arXiv subjects

Nayantara Gupta

Publications and source records attributed to Nayantara Gupta.

At least 73 records · Page 4Linked to original sources

arXiv2010

Tracing Cosmic accelerators with Decaying Neutrons

Ultrahigh energy neutrons and pions are likely to be produced in particle interactions inside cosmic ray sources and subsequently decay to neutrinos and other secondary particles ($π^\pm \rightarrow μ^\pmν_μ(\bar ν_μ),μ^\pm \rightarrow e^\pm \barν_μ(ν_μ) ν_e(\barν_e)$). In high magnetic fields of the cosmic acceleration sites, the ultrahigh energy charged particles may lose energy significantly due to synchrotron radiation before decay. We show that for gamma ray bursts in the internal shock model the flux of very high energy antineutrinos ($\bar ν_e$) produced from decaying ultrahigh energy neutrons can be more than the total neutrino flux produced in pion decay depending on the values of their Lorentz factors, luminosities and variability times.

astro-ph.HE↗

arXiv2009

Probing Lorentz Invariance at EeV Energy

Pierre Auger experiment has detected at least a couple of ray events above energy 60 EeV from the direction of the radio-galaxy Centaurus A. Assuming those events are from Centaurus A, we have calculated the number of neutral cosmic ray events from this source for small values of the degree of violation in Lorentz invariance. Our results show that a comparison of our calculated numbers of events with the observed number of events at EeV energy from the direction of the source can probe extremely low value of the degree of this violation.

astro-ph.HE↗

arXiv2009

On the Gamma Ray Burst Origin of Extremely Energetic Cosmic Rays

Air shower experiments have detected cosmic ray events of energies upto 300 EeV. Most likely these cosmic rays have originated from compact objects. Their exact sources are yet to be identified. It has been suggested before that gamma ray bursts are possible sources of ultra-high energy cosmic rays. The two models of gamma ray burst emissions most often discussed are the internal and external shock models. We have calculated the proton spectrum above 60EeV from all gamma ray bursts distributed upto a redshift of 0.02 in the internal shock model assuming redshift and luminosity distributions consistent with observations, log normal distributions for their values of Lorentz factors, variability times and duration of bursts. Within the external shock model we have calculated the proton flux above 60EeV from all nearby gamma ray bursts assuming log normal distributions in their values of total energies, Lorentz factors at the deceleration epoch and compared with the observed data. We find that gamma ray bursts can produce cosmic ray proton flux comparable to the flux observed by the Pierre Auger experiment both within the internal and external shock models. We have also studied the dependence of the maximum proton energies and the cooling breaks in the proton spectrum on the various parameters like Lorentz factor, energy of the GRB fireball, variability time (in case of internal shocks), ambient particle density (in case of external shocks). Our results are important to understand how the various observable parameters determine which mechanism e.g. $pγ$ interactions, synchrotron cooling of protons will dominate over one another inside these sources.

astro-ph.CO↗

arXiv2008

Gamma Rays from Centaurus A

Centaurus A, the cosmic ray accelerator a few Mpc away from us is possibly one of the nearest sources of extremely high energy cosmic rays. We investigate whether the gamma ray data currently available from Centaurus A in the GeV-TeV energy band can be explained with only proton proton interactions. We show that for a single power law proton spectrum, mechanisms of $γ$-ray production other than proton proton interactions are needed inside this radio-galaxy to explain the gamma ray flux observed by EGRET, upper limits by H.E.S.S./CANGAROO-III and the correlated extremely energetic cosmic ray events observed by the Pierre Auger experiment. In future with better $γ$-ray data, simultaneous observation with $γ$-ray and cosmic ray detectors, it would be possible to carry out such studies on different sources in more detail.

