Measurement of the Crab Flux above 60 GeV with the CELESTE Cerenkov Telescope

Abstract

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmosphericČerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 10 24 Hz). Here we present the first sub-100 GeV -2detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospherič Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma ray rate of 6.1 ± 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E > 60 GeV) = 6.2 +5.3 −2.3 × 10 −6 photons m −2 s −1 . The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be ¡12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e −E/E 0 then E 0 < 26 GeV. This is the lowest energy probed by ǎ Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

Search for diffuse gamma rays from the Galactic anticentre region above 100 GeV with CELESTE

“…The MC simulations of a γ-ray point source reproduce the Crab signal obtained by ON-OFF subtraction (de Naurois et al 2002;Britto 2006). This good agreement between our simulations and true data allowed us to optimize the hadron rejection cuts for different diffuse emission models by simulating the corresponding spectra.…”

“…Figure 3 shows that the OFF-OFF analysis presented here yields significances that are consistent with 0 for the 11 selected pairs (corresponding to 150 mn of data) where no signal is expected. In the right panel, we also compare our x eff analysis results with those of the standard analysis completed by applying a separate cut to each variable (de Naurois et al 2002).…”

“…Large collection area and fast acquisition electronics system (∼940 MHz) for sampling the Cherenkov pulses allowed CELESTE to be the first ground-based Cherenkov telescope with a trigger threshold of below 100 GeV (de Naurois et al 2002).…”

Precision measurement of optical pulsation using a Cherenkov telescope

“…The major new air-Cherenkov experiments for ground based γ-ray astronomy, such as H.E.S.S. [5], MAGIC [6] and VERITAS [7] exhibit greatly improved sensitivity around 100 GeV compared to previous instruments [8,9]. Detections or upper limits on pulsed emission from these new instruments will likely be based on data taken over months or even years.…”

“…The potential advantages of this approach include smaller fluctuations due to seeing (intensity fluctuations on small angular scales caused be atmospheric turbulence), potentially deadtime free operation and in general a different set of systematic effects. The idea of measuring the optical pulsations of pulsars with Cherenkov telescopes is at least a decade old and pulses from the Crab pulsar have been measured successfully by several instruments: Whipple [14], STACEE [15], CELESTE [9], and HEGRA [16], with a typical sensitivity of 0.1 σ/ t/s (i.e. a one second observation leads on average to an 0.1 standard deviation detection significance, increasing with the square-root of time, t).…”