Pachmarhi Array of Cherenkov Telescopes (PACT)

PACT was operational at Pachmarhi (longitude 78°26' E. Latitude 22°28' N & altitude 1075m asl) in Madhya Pradesh during 2000-2012. This array consisted of 24 telescopes spread over the region of 80m * 100m. Array was divided into fours sectors of six telescopes each.

PACT array schematic

Each telescope consisted of seven para-axially mounted parabolic mirrors of diameter 0.9 m each with f/D=1. A fast Photo-Multiplier Tube of type EMI9807B was mounted at the focus of each mirror, behind a mask restricting field of view to 3 deg FWHM.

Telescopes had equatorial mounts and remote controlled Automated Computerized Telescopes Orientation System (ACTOS) was used for tracking sources as well as for slow or fast movements of telescopes. The hardware of ACTOS was designed in-house and consisted of a semi-intelligent closed loop feedback system with built-in safety features. This system could orient the telescopes to source in the sky from an arbitrary initial position with an accuracy of 0°.05.

Each of the four sector of PACT could be operated as an independent unit and had its own data acquisition system. These four field signal processing centers (FSPCs) processed and recorded data from telescopes in the sector. A central control room located at the centre of the array housed the master signal processing center (MSPC). Sector trigger was generated based on a coincidence of 4 out of 6 telescope pulses in 75 ns window. Data were recorded by MSPC on trigger from any of the sectors.

Pachmarhi Array of Cherenkov Telescopes
One of the telescopes from PACT
Closer view of Telescope
Field Signal Processing Center
Master Signal Processing Center

Angular resolution of PACT i.e. the error in the determination of the arrival direction of the primary was estimated using the split array method and was found to be ~ 2.4' for primaries of energy > 3 TeV.

(Ref. Majumdar et al., Astroparticle Physics, 18, 333, 2003).

Energy threshold was PACT was estimated to be 750 GeV (Bose et al. 2007).

Several blazar class AGNs like Mkn 421, Mkn 501, 1ES1426+428 and ON 231 were observed using PACT. Average flux from Mkn 421 during 2001-2005 was estimated to be (4.45 1.9) 10-12 photons/cm2/s at energies above 1.2 TeV. For Mkn 501, 1ES1426+428 and ON 231, upper limits on flux were estimated.

Multiwaveband spectral energy distribution (SED) for Mkn 421 was explained in terms of one zone synchrotron self-Compton (SSC) model.


Multiwaveband SEDs for Mkn 421 for flare and quiescent states

(Ref : D. Bose et al., Astrophysics and Space Science, 309, 111, 2007)

Geminga pulsar was observed extensively using PACT. Search for pulsations near 237 ms did not give statistically significant result and upper limit of pulsed flux was estimated for energies above 825 GeV.

(Ref. B. B. Singh et al., Astroparticle Physics, 32, 120, 2009)


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