astro-ph↗

arXiv2007

Diagnosing GRB Prompt Emission Site with Spectral Cut-Off Energy

The site and mechanism of gamma-ray burst (GRB) prompt emission is still unknown. Although internal shocks have been widely discussed as the emission site of GRBs, evidence supporting other emission sites have been also suggested recently, including the closer-in photosphere where the fireball becomes transparent and further-out radii near the fireball deceleration radius where magnetic dissipation may be important. With the successful operation of the GLAST experiment, prompt high energy emission spectra from many GRBs would be detected in the near future. We suggest that the cut-off energy of the prompt emission spectrum from a GRB depends on both the fireball bulk Lorentz factor and the unknown emission radius from the central engine. If the bulk Lorentz factor could be independently measured (e.g. from early afterglow observations), the observed spectral cutoff energy can be used to diagnose the emission site of gamma-rays. This would provide valuable information to understand the physical origin of the GRB promp emission.

astro-ph↗

arXiv2007

Prompt Emission of High Energy Photons from Gamma Ray Bursts

Within the internal shock scenario we consider different mechanisms of high energy ($>1$ MeV) photon production inside a Gamma Ray Burst (GRB) fireball and derive the expected high energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high energy emission is dominated by the leptonic component if fraction of shock energy carried by electrons is not very small (e.g. $ε_e > 10^{-3}$). For very small values of $ε_e$ the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime. However, in this case a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For a canonical fireball bulk Lorentz factor (e.g. $Γ=400$), emissions above $\sim 10$ GeV are attenuated by two-photon pair production processes. For a fireball with an even higher Lorentz factor, the cutoff energy is higher, and emissions of 10 TeV - PeV due to $π^0$-decay can also escape from the internal shocks. The flux level is however too low to be detected by current TeV detectors, and these photons also suffer attenuation by external soft photons.

astro-ph↗

arXiv2007

Constraining Galactic $pγ$ Interactions with Cosmic Ray Electron and Positron Spectra

High energy protons produced by various sources of cosmic rays, {\it e.g.} supernova remnants, pulsar wind nebulae, active galactic nuclei and gamma-ray bursts participate in $pγ$ and $pp$ interactions. Although $pp$ interactions may be the dominant mechanism in our Galaxy, it is unclear how important $pγ$ process is. We show that the upper bound on the fraction of protons participating in $pγ$ interactions inside all Galactic astrophysical sources of cosmic rays is 10%.

astro-ph↗

arXiv2007

Neutrino Spectra from Low and High Luminosity Populations of Gamma Ray Bursts

The detection of GRB 060218 at z=0.033 by {\em Swift} within 1.5 years of operation, together with the detection of GRB 980425 at $z=0.0085$ by BeppoSAX, suggest that these low-luminosity (LL) GRBs have a much higher event rate than the canonical high-luminosity (HL) GRBs, and they form a distinct new component in the GRB luminosity function. We explore the contribution of this previously neglected GRB population to the diffuse neutrino background within the internal shock model and compare it with that of the canonical HL population. By considering a wide range of distributions of various parameters (e.g. luminosity, spectral break energy, duration, variability time, Lorentz factor, redshift) for both populations, we find that although it is difficult to detect neutrinos from the individual LL GRBs, the contribution of the LL population to the diffuse neutrino background is more than the HL population above about $10^{8}$ GeV.

astro-ph↗

arXiv2005

Prospect of determining the Dirac/Majorana state of neutrino by Multi-OWL experiment

We consider the non-radiative two body decay of a neutrino to a daughter neutrino with degraded energy and a very light particle (Majoron). Ultrahigh energy neutrinos from an astrophysical source like a Gamma Ray Burst undergoing this decay process are found to produce different number of events in the detector depending on whether they are Majorana or Dirac particles. The next generation large scale experiments like Multi-OWL is expected to provide us an accurate determination of the flux of neutrinos from astrophysical sources and this may enable us to distinguish between the Dirac and Majorana nature of neutrino.

