arXiv subjects

U. D. Goswami

Publications and source records attributed to U. D. Goswami.

8 recordsLinked to original sources

arXiv2023

Small-scale cosmic ray anisotropy observed by the GRAPES-3 experiment at TeV energies

GRAPES-3 is a mid-altitude (2200 m) and near equatorial ($11.4^{\circ}$ North) air shower array, overlapping in its field of view for cosmic ray observations with experiments that are located in Northern and Southern hemispheres. We analyze a sample of $3.7\times10^9$ cosmic ray events collected by the GRAPES-3 experiment between 1 January 2013 and 31 December 2016 with a median energy of $\sim16$ TeV for study of small-scale ($<60^{\circ}$) angular scale anisotropies. We observed two structures labeled as A and B, deviate from the expected isotropic distribution of cosmic rays in a statistically significant manner. Structure `A' spans $50^{\circ}$ to $80^{\circ}$ in the right ascension and $-15^{\circ}$ to $30^{\circ}$ in the declination coordinate. The relative excess observed in the structure A is at the level of $(6.5\pm1.3)\times10^{-4}$ with a statistical significance of 6.8 standard deviations. Structure `B' is observed in the right ascension range of $110^{\circ}$ to $140^{\circ}$. The relative excess observed in this region is at the level of $(4.9\pm1.4)\times10^{-4}$ with a statistical significance of 4.7 standard deviations. These structures are consistent with those reported by Milagro, ARGO-YBJ, and HAWC. These observations could provide a better understanding of the cosmic ray sources, propagation and the magnetic structures in our Galaxy.

astro-ph.HE↗

arXiv2018

Parameterisation of lateral density and arrival time distributions of Cherenkov photons in EASs as functions of independent shower parameters for different primaries

The simulation of Cherenkov photon's lateral density and arrival time distributions in Extensive Air Showers (EASs) was performed with the CORSIKA code in the energy range: 100 GeV to 100 TeV. On the basis of this simulation we obtained a set of approximating functions for the primary $γ$-ray photons, protons and iron nuclei incident at zenith angles from 0$^\circ$ to 40$^\circ$ over different altitudes of observation. Such a parameterisation is important for the primary particle identification, for the reconstruction of the shower observables and hence for a more efficient disentanglement of the $γ$-ray showers from the hadronic showers. From our parameterisation analysis, we have found that even though the geometry of the lateral density ($ρ_{ch}$) and the arrival time ($t_{ch}$) distributions is different for different primaries at a particular energy ($E$), at a particular incident angle ($θ$) and at a particular altitude of observation ($H$) up to a given distance from the showe core ($R$), the distributions follow the same mathematical functions $ρ(E,R,θ,H) = a E^{b}\exp[-\{c R + (θ/d)^{2}-f H\}]$ and $t(E,R,θ,H) = l E^{-m}\exp(n/R^{p})(θ^q+s)(u {H}^2+v)$ respectively but with different values of function parameters.

astro-ph.IM↗

arXiv2018

Azimuthal distribution of Cherenkov photons and corresponding electron-positron asymmetry in EASs of different primaries

We study the azimuthal distributions of Cherenkov photons in Extensive Air Showers (EASs) initiated by $γ$-ray, proton and iron primaries of different energies incident at various zenith angles over a high altitude observation level. The azimuthal distributions of electrons and positrons along with their asymmetric behaviour have also been studied here to understand the feature of azimuthal distributions of Cherenkov photons in EASs. The main motivation behind this study is to see whether the azimuthal distribution of Cherenkov photons can provide any means to distinguish the $γ$-ray initiated showers from that of hadron initiated showers in the ground based $γ$-ray astronomy experiment. Apart from this, such study is also important to understand the natures of $γ$-ray and hadronic showers in general. We have used the CORSIKA 6.990 simulation package for generating the showers. The study shows the double peak nature of the azimuthal distribution of Cherenkov photons which is due to the separation of electron and positrons in the azimuthal plane. The pattern of distribution is more sensitive for the energy of the primary particle than it's angle of incidence. There is no significant difference between distributions for $γ$-ray and handron initiated showers.

astro-ph.IM↗

arXiv2018

A simulation study on few parameters of Cherenkov photons in extensive air showers of different primaries incident at various zenith angles over a high altitude observation level

