The search for short-term flares in extended VHE Crab Nebula observations with the Whipple 10m telescope

TL;DR

This study analyzed a decade of Whipple telescope data to search for short-term TeV gamma-ray flares from the Crab Nebula, finding no significant flaring activity but setting upper limits on flare frequency.

Contribution

It is the first comprehensive search for short-term TeV flares in the Whipple archive, establishing upper limits on flare occurrence rates.

Findings

No significant flares detected in the data set.

Upper limits of 0.02 and 0.27 flares per year for five-fold and two-fold flares.

Detection of 1.5-fold flares remains inconclusive due to false-positive rate.

Abstract

In 1989, the Whipple 10m telescope achieved the first indisputable detection of a TeV gamma-ray source, the Crab Nebula. Until its decommissioning in 2011, the Whipple telescope took regular measurements of the nebula. With the recent discovery of GeV gamma-ray flaring activity in the Crab Nebula, it is an opportune time to return to the Whipple telescope data set and search its extensive archive for evidence of TeV flares. A data set on the Crab Nebula spanning ten years, 2000 - 2010, is compiled and searched for day-scale flaring activity using a Bayesian-block binning algorithm. No evidence for significant flaring activity is found. Monte Carlo simulations show that low levels of flux increase on short timescales are difficult to detect. Assuming a flare duration of seven days, 99% confidence level upper limits are calculated for the possible frequency of five-fold, two-fold and 1.5-fold flares in the data set. An upper limit of 0.02 flares per year is found for the five-fold flare, and a limit of 0.27 flares per year is placed on the two-fold flare. The detection of the 1.5-fold flare is consistent with the false-positive rate of the method, and so cannot be excluded.