# 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
10^{5} m^{2} 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.

File translated from T_{E}X by T_{T}H, version 2.32.

On 23 Aug 1999, 11:46.