Calibration of the Sensitivity of Imaging Atmospheric Cherenkov Telescopes using a Reference Light Source

Abstract

The sensitivity of an Imaging Atmospheric Cherenkov telescope is calibrated by shining, from a distant pulsed monochromatic light source, a defined photon flux onto the mirror. The light pulse is captured and reconstructed by the telescope in an identical fashion as real Cherenkov light. The intensity of the calibration light pulse is monitored via a calibrated sensor at the telescope; in order to account for the lower sensitivity of this sensor compared to the Cherenkov telescope, an attenuator is inserted in the light source between the measurements with the calibrated sensor, and with the telescope. The resulting telescope sensitivities have errors of 10%, and compare well with other estimates of the sensitivity. Imaging Atmospheric Cherenkov Telescopes (IACTs) have evolved into the most powerful tool for the study of galactic and extragalactic γ-ray sources in the TeV energy range [1]. In IACTs, a (frequently tesselated) reflector with areas between a few m and almost 100 m is used to image the Cherenkov photons emitted by an air shower onto a camera consisting of photomultiplier (PM) pixels. The elliptical shower image traces the longitudinal development of an air shower. The long axis of the image points to the image of the source. The shape of the image allows to distinguish, to a certain degree, compact γray induced showers from the more diffuse cosmic-ray showers [2]. The power of IACTs can be improved significantly by operating multiple IACTs in a stereo mode, observing the same shower with several IACTs in coincidence. Stereo imaging allows the unambiguous spatial reconstruction of the direction of individual air showers with a precision of 0.1 and better [3–5], and therefore provides the best angular resolution of all tools in γ-ray astronomy. As the field matures, emphasis is shifting from the simple detection of TeV γray sources to the precise determination of source fluxes and their spectra. The spectra contain important clues both concerning the acceleration mechanisms in galactic and extragalactic particle sources, and concerning the propagation of γ-rays and their interaction in particular with extragalactic radiation fields. One of the key issues in the measurement of fluxes and spectra is the energy calibration of an IACT, i.e, the determination of the relation between the signal size (measured, e.g., in units of ADC counts) and the incident photon flux, or ultimately, the energy of the air shower. Because of the steeply falling spectra, calibration errors are amplified in the calculation of integral fluxes above a certain energy threshold. Lacking a suitable monochromatic “test beam”, the energy calibration of IACTs has to be derived indirectly. Presently, uncertainties in the energy calibration of IACT frequently range in the 20% to 30% region and the resulting systematic errors are the dominant term in measurements of the γ-ray flux. In this paper, we describe a technique to calibrate, in one single step, the response of an IACT and its readout chain. The paper is structured as follows: the next section (1) contains a quantitive discussion of the IACT calibration issue, and gives examples of calibration techniques. In the following sections (2 and 3), our technique is introduced, and the implementation is described. A final section (4) is dedicated to the discussion of the results and error sources. 1 Response and energy calibration of IACTs For the calibration of the response of an IACT, two coefficients are relevant. The relation between light intensity (in photons/m) and the digitized telescope signal (in ADC channels) provides an overall scale factor in estimates of shower energies. The signal per single photoelectron is needed in addition to obtain the actual number of photoelectrons in a given image, which determines the size of fluctuations around the mean response. Scope of this paper is the determination of the first of these two coefficients; methods to determine the second are discussed e.g. in [6–8]. Usually, the total intensity M of a Cherenkov image, given in units of ADC channels, is used as a measure of the energy of a γ-ray shower. The radial dependence of the intensity of the Cherenkov light is taken into account by either selecting events with impact points within a certain limited distance range, or by explicit estimation of the impact distance and corresponding correction factors. Let I(ν) be the intensity 1 (photons of frequency ν per unit area) the air shower would have generated at the location of the telescope without intermediate absorption or scattering in the atmosphere, and Tatm(ν) the atmospheric transmission (which of course depends on the height distribution of photon emission). The magnitude M of the image can then be written in 1 In the context of photon emission from air shower, we use the terms “photon flux” and “photon intensity” as synonyms.

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Cite this paper

@inproceedings{Fra1997CalibrationOT, title={Calibration of the Sensitivity of Imaging Atmospheric Cherenkov Telescopes using a Reference Light Source}, author={A. Fra\ss and C. O. K{\"{o}hler and Gunter Hermann and Maurus Hess and Werner Hofmann}, year={1997} }