470 research outputs found
Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons
We discuss the effect of hypothetical violation of Lorentz invariance at high
energies on the formation of atmospheric showers by very-high-energy gamma
rays. In the scenario where Lorentz invariance violation leads to a decrease of
the photon velocity with energy the formation of the showers is suppressed
compared to the Lorentz invariant case. Absence of such suppression in the
high-energy part of spectrum of the Crab nebula measured independently by HEGRA
and H.E.S.S. collaborations is used to set lower bounds on the energy scale of
Lorentz invariance violation. These bounds are competitive with the strongest
existing constraints obtained from timing of variable astrophysical sources and
the absorption of TeV photons on the extragalactic background light. They will
be further improved by the next generation of multi-TeV gamma-ray
observatories.Comment: 21 pages, 4 figures. References adde
Lorentz violation at high energy: concepts, phenomena and astrophysical constraints
We consider here the possibility of quantum gravity induced violation of
Lorentz symmetry (LV). Even if suppressed by the inverse Planck mass such LV
can be tested by current experiments and astrophysical observations. We review
the effective field theory approach to describing LV, the issue of naturalness,
and many phenomena characteristic of LV. We discuss some of the current
observational bounds on LV, focusing mostly on those from high energy
astrophysics in the QED sector at order E/M_Planck. In this context we present
a number of new results which include the explicit computation of rates of the
most relevant LV processes, the derivation of a new photon decay constraint,
and modification of previous constraints taking proper account of the helicity
dependence of the LV parameters implied by effective field theory.Comment: v.1 56 pages, 3 figures, Invited article for Annals of Physics; v.2:
60 pages, 3 figures. Typos fixed, references added, minor editing for clarity
and accuracy; discussion of fermion pair emission added. To appear in January
2006 special issue of Annals of Physic
Astrophysics with High Energy Gamma Rays
Recent results, the present status and the perspectives of high energy
gamma-ray astronomy are described. Since the satellite observations by the
Compton Gamma Ray Observatory and its precursor missions have been reviewed
extensively, emphasis is on the results from the ground-based gamma-ray
telescopes. They concern the physics of Pulsar Nebulae, Supernova Remnants in
their assumed role as the Galactic sources of Cosmic Rays, Jets from Active
Galactic Nuclei, and the Extragalactic Background radiation field due to stars
and dust in galaxies. Since the gamma-ray emission is nonthermal, this kind of
astronomy deals with the pervasive high-energy nonequilibrium states in the
Universe. The present build-up of larger and more sensitive instruments, both
on the ground and in space, gives fascinating prospects also for observational
cosmology and astroparticle physics. Through realistically possible further
observational developments at high mountain altitudes a rapid extension of the
field is to be expected.Comment: 23 pages, 11 figures. To appear in "Astronomy, Cosmology and
Fundamental Physics", ed. P. A. Shaver, L. Di Lella, and A. Gimenez, Proc.
ESA-CERN-ESO Symposium, Garching, March 2002. Springer-Verlag, Berlin,
Heidelberg, series "ESO Astrophysics Symposia
Future Extensive Air Shower arrays: from Gamma-Ray Astronomy to Cosmic Rays
Despite large progresses in building new detectors and in the analysis
techniques, the key questions concerning the origin, acceleration and
propagation of Galactic Cosmic Rays are still open. A number of new EAS arrays
is in progress. The most ambitious and sensitive project between them is
LHAASO, a new generation multi-component experiment to be installed at very
high altitude in China (Daocheng, Sichuan province, 4400 m a.s.l.). The
experiment will face the open problems through a combined study of photon- and
charged particle-induced extensive air showers in the wide energy range
10 - 10 eV. In this paper the status of the experiment will be
summarized, the science program presented and the outlook discussed in
comparison with leading new projects.Comment: Invited plenary talk at RICAP 201
IceCube Constraints on Fast-Spinning Pulsars as High-Energy Neutrino Sources
Relativistic winds of fast-spinning pulsars have been proposed as a potential
site for cosmic-ray acceleration from very high energies (VHE) to ultrahigh
energies (UHE). We re-examine conditions for high-energy neutrino production,
considering the interaction of accelerated particles with baryons of the
expanding supernova ejecta and the radiation fields in the wind nebula. We make
use of the current IceCube sensitivity in diffusive high-energy neutrino
background, in order to constrain the parameter space of the most extreme
neutron stars as sources of VHE and UHE cosmic rays. We demonstrate that the
current non-observation of eV neutrinos put stringent constraints on
the pulsar scenario. For a given model, birthrates, ejecta mass and
acceleration efficiency of the magnetar sources can be constrained. When we
assume a proton cosmic ray composition and spherical supernovae ejecta, we find
that the IceCube limits almost exclude their significant contribution to the
observed UHE cosmic-ray flux. Furthermore, we consider scenarios where a
fraction of cosmic rays can escape from jet-like structures piercing the
ejecta, without significant interactions. Such scenarios would enable the
production of UHE cosmic rays and help remove the tension between their EeV
neutrino production and the observational data.Comment: 23 pages, 8 figures; published in JCAP04(2016)01
Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics
Very-high energy (VHE) gamma quanta contribute only a minuscule fraction -
below one per million - to the flux of cosmic rays. Nevertheless, being neutral
particles they are currently the best "messengers" of processes from the
relativistic/ultra-relativistic Universe because they can be extrapolated back
to their origin. The window of VHE gamma rays was opened only in 1989 by the
Whipple collaboration, reporting the observation of TeV gamma rays from the
Crab nebula. After a slow start, this new field of research is now rapidly
expanding with the discovery of more than 150 VHE gamma-ray emitting sources.
Progress is intimately related with the steady improvement of detectors and
rapidly increasing computing power. We give an overview of the early attempts
before and around 1989 and the progress after the pioneering work of the
Whipple collaboration. The main focus of this article is on the development of
experimental techniques for Earth-bound gamma-ray detectors; consequently, more
emphasis is given to those experiments that made an initial breakthrough rather
than to the successors which often had and have a similar (sometimes even
higher) scientific output as the pioneering experiments. The considered energy
threshold is about 30 GeV. At lower energies, observations can presently only
be performed with balloon or satellite-borne detectors. Irrespective of the
stormy experimental progress, the success story could not have been called a
success story without a broad scientific output. Therefore we conclude this
article with a summary of the scientific rationales and main results achieved
over the last two decades.Comment: 45 pages, 38 figures, review prepared for EPJ-H special issue "Cosmic
rays, gamma rays and neutrinos: A survey of 100 years of research
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