P. N. Bhat

Angular resolution of the Pachmarhi array of Čerenkov telescopes

Abstract The Pachmarhi array of Cerenkov telescopes consists of a distributed array of 25 telescopes that are used to sample the atmospheric Cerenkov photon showers. Each telescope consists of seven parabolic mirrors each viewed by a single photo-multiplier tube (PMT). Reconstruction of photon showers are carried out using fast timing information on the arrival of pulses at each PMT. The shower front is fitted to a plane and the direction of arrival of primary particle initiating the shower is obtained. The error in the determination of the arrival direction of the primary has been estimated using the split array method. It is found to be ∼2.4 ′ for primaries of energy>3 TeV. The dependence of the angular resolution on the separation between the telescopes and the number of detectors are also obtained from the data.

Rise and decay time of subpeaks in short duration bursts

Temporal profiles of subpeaks detected in GRBs, belonging to a sample of 65 short duration bursts selected from the 3B catalog, have been studied by fitting the former with lognormal functions since most subpeaks exhibit fast rise and slow decay. We present statistical evidence for a systematic increase in the ratio of rise to decay time with peak position. Using weighted correlation analysis we find that decay time displays a strong anti-correlation with peak position. On the other hand, no significant correlation between rise time and instant of peak maximum has been observed. If subpeaks in a burst arise due to emission from distinct shells then those scenarios in which later shells either suffer less deceleration or have larger speeds so that they catch up with slower matter earlier, are favored as they provide the required explanation for the observed decay time versus peak position anti-correlation.

Pachmarhi Array of Čerenkov Telescopes

Pachmarhi Array of Cerenkov Telescopes (PACT) has been designed to search for celestial TeV γ-rays using the wavefront sampling technique. PACT consists of 25 telescopes deployed over an area of 80u200am×100u200am. A telescope consists of 7 parabolic reflectors, each viewed by a fast phototube behind a 3° mask at the focus. The density and arrival time of photons at the PMT are measured. The energy threshold and collection area of the array are estimated, from Monte Carlo simulations, to be ∼1 TeV and 105u200am2 respectively. The accuracy in determination of arrival angle of a shower was estimated to be 0.1° in the near vertical direction. About 99% of the off-axis hadronic events could be rejected from directional information alone. Further, nearly 75% of the on-axis hadronic events could be rejected using species sensitive measurements like the photon density fluctuations. These cuts on data to reject back-ground would retain ∼44% of the γ-ray signal. The sensitivity of the array for a 5σ detection of γ-ray signal ...

Asymmetric subpeaks in short duration bursts

Subpeaks in 65 short duration gamma ray bursts belonging to the 3B catalogue have been identified and are fitted with lognormal functions as most of them are of FRED kind. Characterising the symmetry of a subpeak by the ratio of rise and decay time, we find that statistically the first peak of a burst tends to be more asymmetric than the subsequent ones. In this work we have also studied the correlations between various parameters like duty cycle, hardness ratio, rapidity index etc. that characterize a burst.

Gamma Ray and Hadron generated Čerenkov Photon Spectra at Various Observation Altitudes

We study the propagation of Čerenkov photons generated by Very HighEnergy γ-rays and hadrons in the atmosphere. The photon productionheight distributions are estimated from semi-empirical methods andcompared with those derived by standard simulation techniques. Incidentspectra at various observation altitudes are then derived after applyingwavelength dependent corrections due to photon attenuation in theatmosphere during the propagation of photons from the height of productionto the height of observation. These are generated both for γ-ray and hadron primaries of various energies. The derivedproduction height distributions agree very well with those generated bythe simulation package `CORSIKA at all energies and for both γ-ray and proton primaries. The incident photon spectra are found to beboth altitude and primary energy dependent. The peak ofthe incident spectrum shifts towards the shorter wavelength withincreasing altitude of observation for a given primary. Also the peak ofthe photon spectrum shifts towards the shorter wavelength withincreasing energy of the primary at a given altitude. The fraction of the UVcomponent in the incident Čerenkov spectrum is estimated both forγ-ray and hadronic primaries at various observation altitudes andenergies. Hadron generated Čerenkov spectra are marginally richer in UVlight and the difference increases slightly at higher altitudes. The fraction of the UV to the visible light in the Čerenkov spectrum could be a usefulparameter to separate γ-rays from cosmic ray background only if onecan measure this fraction very accurately.

Search for UHE counter-parts of Gamma Ray Bursts

Several Gamma Ray Bursts detected in the keV energy range by the Burst and Transient Source Experiment (BATSE) have been observed by the Energetic Gamma Ray Experiment Telescope (EGRET) in the MeV/GeV energy range. The energy spectra of γ-rays in some of the bursts continues from keV to GeV without a break or any evidence for a high energy cut-off. If the same slope continues up to the PeV range the expected γ-ray flux falls within the sensitivity of the Extensive Air Shower array operating at the Kolar Gold Fields, India. In view of the enormous astrophysical implications of the results, we have conducted a systematic search for PeV γ-rays from the locations of about 39 high fluence GRB’s detected during April 1991 and March 1993. The results of a search from 8 highest fluence GRB’s are presented here.