77 research outputs found
Mapping the most energetic cosmic rays
The correlation between the directions of optically-detected AGNs within 75
Mpc and the arrival directions of cosmic rays above 57 EeV detected by the
Auger collaboration up to August 2007 is examined using uniform-exposure plots
and a form of right ascension resonance which does not require choice of an
association window radius. Using the latter, the chance of accidental
correlation is found to be well below 1 in 10^5 even without using 3.2 degree
windows; the correlation with FRI-like radio galaxies within 75 Mpc, listed by
Nagar & Matulich (which are in AGN clusters), is separately just as
significant; and a correlation can also be found in other data at a lower
energy. Cen A is currently inactive at this energy, as more distant radio
galaxies are so prominent. The efficacy of a 57 EeV cut to select this
revelatory (proton) sample of the Auger data may be almost accidental and not
robust. The cosmic rays in the Auger sample seem to be scattered by ~3-4
degrees en route, from about 50 Mpc, and in one relatively well probed sky
region there may be a 4 degree systematic deflection in a Bz component of the
magnetic field in the galactic halo. The sources appear to be mostly within 120
Mpc. This is compatible with a GZK survival horizon, but only if (a) the Auger
energies are underestimated by ~25% and (b) the sudden fall in the energy
spectrum is not simply a GZK effect but essentially reflects an energy cut-off
in the sources.Comment: Submitted to Astroparticle Physics; 27 pages, 13 figures (20 panels
A Constraint on Electromagnetic Acceleration of Highest Energy Cosmic Rays
The energetics of electromagnetic acceleration of ultra-high-energy cosmic
rays (UHECRs) is constrained both by confinement of a particle within an
acceleration site and by radiative energy losses of the particle in the
confining magnetic fields. We demonstrate that the detection of ~ 3 x 10^{20}
eV events is inconsistent with the hypothesis that compact cosmic accelerators
with high magnetic fields can be the sources of UHECRs. This rules out the most
popular candidates, namely spinning neutron stars, active galactic nuclei
(AGNs), and gamma-ray burst blast waves. Galaxy clusters and, perhaps, AGN
radio lobes remain the only possible (although not very strong) candidates for
UHECR acceleration sites. Our analysis places no limit on linear accelerators.
With the data from the future Auger experiment one should be able to answer
whether a conventional theory works or some new physics is required to explain
the origin of UHECRs.Comment: 5 pages, 2 figures. Accepted for publication in PR
Recommended from our members
Results from the CACTI experiment: Air-Cerenkov and particle measurements of PeV air showers at Los Alamos
An array of six wide angle Cerenkov detectors was constructed amongst the scintillator and muon detectors of the CYGNUS II array at Los Alamos National Laboratory to investigate cosmic ray composition in the PeV region through measurements of the shape of Cerenkov lateral distributions. Data were collected during clear, moonless nights over three observing periods in 1995. Estimates of depths of shower maxima determined from the recorded Cerenkov lateral distributions align well with existing results at higher energies and suggest a mixed to heavy composition in the PeV region with no significant variation observed around the knee. The accuracy of composition determination is limited by uncertainties in the expected levels of depth of maximum predicted using different Monte-Carlo shower simulation models
Constraints on the Ultra High Energy Photon flux using inclined showers from the Haverah Park array
We describe a method to analyse inclined air showers produced by ultra high
energy cosmic rays using an analytical description of the muon densities. We
report the results obtained using data from inclined events
(60^{\circ}<\theta<80^{\circ}) recorded by the Haverah Park shower detector for
energies above 10^19 eV. Using mass independent knowledge of the UHECR spectrum
obtained from vertical air shower measurements and comparing the expected
horizontal shower rate to the reported measurements we show that above 10^19 eV
less than 48 % of the primary cosmic rays can be photons at the 95 % confidence
level and above 4 X 10^19 eV less than 50 % of the cosmic rays can be photonic
at the same confidence level. These limits place important constraints on some
models of the origin of ultra high-energy cosmic rays.Comment: 45 pages, 25 figure
Cosmic ray knee and new physics at the TeV scale
We analyze the possibility that the cosmic ray knee appears at an energy
