arXiv subjects

S. Bhattacharyya

Publications and source records attributed to S. Bhattacharyya.

107 records · Page 6Linked to original sources

arXiv2007

Characterization of fragment emission in ^{20}Ne (7 - 10 MeV/nucleon) + ^{12}C reactions

The inclusive energy distributions of the complex fragments (3 $\leq$ Z $\leq$ 7) emitted from the bombardment of ^{12}C by ^{20}Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range 10$^{o} \leq θ_{lab} \leq$ 50^{o}. Damped fragment yields in all the cases have been found to be the characteristic of emission from fully energy equilibrated composites. The binary fragment yields are compared with the standard statistical model predictions. Enhanced yields of entrance channel fragments (5 $\leq$ Z $\leq$ 7) indicate the survival of orbiting-like process in ^{20}Ne + ^{12}C system at these energies.

nucl-ex↗

arXiv2007

The TACTIC atmospheric Cherenkov Imaging telescope

The TACTIC $γ$-ray telescope, equipped with a light collector of area $\sim$9.5m$^2$ and a medium resolution imaging camera of 349-pixels, has been in operation at Mt.Abu, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11$\times$11 pixels ($\sim$ 3.4$^\circ$$\times3.4^\circ$), the telescope records a cosmic ray event rate of $\sim$2.5 Hz at a typical zenith angle of 15$^\circ$. Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above $\sim$1.2 TeV energy, at a sensitivity level of $\sim5.0σ$ in $\sim$25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong $γ$-ray signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during 2001 and 2005-2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV $γ$-ray emission activity from other active galactic nuclei on a long term basis.

astro-ph↗

arXiv2007

Very High Energy $γ$-ray and Near Infrared observations of 1ES2344+514 during 2004-05

We have observed the BL Lac object 1ES2344+514 (z = 0.044) in Very High Energy (VHE) gamma-ray and near-infrared wavelength bands with TACTIC and MIRO telescopes respectively. The observations were made from 18th October to 9th December 2004 and 27th October 2005 to 1st January 2006. Detailed analysis of the TACTIC data indicates absence of a statistically significant gamma-ray signal both in overall data and on a nightly basis from the source direction. We estimate an upper limit of I($\geq$1.5 TeV)$\leq 3.84 \times 10^{-12}$ photons cm$^{-2}$ s$^{-1}$ at a 3$σ$ confidence level on the integrated $γ$-ray flux. In addition, we have also compared TACTIC TeV light curves with those of the RXTE ASM (2-12keV) for the contemporary period and find that there are no statistically significant increases in the signal strengths from the source in both these energy regions. During 2004 IR observations, 1ES2344+514 shows low level (~0.06 magnitude) day-to-day variation in both, J & H bands. However, during 2005 observation epoch, the source brightens up by about 0.41 magnitude from its October 2005 level J magnitude= 12.64 to J = 12.23 on December 6, 2005. It then fades by about 0.2 magnitude during 6 to 10 December, 2005. The variation is seen in both, J & H, bands simultaneously. The light travel time arguments suggest that the emission region size is of the order of $10^{17}$ cms.

astro-ph↗

arXiv2007

Observations of TeV gamma-rays from Mrk 421 during Dec. 2005 to Apr. 2006 with the TACTIC telescope

The TACTIC $γ$-ray telescope has observed Mrk 421 on 66 clear nights from Dec. 07, 2005 to Apr. 30, 2006, totalling $\sim$ 202 hours of on-source observations. Here, we report the detection of flaring activity from the source at $\geq$ 1 TeV energy and the time-averaged differential $γ$-ray spectrum in the energy range 1-11 TeV for the data taken between Dec. 27, 2005 to Feb. 07, 2006 when the source was in a relatively higher state as compared to the rest of the observation period. Analysis of this data spell, comprising about $\sim$97h reveals the presence of a $\sim 12.0 σ$ $γ$-ray signal with daily flux of $>$ 1 Crab unit on several days. A pure power law spectrum with exponent $-3.11\pm0.11$ as well as a power law spectrum with an exponential cutoff $(Γ= -2.51\pm0.26$ and $E_0=(4.7\pm2.1) TeV)$ are found to provide reasonable fits to the inferred differential spectrum within statistical uncertainties. We believe that the TeV light curve presented here, for nearly 5 months of extensive coverage, as well as the spectral information at $γ$-ray energies of $>$ 5 TeV provide a useful input for other groups working in the field of $γ$-ray astronomy.

