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Search Results: 1 - 10 of 365660 matches for " P. N. Bhat "
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The role of fluctuations in atmospheric Cerenkov technique
P. N. Bhat
Physics , 1997,
Abstract: In the absence of a standard source of gamma rays or hadrons of known energy one has to study the details of production of \v Cerenkov light at the observation level only through detailed simulation studies. Recently such studies have become all the more important in view of the various techniques resulting from such studies, to distinguish between gamma ray initiated events from those generated by much more abundant hadronic component of cosmic rays. We have carried out a detailed simulation studies using the CORSIKA package in order to study the Cerenkov photon density fluctuations at various core distances both for photon and proton primaries incident vertically at the top of the atmosphere. It is found that the density fluctuations are significantly non-statistical. Such fluctuations are much more pronounced in the case of proton primaries as compared to photon primaries at all energies. Several statistical parameters have been computed some which might lead to a technique in distinguishing photon primaries.
Pachmarhi Array of Cerenkov Telescopes
P. N. Bhat
Physics , 1997,
Abstract: This talk is based on the Very High Energy Gamma Ray Astronomy observations planned to be carried out at Pachmarhi in the central Indian state of Madhya Pradesh using the well known atmospheric \v Cerenkov technique. The development of a ground based array of 25 \v Cerenkov telescopes is currently underway at Pachmarhi situated at an altitutde of about a kilometer. Using this array it is proposed to sample the \v Cerenkov light pool at various distances from the shower core in order to estimate the lateral distribution parameters of the shower. Simulation studies have shown that these parametrs would enable one to distinguish gamma ray initiated showers from those by cosmic ray charged particles, thus significantly improving the signal to noise ratio. After summarizing the genesis of VHE gamma ray astronomy in our institute we will discuss the scientific motivation of this concept of enriching the gamma ray signal as compared to the standard imaging technique. The current status of the detector development and the expected results will be presented.
Variability Time Scales of Long and Short GRBs
P. N. Bhat
Physics , 2013,
Abstract: Gamma-ray bursts (GRB) are extremely energetic events and produce highly diverse light curves. Light curves are believed to be resulting from internal shocks reflecting the activities of the GRB central engine. Hence their temporal studies can potentially lead to an understanding of the GRB central engine and its evolution. The light curve variability time scale is an interesting parameter which most models attribute to a physical origin e.g., central engine activity, clumpy circum-burst medium, or relativistic turbulence. We develop a statistical method to estimate the GRB minimum variability time scale (MVT) for long and short GRBs detected by GBM. We find that the MVT of short bursts is distinctly shorter than that of long GRBs supporting the possibility of a more compact central engine of the former. We also find that MVT estimated by this method is consistent with the shortest rise time of the fitted pulses. Hence we use the fitted pulse rise times to study the evolution of burst variability time scale. Variability time coupled with the highest energy photon detected in turn related to the minimum bulk Lorentz factor of the relativistic shell emitted by the inner engine. %Using this we relate the GRB spectral evolution to the evolution of the variability time scale.
An overview of the current understanding of Gamma-ray Bursts in the Fermi era
P. N. Bhat,S. Guiriec
Physics , 2011,
Abstract: Gamma-ray bursts are the most luminous explosions in the Universe, and their origin as well as mechanism are the focus of intense research and debate. More than three decades since their serendipitous discovery, followed by several breakthroughs from space-borne and ground-based observations, they remain one of the most interesting astrophysical phenomena yet to be completely understood. Since the launch of Fermi with its unprecedented energy band width spanning seven decades, the study of gamma-ray burst research has entered a new phase. Here we review the current theoretical understanding and observational highlights of gamma-ray burst astronomy and point out some of the potential promises of multi-wavelength observations in view of the upcoming ground based observational facilities.
Possible Discrimination between Gamma Rays and Hadrons using Cerenkov Photon Timing Measurements
V. R. Chitnis,P. N. Bhat
Physics , 2000, DOI: 10.1016/S0927-6505(00)00137-7
Abstract: Atmospheric \v{C}erenkov Technique is an established methodology to study $TeV$ energy gamma rays. However the challenging problem has always been the poor signal to noise ratio due to the presence of abundant cosmic rays. Several ingenious techniques have been employed to alleviate this problem, most of which are centred around the \v{C}erenkov image characteristics. However there are not many techniques available for improving the signal to noise ratio of the data from wavefront sampling observations. One such possible technique is to use the \v{C}erenkov photon arrival times and identify the species dependent characteristics in them. Here we carry out systematic monte carlo simulation studies of the timing information of \v{C}erenkov photons at the observation level. We have parameterized the shape of the \v{C}erenkov shower front as well as the pulse shapes in terms of experimentally measurable quantities. We demonstrate the sensitivity of the curvature of the shower front, pulse shape parameters as well as the photon arrival time jitter to primary species and show their efficiency in improving the signal to noise ratio. The effect of limiting the \v{C}erenkov telescope opening angle by using a circular focal point mask, onthe efficacy of the parameters has also been studied for each of the parameters. Radius of the shower front, pulse decay time and photon arrival time jitter have been found to be the most promising parameters which could be used to discriminate $\gamma -$ray events from the background. We also find that the efficiency of the first two parameters increases with zenith angle and efficiency of pulse decay time decreases with increasing altitude of observation.