threshold where the proton-dark matter cross section becomes large due to new
TeV physics. It has been shown that such interactions could break the proton
and produce a diffuse gamma ray flux consistent with MILAGRO observations. We
argue that this hypothesis implies knees that scale with the atomic mass for
the different nuclei, as KASKADE data seem to indicate. We find that to explain
the change in the spectral index in the flux from E^{-2.7} to E^{-3.1} the
cross section must grow like E^{0.4+\beta} above the knee, where \beta=0.3-0.6
parametrizes the energy dependence of the age (\tau \propto E^{-\beta}) of the
cosmic rays reaching the Earth. The hypothesis also requires mbarn cross
sections (that could be modelled with TeV gravity) and large densities of dark
matter (that could be clumped around the sources of cosmic rays). We argue that
neutrinos would also exhibit a threshold at E=(m_\chi/m_p)E_{knee}\approx 10^8
GeV where their interaction with a nucleon becomes strong. Therefore, the
observation at ICECUBE or ANITA of standard neutrino events above this
threshold would disprove the scenario.Comment: 10 pages, version to appear in JCA
A high resolution imaging detector for TeV gamma-ray astronomy
Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques
Particle Acceleration at Relativistic Shocks
I review the current status of Fermi acceleration theory at relativistic
shocks. I first discuss the relativistic shock jump conditions, then describe
the non-relativistic Fermi mechanism and the differences introduced by
relativistic flows. I present numerical calculations of the accelerated
particle spectrum, and examine the maximum energy attainable by this process. I
briefly consider the minimum energy for Fermi acceleration, and a possible
electron pre-acceleration mechanism.Comment: 17 pages, 4 figures. To appear in "Relativistic Flows in
Astrophysics", A.W. Guthmann, M. Georganopoulos, A. Marcowith and K.
Manolokou, eds., Lecture Notes in Pysics, Springer Verla
A first EGRET-UNID-related agenda for the next-generation Cherenkov telescopes
The next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) will
open the regime between approx. 30 GeV and 200 GeV to ground-based gamma
observations with unprecedented point source sensitivity and source location
accuracy. I examine the prospects of observing the unidentified objects (UNIDs)
of the Third EGRET Catalog using the IACT observatories currently under
construction by the CANGAROO, HESS, MAGIC and VERITAS collaborations. Assuming
a modest spectral steepening similar to that observed in the inverse Compton
component of the Crab Nebula spectrum and taking into account the sensitivity
of the instruments and its zenith angle dependence, a detailed list of 78
observable objects is derived which is then further constrained to 38 prime
candidates. The characteristics of this agenda are discussed.Comment: 21 pages, 5 figures, to be published in Carraminana, Reimer &
Thompson (eds.) Proc. "The nature of unidentified high-energy gamma-ray
sources (Tonantzintla, Mexico, October 2000)", Kluwer Academi
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Dark Matter signals from Draco and Willman 1: Prospects for MAGIC II and CTA
The next generation of ground-based Imaging Air Cherenkov Telescopes (IACTs)
will play an important role in indirect dark matter searches. In this article,
we consider two particularly promising candidate sources for dark matter
annihilation signals, the nearby dwarf galaxies Draco and Willman 1, and study
the prospects of detecting such a signal for the soon-operating MAGIC II
telescope system as well as for the planned installation of CTA, taking special
care of describing the experimental features that affect the detectional
prospects. For the first time in such a study, we fully take into account the
effect of internal bremsstrahlung, which has recently been shown to
considerably enhance, in some cases, the gamma-ray flux at the high energies
where Atmospheric Cherenkov Telescopes operate, thus leading to significantly
harder annihilation spectra than traditionally considered. While the detection
of the spectral features introduced by internal bremsstrahlung would constitute
a smoking gun signature for dark matter annihilation, we find that for most
models the overall flux still remains at a level that will be challenging to
detect unless one adopts rather (though by no means overly) optimistic
astrophysical assumptions about the distribution of dark matter in the dwarfs.Comment: 10 pages, 4 figures, minor changes, matches the published version
(JCAP
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