arXiv subjects

P. N. Bhat

Publications and source records attributed to P. N. Bhat.

At least 55 records · Page 3Linked to original sources

arXiv2010

Time-Resolved Spectroscopy of the 3 Brightest and Hardest Short Gamma-Ray Bursts Observed with the FGST Gamma-Ray Burst Monitor

From July 2008 to October 2009, the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope (FGST) has detected 320 Gamma-Ray Bursts (GRBs). About 20% of these events are classified as short based on their T90 duration below 2 s. We present here for the first time time-resolved spectroscopy at timescales as short as 2 ms for the three brightest short GRBs observed with GBM. The time-integrated spectra of the events deviate from the Band function, indicating the existence of an additional spectral component, which can be fit by a power-law with index ~-1.5. The time-integrated Epeak values exceed 2 MeV for two of the bursts, and are well above the values observed in the brightest long GRBs. Their Epeak values and their low-energy power-law indices (α) confirm that short GRBs are harder than long ones. We find that short GRBs are very similar to long ones, but with light curves contracted in time and with harder spectra stretched towards higher energies. In our time-resolved spectroscopy analysis, we find that the Epeak values range from a few tens of keV up to more than 6 MeV. In general, the hardness evolutions during the bursts follows their flux/intensity variations, similar to long bursts. However, we do not always see the Epeak leading the light-curve rises, and we confirm the zero/short average light-curve spectral lag below 1 MeV, already established for short GRBs. We also find that the time-resolved low-energy power-law indices of the Band function mostly violate the limits imposed by the synchrotron models for both slow and fast electron cooling and may require additional emission processes to explain the data. Finally, we interpreted these observations in the context of the current existing models and emission mechanisms for the prompt emission of GRBs.

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arXiv2010

Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B

Observations of GRB 100724B with the Fermi Gamma-Ray Burst Monitor (GBM) find that the spectrum is dominated by the typical Band functional form, which is usually taken to represent a non-thermal emission component, but also includes a statistically highly significant thermal spectral contribution. The simultaneous observation of the thermal and non-thermal components allows us to confidently identify the two emission components. The fact that these seem to vary independently favors the idea that the thermal component is of photospheric origin while the dominant non-thermal emission occurs at larger radii. Our results imply either a very high efficiency for the non-thermal process, or a very small size of the region at the base of the flow, both quite challenging for the standard fireball model. These problems are resolved if the jet is initially highly magnetized and has a substantial Poynting flux.

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arXiv2010

Observations of Soft Gamma Ray Sources >100 keV Using Earth Occultation with GBM

The NaI and BGO detectors on the Gamma ray Burst Monitor (GBM) on Fermi are now being used for long term monitoring of the hard X-ray/low energy gamma ray sky. Using the Earth occultation technique demonstrated previously by the BATSE instrument on the Compton Gamma Ray Observatory, GBM produces multiband light curves and spectra for known sources and transient outbursts in the 8 keV - 1 MeV band with its NaI detectors and up to 40 MeV with its BGO. Coverage of the entire sky is obtained every two orbits, with sensitivity exceeding that of BATSE at energies below ~25 keV and above ~1.5 MeV. We describe the technique and present preliminary results after the first ~17 months of observations at energies above 100 keV. Seven sources are detected: the Crab, Cyg X-1, Swift J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient source XTE J1752-223.

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arXiv2009

The Fermi Gamma-Ray Burst Monitor

The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of Gamma-Ray Bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data exist. A secondary objective is to compute burst locations on-board to allow re-orientiong the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of twelve sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The on-board trigger threshold is ~0.7 photons/cm2/s (50-300 keV, 1 s peak). GBM generates on-board triggers for ~250 GRBs per year.

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arXiv2003

Gamma-Hadron Separation using Cerenkov Photon Density Fluctuations

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.

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arXiv2002

Study of Spectral Lag for Short GRBs

This paper reports statistically significant correlations between various burst parameters, observed in a sample of 156 GRBs belonging to BATSE 4B catalog with T90 less than 2 s. The number of subpulses in a burst is strongly correlated not only with the object's duration but also with its fluence and hardness ratio, suggesting that when the central engine is more powerful, ejecting matter with typically higher values of Lorentz factor, the bulk energy is dissipated on longer time scales in the form of larger number of gamma pulses. We estimate hard-to-soft lag in bursts by taking the difference between centroids corresponding to time profiles at energies $> 100$ keV and $<100$ keV. The number of short GRBs that show soft-to-hard spectral evolution is slightly over one quarter of the total, in the sample considered here. Bursts that exhibit hard-to-soft spectral change appear to form a distinct class, with strength as well as hardness of individual subpeaks tending to decrease with peak position. Opposite is true for objects with softer photons arriving earlier than the harder ones, implying some kind of a rejuvenation of the central engine (may be due to enhanced accretion of matter towards the end). The two classes also show other diverging trends. For instance, objects belonging to the larger of the two classes display strong correlations between spectral lag and the fluence, the hardness ratio as well as the number of pulse, respectively. While no such correlations are seen in bursts that evolve from soft to hard. However, the magnitude of lag is strongly correlated with burst duration in both the classes.

