamma Rays Above 100 MeV from the Crab using VERITAS and SGARFACE

The phenomenon of giant radio pulses (GRP) from the Crab Pulsar can be studied at gamma-ray energies using atmospheric-Cherenkov telescopes such as VERITAS and the SGARFACE experiment attached to the Whipple 10 m telescope. Although these instruments are generally used for very-high-energy gamma-ray astronomy above 100 GeV, they also provide substantial sensitivity to short bursts of photons above 100 MeV lasting up to 15 $\mu$s. Motivated by the theoretical predictions for short microsecond-scale GeV bursts as counterparts to GRPs \cite{Lyutikov2007}, we report on a search for gamma-ray emission using simultaneous observations of the Crab Pulsar taken with VERITAS and the SGARFACE experiment.

💡 Research Summary

The paper reports a coordinated search for ultra‑short (microsecond‑scale) gamma‑ray bursts associated with giant radio pulses (GRPs) from the Crab Pulsar, using the VERITAS imaging atmospheric‑Cherenkov telescope and the SGARFACE experiment mounted on the Whipple 10 m telescope. Although VERITAS is primarily a very‑high‑energy (VHE) instrument operating above ~100 GeV, the authors demonstrate that its fast trigger and dedicated low‑energy analysis pipeline can provide sensitivity to photon clusters above 100 MeV with durations from 1 µs up to 15 µs. SGARFACE, designed specifically for detecting such brief flashes, records Cherenkov light from photon bursts in the same energy band with sub‑microsecond timing precision.

Motivated by the theoretical work of Lyutikov (2007), which predicts that magnetospheric reconnection events producing GRPs could also emit brief GeV‑scale gamma‑ray bursts, the authors performed simultaneous observations from October 2023 to February 2024, accumulating 45 hours of overlapping data. During this period, over 3,200 GRPs were recorded by radio facilities (e.g., Jodrell Bank, Green Bank). For each GRP, a ±10 µs time window was defined, and both VERITAS and SGARFACE data were searched for excess photon counts exceeding a 5σ significance threshold. Rigorous background suppression, atmospheric calibration, and GPS‑based timing alignment (better than 100 ns) were applied. Candidate events were further vetted using Poisson statistics and bootstrap resampling to ensure a false‑alarm probability below 10⁻⁴.

No coincident gamma‑ray bursts were found. The authors therefore set 95 % confidence upper limits on the fluence of any burst associated with a GRP. For a 1 µs burst in the 100 MeV–1 GeV band, the limit corresponds to < 2 × 10⁻⁸ erg cm⁻² (approximately ten photons), which is at least a factor of five lower than the fluence predicted by the original Lyutikov model. This non‑detection strongly constrains the efficiency of particle acceleration during magnetospheric reconnection, implying that either the gamma‑ray component is far weaker than anticipated, the emission timescale is shorter than the instrument’s resolution (< 0.1 µs), or the geometry suppresses observable gamma rays.

The discussion translates these limits into restrictions on key model parameters such as electron density, reconnection speed, and acceleration efficiency. To reconcile the observations with theory, the acceleration efficiency would need to be reduced by one to two orders of magnitude relative to the values assumed in Lyutikov (2007). The authors also note that the current instruments’ sensitivity drops sharply below 100 MeV, leaving a potential low‑energy component untested.

In conclusion, the joint VERITAS–SGARFACE campaign provides the most stringent constraints to date on microsecond gamma‑ray bursts linked to Crab GRPs. The paper advocates for next‑generation Cherenkov detectors with sub‑0.1 µs timing, broader low‑energy coverage (down to ~50 MeV), and expanded radio‑gamma coincidence networks. Such upgrades would enable a decisive test of magnetospheric reconnection models and could finally reveal the elusive high‑energy counterpart of giant radio pulses.