CHERCAM: A Cherenkov imager for the CREAM experiment

International audienceThe CREAM experiment (Cosmic Ray Energetics and Mass) is dedicated to the measurement of the energy spectrum of nuclear elements in cosmic rays, over the range 10$^{12}$ to 10$^{15}$ eV. The individual elements separation, which is a key feature of CREAM, requires instruments with strong identification capabilities. A proximity focused type of Cherenkov imager, CHERCAM (CHERenkov CAMera), providing both a good signature of downgoing Z=1 particles and good single element separation through the whole range of nuclear charges [Buénerd et al. 28th ICRC, Tsukuba, OG 1.5, 2003, p. 2157], is under development. After a brief introduction, the main features and the construction status of the CHERCAM are being summarized

Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope

The CAT Imaging Telescope has observed the BL Lac object Markarian 501 between March and August 1997. We report here on the variability over this time including several large flares. We present also preliminary spectra for all these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late

Very High Energy Gamma-ray spectral properties of Mrk 501 from CAT Cerenkov telescope observations in 1997

The BL Lac object Mrk 501 went into a very high state of activity during 1997, both in VHE gamma-rays and X-rays. We present here results from observations at energies above 250 GeV carried out between March and October 1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum between 30 GeV and 13 TeV shows significant curvature and is well represented by phi_0 * E_TeV^{-(alpha + beta*log10(E_TeV))}, with: phi_0 = 5.19 +/- 0.13 {stat} +/- 0.12 {sys-MC} +1.66/-1.04 {sys-atm} * 10^-11 /cm^2/s/TeV alpha = 2.24 +/- 0.04 {stat} +/- 0.05 {sys} beta = 0.50 +/- 0.07 {stat} (negligible systematics). The TeV spectral energy distribution of Mrk 501 clearly peaks in the range 500 GeV-1 TeV. Investigation of spectral variations shows a significant hardness-intensity correlation with no measurable effect on the curvature. This can be described as an increase of the peak TeV emission energy with intensity. Simultaneous and quasi-simultaneous CAT VHE gamma-ray and BeppoSAX hard X-ray detections for the highest recorded flare on 16th April and for lower-activity states of the same period show correlated variability with a higher luminosity in X-rays than in gamma-rays. The observed spectral energy distribution and the correlated variability between X-rays and gamma-rays, both in amplitude and in hardening of spectra, favour a two-component emission scheme where the low and high energy components are attributed to synchrotron and inverse Compton (IC) radiation, respectively.Comment: Submitted to Astronomy and Astrophysics, 8 pages including 6 figures. Published with minor change

A Cherenkov imager for charge measurements of Nuclear Cosmic Rays in the CREAM II instrument

A proximity focusing Cherenkov imager for the charge measurement of nuclear cosmic rays in the CREAM II instrument, called CHERCAM, is under construction. This imager consists of a silica aerogel radiator plane facing a detector plane equipped with standard photomultipliers. The two planes are separated by a minimal ring expansion gap. The Cherenkov light yield is proportional to the squared charge of the detected particle. The expected relative light collection accuracy is in the few percents range. It should lead to single element separation over the range of nuclear charge Z of main interest 1 $leq$ Z $<$\approx$ 26

Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope

The CAT imaging telescope on the site on the former solar plant Themis has been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997. This source is shown to be highly variable and the light curve is presented. The detected gamma-ray rate for the most intense flare is in excess of 10 per minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba

Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope

The CAT imaging telescope, recently built on the site of the former solar plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from October 1996 to March 1997. This steady source, often considered as the standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam to probe the performances of the new telescope, particularly its energy threshold (220 GeV at 20 degrees zenith angle) and the stability of its response. Due to the fine-grain camera, an accurate analysis of the longitudinal profiles of shower images is performed, yielding the source position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba

The CAT Imaging Telescope for Very-High-Energy Gamma-Ray Astronomy

The CAT (Cherenkov Array at Themis) imaging telescope, equipped with a very-high-definition camera (546 fast phototubes with 0.12 degrees spacing surrounded by 54 larger tubes in two guard rings) started operation in Autumn 1996 on the site of the former solar plant Themis (France). Using the atmospheric Cherenkov technique, it detects and identifies very high energy gamma-rays in the range 250 GeV to a few tens of TeV. The instrument, which has detected three sources (Crab nebula, Mrk 421 and Mrk 501), is described in detail.Comment: 24 pages, 15 figures. submitted to Elsevier Preprin

CHERCAM: the Cherenkov imager of the CREAM experiment, results in Z=1 test beams

International audienceThe CREAM experiment investigates the high energy spectrum of nuclear elements from H to Fe in the cosmic ray flux up to $10^{15}$ eV, with an instrument designed to achieve individual elements separation over the whole mass range. A proximity focused Cherenkov imager, CHERCAM (CHERenkov CAMera), will provide both a good topological signature (Cherenkov ring) for downgoing Z=1 particles, and a charge independent individual element separation through the considered range of nuclear charges. It will be implemented in the forthcoming CREAM flight 3. The contribution reports on the CHERCAM main features and on the preliminary results from in-beam tests at CERN

Detection of the BL Lac object 1ES1426+428 in the Very High Energy gamma-ray band by the CAT Telescope from 1998-2000

The BL Lac Object 1ES 1426+428, at a red-shift of z=0.129, has been monitored by the CAT telescope from February 1998 to June 2000. The accumulation of 26 hours of observations shows a gamma-ray signal of 321 events above 250 GeV at 5.2 standard deviations, determined using data analysis cuts adapted to a weak, steep-spectrum source. The source emission has an average flux of Phi_diff(400 GeV) = 6.73 +/- 1.27stat +/- 1.45syst x 10^-11 /cm^-2/s/TeV, and a very steep spectrum, with a differential spectral index of gamma = -3.60 +/- 0.57 which can be refined to gamma = -3.66 +/- 0.41 using a higher flux data subset. If, as expected from its broad-band properties, the Very High Energy emission is hard at the source, these observations support a strong absorption effect of gamma-rays by the Intergalactic Infrared field.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter

Temporal and spectral gamma-ray properties of Mkn421 above 250 GeV from CAT observations between 1996 and 2000

The gamma-ray emission above 250 GeV from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source produced a series of small flares, making it the second extragalactic source detected by CAT. The time-averaged differential spectrum has been measured from 0.3 to 5 TeV, which is well fitted with a power law with an index of -2.88+-0.12(stat)+-0.06(syst). In 2000, the source showed an unprecedented activity, with variability time-scales as short as one hour, as for instance observed during the night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvature is found between 0.3 and 5 TeV. The possibility of TeV spectral hardening during flares is also discussed, and the results are compared to those obtained on the other TeV BL Lac, Markarian 501.Comment: Latex2e, 11 pages, 7 figures, accepted for publication in Astronomy & Astrophysic