We have studied the distribution patterns of lateral density, arrival time and angular position of Cherenkov photons generated in Extensive Air Showers (EASs) initiated by $γ$-ray, proton and iron primaries incident with various energies and at various zenith angles. This study is the extension of our earlier work \cite{Hazarika} to cover a wide energy range of ground based $γ$-ray astronomy with a wide range of zenith angles ($\le 40^\circ$) of primary particles, as well as the extension to study the angular distribution patterns of Cherenkov photons in EASs. This type of study is important for distinguishing the $γ$-ray initiated showers from the hadronic showers in the ground based $γ$-ray astronomy, where Atmospheric Cherenkov Technique (ACT) is being used. Importantly, such study gives an insight on the nature of $γ$-ray and hadronic showers in general. In this work, the CORSIKA 6.990 simulation code is used for generation of EASs. Similarly to the case of Ref.\cite{Hazarika}, this study also revealed that, the lateral density and arrival time distributions of Cherenkov photons vary almost in accordance with the functions: $ρ_{ch}(r) = ρ_{0}\;e^{-βr}$ and $t_{ch}(r) = t_{0}e^{Γ/r^λ}$ respectively by taking different values of the parameters of functions for the type, energy and zenith angle of the primary particle. The distribution of Cherenkov photon's angular positions with respect to shower axis shows distinctive features depending on the primary type, its energy and the zenith angle.

astro-ph.IM↗

arXiv2016

Vortex dynamics and specific heat of type II superconductor with quasi-periodic geometry

The vortex dynamics and the specific heat of a type II superconducting system with quasi-periodic geometry is studied theoretically for different values of interaction parameters using the numerical simulation technique, where the vortex-vortex interaction potential is considered in the form of the modified Bessel's function of first kind. The dynamics of the system is analysed by phase space trajectories of the vortex for both high and low values as well as for both high and low mismatch of vortex-vortex and vortex-pinning interaction parameters. The specific heat variation with temperature is analysed statistically for different values of interaction parameters. It is observed that for low values and lower mismatch of interaction parameters, the system is highly chaotic and shows a bifurcation pattern similar to Hopf bifurcation. The specific heat also shows a highly divergent character in this situation. However for high values and higher mismatch, the superconducting system tends to be a very regular one. The trajectory of the vortices will also be very stable in this situation. Similar situations are also observed respectively for low and high values of the quasi-periodic parameter.

cond-mat.supr-con↗

arXiv2015

Lateral density and arrival time distributions of Cherenkov photons in extensive air showers: a simulation study

We have investigated some features of the density and arrival time distributions of Cherenkov photons in extensive air showers using the CORSIKA simulation package. The main thrust of this study is to see the effect of hadronic interaction models on the production pattern of Cherenkov photons with respect to distance from the shower core. Such studies are very important in ground based $γ$-ray astronomy for an effective rejection of huge cosmic ray background, where the atmospheric Cherenkov technique is being used extensively within the energy range of some hundred GeV to few TeV. We have found that for all primary particles, the density distribution patterns of Cherenkov photons follow the negative exponential function with different coefficients and slopes depending on the type of primary particle, its energy and the type of interaction model combinations. Whereas the arrival time distribution patterns of Cherenkov photons follow the function of the form $t (r) = t_{0}e^{Γ/r^λ}$, with different values of the function parameters. There is no significant effect of hadronic interaction model combinations on the density and arrival time distributions for the $γ$-ray primaries. However, for the hadronic showers, the effects of the model combinations are significant under different conditions.

astro-ph.IM↗

arXiv2010

Formation of caustics in Dirac-Born-Infeld type scalar field systems

We investigate the formation of caustics in Dirac-Born-Infeld type scalar field systems for generic classes of potentials, viz., massive rolling scalar with potential, $V(ϕ)=V_0e^{\pm \frac{1}{2} M^2 ϕ^2}$ and inverse power-law potentials with $V(ϕ)=V_0/ϕ^n,~0<n<2$. We find that in the case of\texttt{} exponentially decreasing rolling massive scalar field potential, there are multi-valued regions and regions of likely to be caustics in the field configuration. However there are no caustics in the case of exponentially increasing potential. We show that the formation of caustics is inevitable for the inverse power-law potentials under consideration in Minkowski space time whereas caustics do not form in this case in the FRW universe.

hep-th↗

arXiv2008

Forbush decreases and turbulence levels at CME fronts

We seek to estimate the average level of MHD turbulence near coronal mass ejection (CME) fronts as they propagate from the Sun to the Earth. We examine the cosmic ray data from the GRAPES-3 tracking muon telescope at Ooty, together with the data from other sources for three well observed Forbush decrease events. Each of these events are associated with frontside halo Coronal Mass Ejections (CMEs) and near-Earth magnetic clouds. In each case, we estimate the magnitude of the Forbush decrease using a simple model for the diffusion of high energy protons through the largely closed field lines enclosing the CME as it expands and propagates from the Sun to the Earth. We use estimates of the cross-field diffusion coefficient $D_{\perp}$ derived from published results of extensive Monte Carlo simulations of cosmic rays propagating through turbulent magnetic fields. Our method helps constrain the ratio of energy density in the turbulent magnetic fields to that in the mean magnetic fields near the CME fronts. This ratio is found to be $\sim$ 2% for the 11 April 2001 Forbush decrease event, $\sim$ 6% for the 20 November 2003 Forbush decrease event and $\sim$ 249% for the much more energetic event of 29 October 2003.

astro-ph↗