astro-ph↗

arXiv2005

Probing neutrino mixing angles with ultrahigh energy neutrino telescopes

We point out that detecting $\nuebar$'s from distant astrophysical sources with the up-coming and future neutrino telescopes using the Glashow resonance channel $\nuebar e^{-}\to W^{-} \to$ anything, which occurs over a small energy window around the $\nuebar$ energy of $\sim6.3\pev$, offers a new way of measuring or setting limits on neutrino mixing angles, in particular the angle $θ_{12}$, thereby providing an independent experimental probe of neutrino mixing angles. We also discuss how this exercise may throw light on the nature of the neutrino production mechanism in individual astrophysical sources.

hep-ph↗

arXiv2004

Ultrahigh Energy Cosmic Rays from Gamma Ray Bursts: Implications of the Recent Observational Results by Milagro

It has been speculated earlier that Gamma Ray Bursts are sources of ultrahigh energy cosmic rays. Recently, the search for high energy photons from Gamma Ray Bursts by Milagro group has put limits on the isotropic luminosity of these transient sources in very high energy photons. The implications of the results obtained by Milagro to our understanding of the ultrahigh energy cosmic ray spectrum from these sources have been discussed in the present work.

astro-ph↗

arXiv2003

Observability of neutron events above the Greisen-Zatsepin-Kuzmin cut-off due to violation of Lorentz invariance

The clustering of ultra high energy cosmic ray events suggests that they have originated from compact sources. One of the possible physical mechanisms by which ultra high energy nuclei reach the Earth from far away astrophysical sources (quasars or BL Lac objects) evading the Greisen-Zatsepin-Kuzmin (GZK) cut-off is by violation of Lorentz invariance. Assuming that there is violation of Lorentz invariance, we calculate the expected number of neutron events from some of the EGRET sources (including $γ$-ray loud BL Lac objects) which can be correlated in direction with ultra high energy cosmic ray events observed by AGASA above energy $4\times10^{19}$eV. We present in this paper what AGASA should see in future if violation of Lorentz invariance is responsible for the propagation of ultra high energy cosmic rays having energies above the GZK cut-off when there is a correlation of EGRET sources with the ultra high energy cosmic ray events.

astro-ph↗

arXiv2003

A study of the appearance of tau neutrinos from a gamma ray burst by detecting their horizontal electromagnetic showers

We explore the possibilty of detecting horizontal electromagnetic showers of tau neutrinos from individual gamma ray bursts, in large scale detectors like HiRes and Telescope Array. We study the role of the parameters of a gamma ray burst in determining the expected number of tau events from that burst. The horizontal beam of tau leptons produce visible signals in the atmosphere. We find that there is a slim chance of observing tau lepton appearances from GRBs with Telescope Array. The number of signals is strongly dependent on the Lorentz factor $Γ$, redshift $z$ of a GRB, energy emitted in muon neutrinos and antineutrinos $E_{ν,HE}$ and also on some other parameters of a GRB. It is possible to understand neutrino oscillations in astrophysical neutrinos and the mechanism or model of neutrino production inside a GRB by detection or non detection of tau lepton signals from it.

astro-ph↗

arXiv2003

Ultrahigh Energy Cosmic Rays and Prompt TeV Gamma Rays from Gamma Ray Bursts

Gamma Ray Bursts (GRBs) have been proposed as one {\it possible} class of sources of the Ultrahigh Energy Cosmic Ray (UHECR) events observed up to energies $\gsim10^{20}\ev$. The synchrotron radiation of the highest energy protons accelerated within the GRB source should produce gamma rays up to TeV energies. Here we briefly discuss the implications on the energetics of the GRB from the point of view of the detectability of the prompt TeV gamma rays of proton-synchrotron origin in GRBs in the up-coming ICECUBE muon detector in the south pole.

astro-ph↗

arXiv2003

Detectability of Prompt TeV Gamma Rays of Proton Synchrotron Origin from Gamma Ray Bursts by AMANDA/ICECUBE Detectors

