T. Sako

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

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 1024 Hz). Here we present the first sub-100 GeV detection by a ground-based telescope of a gamma-ray source, the Crab Nebula, in the energy region between satellite measurements and imaging atmospheric Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma-ray rate of 6.1 ± 0.8 minute-1. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma-ray flux of I(E > 60GeV) = 6.2 × 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 less than 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, then E0 < 26 GeV. This is the lowest energy probed by a Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

Very High Energy Gamma-Ray Observations of PSR B1509–58 with the CANGAROO 3.8 Meter Telescope

The gamma-ray pulsar PSR B1509-58 and its surrounding nebulae have been observed with the Collaboration of Australia and Nippon for a Gamma-Ray Observatory in the Outback (CANGAROO) 3.8 m imaging atmospheric Cerenkov telescope. The observations were performed from 1996 to 1998 in Woomera, South Australia, under different instrumental conditions with estimated threshold energies of 4.5 (1996), 1.9 (1997), and 2.5 TeV (1998) at zenith angles of ~30°. Although no strong evidence of gamma-ray emission was found, the lowest energy threshold data of 1997 showed a marginal excess of gamma-ray-like events at the 4.1 σ significance level. The corresponding gamma-ray flux is calculated to be (2.9 ± 0.7) × 10-12 cm-2 s-1 above 1.9 TeV. The observations of 1996 and 1998 yielded only upper limits (99.5% confidence level) of 1.9 × 10-12 cm-2 s-1 above 4.5 TeV and 2.0 × 10-12 cm-2 s-1 above 2.5 TeV. Assuming that the 1997 excess is due to very high energy (VHE) gamma-ray emission from the pulsar nebula, our result, when combined with the X-ray observations, leads to a value of the magnetic field strength 5 μG. This is consistent with the equipartition value previously estimated in the X-ray nebula surrounding the pulsar. No significant periodicity at the 150 ms pulsar period has been found in any of the three years data. The flux upper limits set from our observations are 1 order of magnitude below previously reported detections of pulsed TeV emission.

CELESTE: an atmospheric Cherenkov telescope for high energy gamma astrophysics

Abstract CELESTE is an atmospheric Cherenkov telescope based on the sampling method which makes use of the de-commissioned THEMIS solar electrical plant in the French Pyrenees. A large (2000 m 2 ) mirror surface area from 40 independent heliostats followed by a secondary optic, a trigger system using analog summing techniques and signal digitization with 1 GHz flash ADCs make possible the detection of cosmic γ-rays down to 30 GeV . This paper provides a detailed technical description of the CELESTE installation.

The CANGAROO-III project

The CANGAROO-III project, which consists of an array of four 10 m imaging Cherenkov telescopes, has just started being constructed in Woomera, South Australia, in a collaboration between Australia and Japan. The first stereoscopic observation of celestial high-energy gamma-rays in the 100 GeV region with two telescopes will start in 2002, and the four telescope array will be completed in 2004. The concept of the project and the expected performance are discussed.

Mrk 421, Mrk 501, and 1ES 1426+428 at 100 GeV with the CELESTE Cherenkov telescope

We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in the energy range between 50 and 350 GeV (1.2 to 8.3 x 10^25 Hz). The detector, called CELESTE, used first 40, then 53 heliostats of the former solar facility Themis in the French Pyrenees to collect Cherenkov light generated in atmospheric particle cascades. The signal from Mrk 421 is often strong. We compare its flux with previously published multi-wavelength studies and infer that we are straddling the high energy peak of the spectral energy distribution. The signal from Mrk 501 in 2000 was weak (3.4 sigma). We obtain an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab flux near 100 GeV. The data analysis and understanding of systematic biases have improved compared to previous work, increasing the detectors sensitivity.

First detection of gamma rays from the crab nebula with the CELESTE “solar farm” Cherenkov detector

Abstract We have converted the THEMIS solar array (French Pyrenees) into an atmospheric Cherenkov telescope, called CELESTE, sensitive to astrophysical gamma rays above 30 GeV (7×10 24 Hz). In early 1998 the Crab nebula was detected at 80 GeV with a preliminary 18 heliostat setup. The full 40 heliostat array has since been commissioned. The STACEE experiment using the same technique in New Mexico is also analysing their first data. Thus, the window between the EGRET instrument and the Cherenkov imagers has been opened. We describe the CELESTE detector and the data analysis, and discuss the prospects for studying AGN (specifically, blazars) and galactic sources in this energy range.

Initial performance of CANGAROO-II 7m telescope

CANGAROO group constructed an imaging air Cherenkov telescope (CANGAROO-II) in March 1999 at Woomera, South Australia to observe celestial gamma-rays in hundreds GeV region. It has a 7m parabolic mirror consisting of 60 small plastic spherical mirrors, and the prime focus is equipped with a multi-pixel camera of 512 PMTs covering the field of view of 3 degrees. We report initial performance of the telescope.