astro-ph↗

arXiv2006

Self-consistent computation of gamma-ray spectra due to proton-proton interactions in black hole systems

In the inner regions of an accretion disk around a black hole, relativistic protons can interact with ambient matter to produce electrons, positrons and $γ$-rays. The resultant steady state electron and positron particle distributions are self-consistently computed taking into account Coulomb and Compton cooling, $e^-e^+$ pair production (due to $γ-γ$ annihilation) and pair annihilation. While earlier works used the diffusion approximation to obtain the particle distributions, here we solve a more general integro-differential equation that correctly takes into account the large change in particle energy that occur when the leptons Compton scatter off hard X-rays. Thus this formalism can also be applied to the hard state of black hole systems, where the dominant ambient photons are hard X-rays. The corresponding photon energy spectrum is calculated and compared with broadband data of black hole binaries in different spectral states. The results indicate that the $γ$-ray spectra ($E > 0.8$ MeV) of both the soft and hard spectral states and the entire hard X-ray/$γ$-ray spectrum of the ultra-soft state, could be due to $p-p$ interactions. These results are consistent with the hypothesis that there always exists in these systems a $γ$-ray spectral component due to $p-p$ interactions which can contribute between 0.5 to 10% of the total bolometric luminosty. The model predicts that {\it GLAST} would be able to detect black hole binaries and provide evidence for the presence of non-thermal protons which in turn would give insight into the energy dissipation process and jet formation in these systems.

astro-ph↗

arXiv2005

Survival of orbiting in $^{20}$Ne (7 - 10 MeV/nucleon) + $^{12}$C reactions

The inclusive energy distributions of fragments with Z $\geq$ 3 emitted from the bombardment of $^{12}$C by $^{20}$Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range $θ_{lab} \sim$ 10$^\circ$ - 50$^\circ$. Damped fragment yields in all cases have been found to be characteristic of emission from fully energy equilibrated composites; for B, C fragments, average Q-values, $ $, were independent of the centre of mass emission angle ($θ_{c.m}$), and the angular distributions followed $\sim$1/sin$θ_{c.m}$ like variation, signifying long life times of the emitting di-nuclear systems. Total yields of these fragments have been found to be much larger compared to the standard statistical model predictions of the same. This may be indicative of the survival of orbiting like process in $^{12}$C + $^{20}$Ne system at these energies.

nucl-ex↗

arXiv2004

Rapidity Spectra of the Secondaries Produced in Heavy Ion Collisions and the Constituent Picture of the Particles

The present study is both a reanalysis and an extension of the approach initiated by Eremin and Voloshin(referenced in the text). We attempt to interpret here the rapidity-spectra of the various particles produced in both $Pb+Pb$ and $Au+Au$ collisions at CERN-SPS and RHIC-BNL. The study made here is wider in scope and is more species-specific than the earlier ones for which the results obtained here have been compared with those suggested by some previous works based on HIJING, VENUS etc, at various centralities. The study reconfirms that the constituent parton picture of the particles provides a better and more unified description of the rapidity-density yields for the various secondaries, including some light cluster particles like deuteron even in heavy ion collisions.

nucl-th↗

arXiv2004

Indian monsoon rainfall is higher in epochs of higher solar activity: a wavelet cross-spectral analysis

