PACHMARHI ARRAY OF CERENKOV TELESCOPES

Bhat, P. N.

B. S. Acharya, V. R. Chitnis, P. Mazumdar, M. A. Rahman, M. R. Krishnaswamy and P. R. Vishwanath

Abstract

Pachmarhi Array of Cerenkov Telescopes (PACT) has been designed to search for TeV  g- rays using the wavefront sampling technique. PACT consists of 25 telescopes deployed over an area of 80 m×100 m, while each telescopes consists of 7 parabolic reflectors with a fast phototube behind a 3 (FWHM) mask at the focus of each of them. Photon density and arrival time (correct to a quarter of a nanosecond) are measured at six of the seven mirrors. The energy threshold and collection area of the array are estimated from a Monte Carlo technique using the measured event rate to be ~ 1  TeV and 105 m2 respectively. Recent engineering runs taken from this array enable us to calculate the accuracy by which arrival angle for a near vertical shower can be estimated to be 0.1 around the threshold energy. This could deteriorate to ~ 0.12 for showers at around 30. The limiting angular accuracy will improve with increasing primary energy because of the increased number of degrees of freedom. The sensitivity of the array for a 5s detection of g-ray signal has been estimated to be ~ 4.1×10-12  photons cm-2 s-1, for an on source exposure of 50 hrs. While calculating this it is asumed that 99.75% of the off-axis hadronic events could be rejected from directional information and nearly 75% of the on-axis hadronic events could be rejected using species sensitive measurements like the photon density fluctuations. In addition, it is assumed that ~ 66% of the g-rays are lost in the hadron rejection process. Consequently, the continuum emission from the Crab nebula could be detected at 5s level during an on-source exposure of 4 hours.


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On 23 Aug 1999, 11:46.