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arXiv2002

Angular Resolution of the Pachmarhi Array of Cerenkov Telescopes

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 7 parabolic mirrors each viewed by a single photo-multiplier tube. 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 {\it split} array method. It is found to be $\sim 2.4^\prime ~$ 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.

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arXiv2001

Distributed Data Acquisition System for Pachmarhi Array of Cerenkov Telescopes

Pachmarhi Array of \v Cerenkov Telescopes (PACT) consists of a 5$\times$5 array of \v Cerenkov telescopes deployed over an area of 100 $m$ $\times$ 80 $m$, in the form of a rectangular matrix. The experiment is based on atmospheric \v Cerenkov technique using wavefront sampling technique. Each telescope consists of 7 parabolic mirrors mounted para-axially on an equatorial mount. At the focus of each mirror a fast phototube is mounted. In this experiment a large number of parameters have to be measured and recorded from each of the 175 phototubes in the shortest possible time. Further, the counting rates from each phototube as well as the analog sum of the 7 phototubes from each telescope (royal sum) need to be monitored at regular intervals during the run. In view of the complexity of the system, the entire array is divided into four smaller sectors each of which is handled by an independent field signal processing centre (FSPC) housed in a control room that collects, processes and records information from nearby six telescopes that belong to that sector. The distributed data acquisition system (DDAS) developed for the purpose consists of stand-alone sector data acquisition system (SDAS) in each of the four FSPC's and a master data acquisition system (MDAS). MDAS running in the master signal processing centre (MSPC) records data from each of the 25 telescopes. The data acquisition and monitoring PCs (SDAS and MDAS) are networked through LAN. The entire real time software for DDAS is developed in C under $linux$ environment. Most of the hardware in DDAS are designed and fabricated in-house. The design features and the performance of the entire system along with some other auxiliary systems to facilitate the entire observations will be presented.

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arXiv2001

Pachmarhi Array of Čerenkov Telescopes and its Sensitivity

Pachmarhi Array of Čerenkov Telescopes (PACT) has been designed to search for celestial TeV $γ-$rays using the wavefront sampling technique. PACT, located at Pachmarhi, (latitude 22$^\circ$ 28$^\prime$ N, longitude 76$^\circ$ 26$^\prime$ E, altitude 1075 m) consists of 25 telescopes deployed over an area of 80 m $\times$ 100m. Each telescopes consists of 7 parabolic reflectors, each viewed by a fast phototube behind a 3$^\circ$ mask at the focus. The density and the arrival time of the photons at the PMT are recorded for each shower. The energy threshold and collection area of the array are estimated, from Monte Carlo simulations, to be $\sim$ 900 GeV and 10$^5$ m$^2$ respectively. The accuracy in determination of arrival angle of a shower is estimated to be about 0.1$^\circ$ in the near vertical direction. About 99% of the off-axis hadronic events could be rejected from directional information alone. Further, at least 75% of the on-axis hadronic events could be rejected using species sensitive parameters derived from timing and density measurements. These cuts on data to reject background would retain $\sim$ 44% of the $γ-$ray signal. The sensitivity of the array for a 5$σ$ detection of $γ-$ray signal at a threshold energy of 1 TeV has been estimated to be $\sim$ 4.1 $\times$ 10$^{-12}$ photons cm$^{-2}$ s$^{-1}$ for an on source exposure of 50 hours. The PACT set-up has been fully commissioned and is collecting data. The details of the system parameters and sensitivity will be presented.

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arXiv2001

Estimation of Shower Parameters in Wavefront Sampling Technique

Wavefront sampling experiments record arrival times of Čerenkov photons with high precision at various locations in Čerenkov 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 Čerenkov front. Another outstanding problem in the field of atmospheric Čerenkov 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 Čerenkov 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.

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arXiv2001

Gamma-Hadron Separation using Čerenkov Photon Density Fluctuations

In atmospheric Čerenkov technique $γ-$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 Čerenkov photons generated by $γ-$ray and hadron primaries are often used for this purpose. Here we study the differences in Čerenkov 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 $γ-$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.

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arXiv2001

A Possible High Altitude High Energy Gamma Ray Observatory in India

Recently an Indian Astronomical Observatory has been set up at Hanle (32$^\circ$ 46$^\prime$ 46$^{\prime\prime}$ N, 78$^\circ$ 57$^\prime$ 51$^{\prime\prime}$ E, 4515m amsl) situated in the high altitude cold desert in the Himalayas. The Observatory has 2-m aperture optical-infrared telescope, recently built by the Indian Institute of Astrophysics. We have carried out systematic simulations for this observation level to study the nature of Čerenkov light pool generated by gamma ray and proton primaries incident vertically at the top of the atmosphere. The differences in the shape of the lateral distributions of Čerenkov light with respect to that at lower altitudes is striking. This arises primarily due to the proximity of the shower maximum to the observation site. The limited lateral spread of the Čerenkov light pool and near 90% atmospheric transmission at this high altitude location makes it an ideal site for a gamma ray observatory. This results in a decrease in the gamma ray energy threshold by a factor of 2.9 compared to that at sea-level. Several parameters based on density and timing information of Čerenkov photons, including local and medium range photon density fluctuations as well as photon arrival time jitter could be efficiently used to discriminate gamma rays from more abundant cosmic rays at tens of GeV energies.