Using the Morlet continuous wavelet transform on data over the period 1871-1990, it is found that the global wavelet cross spectra between two solar activity indices and seven major Indian monsoon rainfall time series show significant power around the period of the 11 year solar cycle, passing the $χ^2$ test of significance proposed by Torrence and Compo (1998) at levels exceeding 95% in 10 out of the 14 cases studied. Furthermore two distinct epochs are found in the computed cross-spectrum, the transition between them occurring around the decade 1915-1925, beyond which solar activity parameters show a significant increase. By comparison between selected periods of three cycles in solar activity in each of the two epochs, it is shown that the average rainfall is higher in all seven rainfall indices during periods of greater solar activity, at {\it z}-test confidence levels of 95% or greater in three of them.

physics.geo-ph↗

arXiv2003

Centrality-dependence of Particle Production at RHIC and the Combinational Approach

The newly proposed combinational approach, called the grand combinational model (GCM), as would be described in detail in the text, is still under our careful scrutiny. By applying it, we have attempted to analyze here the characteristics of both the transverse momentum($p_{\rm{T}}$)-, and centrality-dependence of production of the main varieties of the secondaries measured in $AuAu$ collisions at BNL-RHIC at both $\sqrt{s_{\rm{NN}}}=130$ GeV and $\sqrt{s_{\rm{NN}}}=200$ GeV by PHENIX Collaboration. Besides, with the help of it, we have also investigated the nature of centrality-dependence of the average transverse momenta of the various major categories of particles in $AuAu$ collisions at RHIC. The model seems to survive quite smoothly the acid tests of the latest PHENIX data, as it accommodates data modestly well on these twin aspects. The study reveals a kind of universality of nature of the hadronic secondaries and also of the basic particle and nuclear interactions at high energies. However, in the end, we precisely point out both the strengths and limitations of the specific model under consideration here.

hep-ph↗

arXiv2003

The $p_T$-Spectra of Some Non-pion Secondaries in High Energy NN to NA/AA Collisions and the Combinational Approach

In continuation of our perusal of the studies on transverse momentum spectra for the main varieties of secondaries from a consistent and comprehensive phenomenological approach, we propose to take up here -- after a successful completion of reporting in detail the results (Ref.[4] in the text) on our analyses of the $p_T$-spectra of pions -- the studies specially on production of kaons, protons and antiprotons in several proton-induced and nucleus-involved collisions at high energies. The measured data on inclusive cross sections of kaons, protons and antiprotons from the various published sources have here been assorted first. Next, data on the $p_T$-spectra of the specific secondaries produced in PP and $P\bar{P}$ reactions have been scanned and analyzed with the help of Hagedorn's model(HM). Thereafter a connector, named here the combinational approach(CA), has been constructed and used to analyze the data on $p_T$-spectra of some major category of non-pion secondaries produced in nucleus-nucleus(AA/AB) collisions at high energies. And these exercises have, finally, led to the modestly successful interpretations of a wide band of data with the revelation of some insightful physical aspects of high energy interactions. The limitations of the approach have also been precisely pointed out in the end.

hep-ph↗

arXiv2003

The Effect of Non-Thermal Protons on the High Energy Spectra of Black Hole Binaries

In the inner regions of an accretion disk around a black hole, the presence of non-thermal protons would via p-p collisions produce electrons, positrons and $γ$-rays. For such a scenario, the steady state electron-positron distribution is computed taking into account Compton cooling, $e^{-} e^{+}$ pair production (due to $γ-γ$ interactions) and pair annihilation. The resultant spectra has a broad feature around $1 - 10$ MeV which may be tested with observations by INTEGRAL and GLAST. Using the available OSSE data for the black hole system, GRS 1915+115 we illustrate that an upper limit on the non-thermal proton fraction can be obtained, which may put constraints on the acceleration process active in this system.

astro-ph↗

arXiv2003

Transverse Mass Distribution Characteristics of $π^0$ Production in $^{208}$Pb-induced Reactions and the Combinational Approach

The nature of invariant cross-sections and multiplicities in some $^{208}Pb$-induced reactions and some important ratio-behaviours of the invariant multiplicities for various centralities of the collision will here be dealt with in the light of a combinational approach which has been built up in the recent past by the present authors. Next, the results would be compared with the outcome of some of the simulation-based standard models for multiple production in nuclear collisions at high energies. Finally, the implications of all this would be discussed.

