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

M. Schroedter|arXiv (Cornell University)|Aug 3, 2009

Gamma-ray bursts and supernovae3 citations

TL;DR

This study searches for short gamma-ray bursts above 100 MeV from the Crab pulsar using simultaneous observations from VERITAS and the SGARFACE experiment. Despite high sensitivity to bursts of 1–15 μs duration, no coincident events were detected over 6.3 hours of observation, setting the most stringent upper limit to date on such bursts above 1 GeV with sub-microsecond timescales.

ABSTRACT

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 $μ$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.

Motivation & Objective

• To test the theoretical prediction that giant radio pulses (GRPs) in the Crab pulsar may produce short gamma-ray bursts of ~30 GeV via curvature radiation.
• To search for gamma-ray bursts above 100 MeV with durations between 10 ns and 15 μs, using ground-based atmospheric-Cherenkov telescopes.
• To improve sensitivity to short, high-energy bursts by combining data from VERITAS and the SGARFACE experiment, which piggybacks on the Whipple 10 m telescope.
• To establish the most stringent upper limits on the rate of such bursts by leveraging coincidence timing and background rejection techniques.
• To assess the feasibility of detecting microsecond-scale GeV bursts from pulsars using current IACT arrays and novel trigger systems.

Proposed method

• Simultaneous observations of the Crab pulsar were conducted using VERITAS and the SGARFACE experiment, which is attached to the Whipple 10 m telescope.
• SGARFACE uses 55 summed PMT channels sampled at 20 ns intervals over a 35.04 μs memory depth, with FADCs designed to preserve charge integration over time.
• A multi-time-scale discriminator (MTD) with six time constants (60 ns to 14.58 μs) and pattern-sensitive logic requiring 7 adjacent triggered channels ensures high background rejection.
• VERITAS triggers on three neighboring PMTs exceeding ~5 photoelectrons within 7 ns, with a minimum detectable fluence of 0.02 γ/m² for 1 GeV bursts over 7 ns.
• Coincident events were searched for using a 1 μs coincidence window, with random coincidence rates estimated by offsetting data sets by 1 second.
• Monte Carlo simulations using KASCADE and GrISU(tah) modeled Cherenkov light patterns and telescope responses to determine sensitivity as a function of burst duration and energy.

Experimental results

Research questions

• RQ1 Can short gamma-ray bursts above 100 MeV, lasting 1–15 μs, be detected from the Crab pulsar using atmospheric-Cherenkov telescopes?
• RQ2 Is there a detectable correlation between giant radio pulses (GRPs) in the Crab pulsar and high-energy gamma-ray bursts via curvature radiation?
• RQ3 What is the sensitivity of VERITAS and SGARFACE to microsecond-scale gamma-ray bursts above 1 GeV?
• RQ4 How effective is the coincidence technique between two independent IACT systems in rejecting cosmic-ray background for burst detection?
• RQ5 What are the most stringent upper limits on the rate of such short gamma-ray bursts from the Crab pulsar?

Key findings

• No coincident gamma-ray burst events were detected in 6.3 hours of simultaneous VERITAS and SGARFACE observations.
• The 99% upper limit on the true burst rate is 0.73 hour⁻¹, assuming a Poisson distribution and accounting for background fluctuations.
• The expected number of random coincidences within a 1 μs window is 0.1, estimated from offset data sets.
• The combined VERITAS-SGARFACE system achieves sufficient sensitivity to detect bursts of 50 GeV fluence predicted by curvature radiation models.
• SGARFACE achieves 98% background rejection for cosmic rays and atmospheric phenomena through time-resolved imaging and pattern-based triggers.
• The sensitivity to bursts decreases as 1/τ, with VERITAS being less sensitive than SGARFACE for durations longer than 100 ns, which limits the upper bound on longer bursts.