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arXiv2001

Very High Energy Gamma Ray Emission from Crab Nebula with the PACT Array

The Crab nebula has proved to be the nearest to a standard candle in VHE $γ-$ ray astronomy. Results on the gamma ray emission from the nebula at various energies have come in the last decade mostly from imaging telescopes. The aim of the new Pachmarhi Atmospheric Cerenkov Telescope (PACT) array has been to use the temporal and spatial distribution of Cerenkov photons in distinguishing between proton and gamma ray showers. The array, with timing information from 175 mirrors, is ideally suited for precise estimation of the arrival direction. Preliminary results from the recent data taken on Crab has shown a 12$σ$ signal with the flux in good agreement with those of the other experiments.

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arXiv2001

Angular Resolution of Pachmarhi Array of Cerenkov Telescopes

Pachmarhi Array of Čerenkov Telescopes(PACT), consisting of a distributed array of 25 telescopes is used to sample the atmospheric Čerenkov Photon showers. The shower front is fitted to a plane and the direction of arrival of primary particle is obtained. The accuracy in the estimation of the arrival direction of showers has been estimated to be $\sim 0^{\circ}.1~$ using `split' array method. The angular resolution is expected to be even better when a spherical front is used for direction reconstruction or correction for the curvature of the front is applied. This is the best angular resolution among all the currently operating atmospheric Čerenkov telescopes in the world.

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arXiv2001

Very High Energy Gamma Ray Emission from Crab and Geminga Pulsars

The main inference from the experiments of the '80s that the time-averaged energy spectra of pulsars had to steepen in the GeV-TeV energy region has been reinforced in the '90s from experiments with higher sensitivities. However, results from several experiments from the past and the more sensitive experiments at present can be reconciled by invoking a possibly different component arising in the TeV region. The results of the preliminary analysis of the data being collected with the PACT array will be presented.

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arXiv2001

Observations of Mkn 421 using Pachmarhi Array of Čerenkov Telescopes

Pachmarhi Array of Čerenkov Telescopes (PACT), based on wavefront sampling technique, has been used for detecting TeV gamma rays from galactic and extra-galactic $γ$-ray sources. The Blazar, Mkn 421 was one such extra-galactic source observed during the winter nights of 2000 and 2001. We have carried out a preliminary analysis of the data taken during the nights of January, 2000 and 2001. Results show a significant gamma ray signal from this source during both these periods above a threshold energy of 900 GeV. The source was contemporaneously observed by CAT imaging telescope during the first episode of January 2000 while HEGRA CT1 was observing the source during the second episode. Both these observations have detected variable $γ$-ray emission this source and they reported that it was flaring during both these periods. The light curve in the TeV gamma ray range derived from the first PACT observations during both these episodes is in agreement with that reported by other experiments. The analysis procedure and the preliminary results will be presented and discussed.

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arXiv2001

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

We study the propagation of Cerenkov photons generated by Very High Energy gamma-rays and hadrons in the atmosphere. The photon production height distributions are estimated from semi-empirical methods and compared with those derived by standard simulation techniques. Incident spectra at various observation altitudes are then derived after applying wavelength dependent corrections due to photon attenuation in the atmosphere during the propagation of photons from the height of production to the height of observation. These are generated both for gamma- and hadron primaries of various energies. The derived production height distributions agree very well with those generated by the simulation package `CORSIKA' at all energies and for both gamma-$ray and proton primaries. The incident photon spectra are found to be both altitude dependent and primary energy dependent. The peak of the incident spectrum shifts towards the shorter wavelength with increasing altitude of observation for a given primary. Also the peak of the photon spectrum shifts towards the shorter wavelength with increasing energy of the primary at given altitude. The fraction of the UV component in the incident Cerenkov spectrum is estimated both for gamma-ray and hadronic primaries at various observation altitudes and energies. Hadron generated Cerenkov spectra are marginally richer in UV light and the difference increases slightly at higher altitudes. The fraction of the UV to the visible light in the Cerenkov spectrum could be a useful parameter to separate gamma-rays from cosmic ray background only if one can measure this fraction very accurately.

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arXiv2000

Possible Discrimination between Gamma Rays and Hadrons using Cerenkov Photon Timing Measurements

Atmospheric Čerenkov 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 Čerenkov 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 Čerenkov 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 Čerenkov photons at the observation level. We have parameterized the shape of the Čerenkov 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 Čerenkov 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 $γ-$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.

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