arXiv subjects

P. N. Bhat

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

62 records · Page 4Linked to original sources

arXiv1999

Dust Properties of NGC4753

We report BVR surface photometry of a lenticular galaxy, NGC4753 with prominent dust lanes. We have used the multicolor broadband photometry to study dust-extinction as a function of wavelength and derived the extinction curve. We find the extinction curve of NGC 4753 to be similar to the Galactic extinction curve in the visible region which implies that the sizes of dust grains responsible for optical extinction are similar to those in our Galaxy. We derive dust mass from optical extinction as well as from the far infrared fluxes observed with IRAS. The ratio of the two dust masses, $\frac{M_{d,IRAS}}{M_{d,optical}}$, is 2.28 for NGC 4753, which is significantly lower than the value of 8.4 \pm 1.3 found previously for a large sample of elliptical galaxies. The total mass of the observed dust within NGC4753 is about a factor of 10 higher than the mass of dust expected from loss of mass from red giant stars and destruction by sputtering and grain-grain collisions in low velocity shocks, and sputtering in supernova driven blast waves. We find evidence for the coexistence of dust and H$α$ emitting gas within NGC4753. The current star formation rate of NGC4753, averaged over past $2\times10^{6} yr$, is estimated to be less than $0.21M_{\sun}yr^{-1}$. A substantial amount of dust within NGC4753 exists in the form of cirrus.

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arXiv1999

Morphological properties of Short Duration GRBs

In this paper, we study a sample of 65 short duration bursts contained in the 3B catalogue. We fit the time profiles of these GRBs with lognormal functions and study various temporal properties. In most of the multi-peaked bursts, our analysis leads to a statistical evidence for the evolution of temporal asymmetry in individual pulses i.e. subsequent pulses in a GRB, on an average, tend to be more symmetric. Using a peak asymmetry evolution parameter, we find that 90% of short duration GRBs with 3 or more peaks exhibit the above trend individually.

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arXiv1999

Simulation Studies on Arrival Time Distributions of Cherenkov Photons in Extensive Air Showers

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.

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arXiv1998

Cerenkov Photon Density Fluctuations in Extensive Air Showers

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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arXiv1997

Pachmarhi Array of Cerenkov Telescopes

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.

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arXiv1997

The role of fluctuations in atmospheric Cerenkov technique

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.

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arXiv1997

Pachmarhi Array of Cerenkov Telescopes

Very High Energy (VHE) gamma -ray Astronomy observations are planned to be carried out at Pachmarhi in the central Indian state of Madhya Pradesh using the well known atmospheric Cerenkov technique. An array of 25 Cerenkov telescopes is currently under construction. Using this array it is proposed to sample the Cerenkov light pool at various distances from the shower core in order to estimate the lateral distribution parameters of the shower. In this paper we 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.

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arXiv1995

A Dust Lane in the Radio galaxy 3C270

We present broad band surface photometry of the radio galaxy 3C270 (NGC~4261). We find a distinct dust lane in the $V-R$ image of the galaxy, and determine its orientation and size. We use the major axis profile of the galaxy to estimate the optical depth of the dust lane, and discuss the significance of the lane to the shape of the galaxy.

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