Gamma-Hadron Separation using Cerenkov Photon Density Fluctuations
V. R. Chitnis,P. N. Bhat
Physics , 2003, DOI: 10.1023/A:1023784403643
Abstract: In the atmospheric Cerenkov technique gamma rays are detected against the abundant background produced by hadronic showers. In order to improve the signal to noise ratio of the experiment, it is necessary to reject a significant fraction of hadronic showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data from imaging telescopes. However, non-imaging Cerenkov telescopes have to develop very different means of statistically identifying and removing cosmic ray events. Some of the parameters which could be potentially important for non-imaging arrays are the temporal and spectral differences, the lateral distributions and density fluctuations of Cerenkov photons generated by gamma ray and hadron primaries. Here we study the differences in fluctuations of Cerenkov photon density in the light pool at the observation level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of about 90% can be achieved. Use of density fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the signal to noise ratio.
Gamma-Hadron Separation using ?erenkov Photon Density Fluctuations
P. N. Bhat,V. R. Chitnis
Physics , 2001,
Abstract: In atmospheric \v{C}erenkov technique $\gamma-$rays are detected against abundant background produced by hadronic showers. In order to improve signal to noise ratio of the experiment, it is necessary to reject a significant fraction of hadronic showers. The temporal and spectral differences, the lateral distributions and density fluctuations of \v{C}erenkov photons generated by $\gamma-$ray and hadron primaries are often used for this purpose. Here we study the differences in \v{C}erenkov photon density fluctuations at the observation level based on Monte Carlo simulations. Various types of density fluctuations like the short range (or local), medium range fluctuations and flatness parameter are studied. The estimated quality factors reflect the efficiencies with which the hadrons can be rejected from the data. It has been found that we can reject around 80% of proton showers while retaining about 70% of $\gamma-$ray showers in the data, based only on the differences in the flatness parameter. Density fluctuations particularly suited for wavefront sampling observations seem to be a good technique to improve the signal to noise ratio.
Estimation of Shower Parameters in Wavefront Sampling Technique
V. R. Chitnis,P. N. Bhat
Physics , 2001,
Abstract: Wavefront sampling experiments record arrival times of \v{C}erenkov photons with high precision at various locations in \v{C}erenkov pool using a distributed array of telescopes. It was shown earlier that this photon front can be fitted with a spherical surface traveling at a speed of light and originating from a single point on the shower axis. Radius of curvature of the spherical shower front ($R$) is approximately equal to the height of shower maximum from observation level. For a given primary species, it is also found that $R$ varies with the primary energy ($E$) and this provides a method of estimating the primary energy. In general, one can estimate the arrival times at each telescope using the radius of curvature, arrival direction of the primary and the core location. This, when compared with the data enables us to estimate the above parameters for each shower. This method of obtaining the arrival direction alleviates the difficulty in the form of systematics arising out of the plane wavefront approximation for the \v{C}erenkov front. Another outstanding problem in the field of atmospheric \v{C}erenkov technique is the difficulty in locating the shower core. This method seems to solve both these problems and provides an elegant method to determine the arrival direction as well as the core location from timing information alone. In addition, using the \v{C}erenkov photon density information and the core position we can estimate the energy of the primary if the nature of the primary is known. Combining these two independent estimates of the primary energy, the energy resolution can be further improved. Application of this methodology to simulated data and the results will be presented. The intrinsic uncertainties on the various estimated parameters also will be discussed.
Simulation Studies on Arrival Time Distributions of Cherenkov Photons in Extensive Air Showers
V. R. Chitnis,P. N. Bhat
Physics , 1999, DOI: 10.1016/S0927-6505(99)00013-4
Abstract: Atmospheric Cherenkov technique is an established methodology to study TeV energy gamma rays. Here we carry out systematic monte carlo simulation studies of the timing information of Cherenkov photons. Extensive studies have already been carried out in this regard. Most of these are carried out at higher energies with the aim of studying the elemental composition of cosmic rays. However not much attention is paid to the species dependent signatures at TeV energies. In this work, functional fits have been carried out to the spherical Cherenkov shower fronts and the radii of curvature have been found to be equal to the height of shower maximum irrespective of the species or the observation level. Functional fits have also been carried out to describe the pulse shapes at various core distances in terms of well known probability density distribution functions (PDF). Two types of PDF's have been tried viz. gamma function and lognormal function. The variation of the pulse shape parameters as a function of primary energy, observation height and incident angles have been studied. The possibility of deriving the pulse shape parameters like the rise & decay times, full width at half maximum from the easily measurable quantities like the mean and RMS variation of photon arrival times offers a very important new technique which can be easily applied in an observation.
Cerenkov Photon Density Fluctuations in Extensive Air Showers
V. R. Chitnis,P. N. Bhat
Physics , 1998, DOI: 10.1016/S0927-6505(98)00007-3
Abstract: The details of Cerenkov light produced by a gamma ray or a cosmic ray incident at the top of the atmosphere is best studied through systematic simulations of the extensive air showers. Recently such studies have become all the more important in view of the various techniques resulting from such studies, to distinguish gamma ray initiated showers from those generated by much more abundant hadronic component of cosmic rays. We have carried out here such systematic simulation studies using CORSIKA package in order to understand the Cerenkov photon density fluctuations for 5 different energies at various core distances both for gamma ray and proton primaries incident vertically at the top of the atmosphere. Such a systematic comparison of shower to shower density fluctuations for gamma ray and proton primaries is carried out for the first time here. It is found that the density fluctuations are significantly non-Poissonian. Such fluctuations are much more pronounced in the proton primaries than gamma ray primaries at all energies. The processes that contribute significantly to the observed fluctuations have been identified. It has been found that significant contribution to fluctuations comes from photons emitted after shower maximum. The electron number fluctuations and correlated emission of Cerenkov photons are mainly responsible for the observed fluctuations.
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