nucl-th↗

arXiv2002

Transverse Momentum Spectra of Pions in Particle and Nuclear Collisions and Some Ratio-Behaviours: Towards A Combinational Approach

The nature of transverse momentum dependence of the inclusive cross-sections for secondary pions produced in high energy hadronic($PP$), hadronuclear($PA$) and nuclear($AA$) collisions has here been exhaustively investigated for a varied range of interactions in a unified way with the help of a master formula. This formula evolved from a new combination of the basic Hagedorn's model for particle(pion) production in PP scattering at ISR range of energies, a phenomenological approach proposed by Peitzmann for converting the results of $NN(PP)$ reactions to those for either $PA$ or $AA$ collisions, and a specific form of parametrization for mass number-dependence of the nuclear cross sections. This grand combination of models(GCM) is then applied to analyse the assorted extensive data on various high energy collisions. The nature of qualitative agreement between measurements and calculations on both the inclusive cross-sections for production of pions, and some ratios of them as well, is quite satisfactory. The modest successes that we achieve here in dealing with the massive data-sets are somewhat encouraging in view of the diversity of the reactions and the very wide range of interaction energies.

nucl-th↗

arXiv2001

Orientational and Translational Hopping in Supercooled Liquids and Glasses : Correlated Dynamics in a Free Energy Landscape

Orientational relaxation (OR) in a viscous, glassy liquid is investigated by carrying out extensive NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture. Near the glass transition, the OR occurs mainly via hopping involving either a ring- or a tunnel like motion, with the participation of several neighboring atoms. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be {\it gated}, restricted primarily by the entropic bottlenecks, with orientation becoming increasingly slower than translation as the pressure is increased. OR is heterogeneous, with a wide distribution of decay times.

cond-mat.soft↗

arXiv2000

Temperature Profiles of Accretion Disks around Rapidly Rotating Neutron Stars in General Relativity and Implications for Cygnus X-2

We calculate the temperature profiles of (thin) accretion disks around rapidly rotating neutron stars (with low surface magnetic fields), taking into account the full effects of general relativity. We then consider a model for the spectrum of the X-ray emission from the disk, parameterized by the mass accretion rate, the color temperature and the rotation rate of the neutron star. We derive constraints on these parameters for the X-ray source Cygnus X-2 using the estimates of the maximum temperature in the disk along with the disk and boundary layer luminosities, using the spectrum inferred from the EXOSAT data. Our calculations suggest that the neutron star in Cygnus X-2 rotates close to the centrifugal mass-shed limit. Possible constraints on the neutron star equation of state are also discussed.

astro-ph↗

arXiv2000

General relativistic spectra of accretion disks around rotating neutron stars

General relativistic spectra from accretion disks around rotating neutron stars in the appropriate space-time geometry for several different equation of state, spin rates and mass of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational red-shifts on the spectra. Light bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one and it is shown that the difference between the two is primarily due to the different radial temperature profile for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus the function can in principle be used to distinguish between the two. In particular, the best fit value of one of the parameters ($β$-parameter) $\approx 0.4$ for the Newtonian case, while it ranges from 0.1 to 0.35 for relativistic case depending upon the inclination angle, EOS, spin rate and mass of the neutron star. Constraining this parameter by fits to future observational data of X-ray binaries will indicate the effect of strong gravity in the observed spectrum.

astro-ph↗

arXiv1999

Observations of the Crab Nebula, Mkn 501 and Mkn 421 using the TACTIC Imaging Element

The results of our observational campaigns on the two extragalactic sources Mkn 501 and Mkn 421, carried out with the Imaging Element of the TACTIC array, during March-May, 1998 and April-May, 1999, are presented. The results indicate that the two BL Lac objects (Mkn 501 and Mkn 421) were in a `low' gamma-ray emission state during both epochs of our observations.

astro-ph↗