The detectability of a possible ``high'' energy ($>100\gev$) component of Gamma-Ray Bursts (GRBs) using AMANDA/ICECUBE large area muon detectors is examined within the context of a specific model of such high energy gamma ray production within GRBs, namely, the proton-synchrotron model, which requires protons to be accelerated to ultrahigh energies $\gsim10^{20}\ev$ within GRBs. In this model, the high energy component is distinct from, but may well be emitted in coincidence with, the usual ``low'' (keV--MeV) energy component observed by satellite-borne detectors. The AMANDA/ICECUBE detectors can detect TeV photons by detecting the secondary muons created by the TeV photons in the Earth's atmosphere. We calculate the muon signal to noise ratio in these detectors due to TeV gamma-rays from individual GRBs for various assumptions on their luminosity, distance (redshift), Lorentz Gamma factor of the underlying fireball model, and various spectral characteristics of the GRBs, including the effect of the absorption of TeV photons within the GRB as well as in the intergalactic infrared radiation background. The intergalactic absorption effect essentially precludes detection of TeV photons in the AMANDA detector for reasonable values of the luminosity in the high energy component, but they may well be detectable in the proposed ICECUBE detector which may have an effective area for downward-going muons a factor of 100 larger than that in AMANDA. However, even in ICECUBE, only relatively close-by GRBs at redshifts $< 0.05$ or so can be expected to be detectable with any reasonable degree of confidence. We discuss the requirement on the luminosity of the GRB in the high energy component for its detectability in ICECUBE.

astro-ph↗

arXiv2002

The Appearance of Tau Neutrinos from a Gamma Ray Burst

The muon neutrinos from the Gamma Ray Bursts(GRBs) during their propagation oscillate to tau neutrinos. The tau neutrinos can be identified by the appearance of tau leptons which decay to muons below the detector. We examine the prospect of detecting these tau neutrinos from individual GRBs in a large area muon detector. We investigate for what range of values of the physical parameters of the GRBs such as Lorentz factor, redshift(distance from the observer) and total energy emitted in neutrino emissions the appearance of tau neutrinos from individual GRBs will be observable in a ground based muon detector of km^2 area. We find it will be possible to detect tau neutrino signals from a single GRB in a ground based muon detector of km^2 area above a muon threshold energy 30 PeV and at a zenith angle 180 degree if the energy emitted in neutrinos is 10^53 erg and its redshift is below 0.01 or if the energy emitted in neutrinos by the GRB is 10^54 erg and its redshift is below 0.03.

astro-ph↗

arXiv2002

Neutrino Induced Upward Going Muons from a Gamma Ray Burst in a Neutrino Telescope of Km^2 Area

The number of neutrino induced upward going muons from a single Gamma Ray Burst (GRB) expected to be detected by the proposed kilometer scale IceCube detector at the South Pole location has been calculated. The effects of the Lorentz factor, total energy of the GRB emitted in neutrinos and its distance from the observer (red shift) on the number of neutrino events from the GRB have been examined. The present investigation reveals that there is possibility of exploring the early Universe with the proposed kilometer scale IceCube neutrino telescope.

astro-ph↗

arXiv2001

Detecting TeV Gamma Rays from Gamma Ray Bursts by Ground Based Muon Detectors

The possibility of detecting Gamma-Ray Bursts (GRB) in TeV energy range using large area muon detectors like AMANDA and Lake Baikal detector is examined. These detectors can detect TeV energy photons by detecting the secondary muons created by the TeV photons in the Earth's atmosphere. We calculate the expected number of muons and the signal to square root of noise ratios in these detectors due to TeV gamma-rays from individual GRBs for various assumption on their luminosity, distance from the observer (redshift), gamma-ray integral spectral index, maximum energy cutoff of the photon spectrum and duration including the effect of the absorption of TeV photons in the intergalactic infrared radiation background. We also calculate the expected rate of detectable TeV-GRB events in these detectors using a recent determination of the luminosity and redshift distributions of GRBs in the Universe.

astro-ph↗