Astrophysics at Ultra-High Energies

PREFACE

CONTENTS

Gamma Ray Bursts (GRBs) are bright, brief flashes of high energy photons and are the most powerful explosions since the Big Bang, with typical energies up to around 1051 ergs. Their outbursts persist for durations ranging from milliseconds to tens of seconds or more. In these brief moments the explosions radiate more energy than the Sun will release in its entire 10 billion year lifetime. They come in two classes: long (¿2 s), softspectrum bursts and short, hard events. Current theories attribute these phenomena to the final collapse of a massive star, or the coalescence of a binary system induced by gravity wave emission.

New results from Swift and related programmes offer fresh understanding of the physics of gamma-ray bursts and of the local environments and host galaxies of burst progenitors. Bursts found at very high red-shifts are new tools for exploring the intergalactic medium, the first stars and the earliest stages of galaxy formation.

Rapid follow-up of gamma-ray burst (GRB) afterglows with the multi-wavelength satellite Swift and other instruments is leading to a reappraisal and expansion of the standard model of the GRB early afterglow and prompt gamma-ray emission. The previously uncharted time range of minutes to hours has revealed systematic X-ray light curve properties such as steep decays, shallow decays and flares. Other discoveries include the localization and follow-up of short GRB afterglows, the detection of long bursts beyond the redfshift z≳6, the detection of prompt optical/IR emission while the gamma-rays are still on, the detection and prompt follow-up of supernovae associated with GRB. We review some of the current theoretical issues.

The BL Lac object 3C 66A was the target of an intensive multiwavelength monitoring campaign organized in 2003–2004. During the campaign, its spectral energy distribution (SED) was measured and flux measurements from radio to X-ray frequencies as well as upper limits in the very high energy (VHE) γ-ray regime were obtained. Here, we reproduce the SED and optical spectral variability pattern observed during our multiwavelength campaign using a time-dependent leptonic jet model. Our model could successfully simulate the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~ 4 GeV.

Between the well known adiabatic and radiative stages of the Supernova remnant (SNR) evolution there is, in fact, a transition stage with a duration comparable to the duration of adiabatic one. Physical existence of the transition stage is motivated by cooling of some part of the downstream hot gas with formation of a thin cold shell that is joined to a shell of swept up interstellar medium (ISM). We give an approximate analytical method for full hydrodynamic description of the transition stage. On its base we investigate the evolution of X-ray and γ-ray radiation during this stage. The role of the transition stage in cosmic ray (CR) re-acceleration is discussed as well.

Dark matter has been recognized as an essential part of matter for over 70 years now, and many suggestions have been made, what it could be. Most of these ideas have centered on Cold Dark Matter, particles that are expected in extensions of standard particle physics, such as supersymmetry. Here we explore the concept that dark matter is sterile neutrinos, a concept that is commonly referred to as Warm Dark Matter. Such particles have keV masses, and decay over a very long time, much longer than the Hubble time. In their decay they produce X-ray photons which modify the ionization balance in the early universe, increasing the fraction of molecular Hydrogen, and thus help early star formation. Sterile neutrinos may also help to understand the baryon-asymmetry, the pulsar kicks, the early growth of black holes, the minimum mass of dwarf spheroidal galaxies, as well as the shape of dark matter halos. As soon as all these tests have been quantitative in its various parameters, we may focus on the creation mechanism of these particles, and could predict the strength of the sharp X-ray emission line, expected from any large dark matter assembly. A measurement of this X-ray emission line would be definitive proof for the existence of may be called weakly interacting neutrinos, or WINs.

The processes of cosmic ray acceleration and transport in the Galaxy are briefly discussed.

The energies of cosmic rays, fully ionized charged nuclei, extend over a wide range up to 10²⁰ eV. A particularly interesting energy region spans from 10¹⁴ to 10¹⁸ eV, where the all-particle energy spectrum exhibits two interesting structures, the ‘knee’ and the ‘second knee’. An explanation of these features is thought to be an important step in understanding of the origin of the high-energy particles. Recent results of air shower experiments in this region are discussed. Special attention is drawn to explain the principle of air shower measurements — a simple Heitler model of (hadronic) air showers is developed.

We briefly discuss ideas related to the origin of the ultrahigh energy cosmic rays. If these extremely high energy particles are generated all around the Universe their production spectrum is modified by interactions in the universal photon fields in their propagation from the sources to us. Cosmogenic neutrinos that are generated in these interactions can help us understand better their origin.

In the standard gamma-ray burst model cosmic rays can be accelerated up to GZK energies E_p ~ 10²⁰ eV, with a flux comparable to that detected in large EAS arrays such as AUGER. Both leptonic, e.g. synchrotron and inverse Compton, as well as photomeson processes can lead to GeV-TeV gamma-rays measurable by GLAST, AGILE, or ACTs, serving as probes of the burst physics and model parameters. Photomeson interactions also produce neutrinos at energies ranging from sub-TeV to EeV, which may yield information about the fundamental interaction physics, as well as the acceleration mechanism, the nature of the sources and their environment. This emission will be probed with forthcoming experiments such as IceCube, ANITA and KM3NeT.

Here in this lecture we will touch on two aspects, one the new radio methods to observe the effects of high energy particles, and second the role that radio galaxies play in helping us understand high energy cosmic rays. We will focus here on the second topic, and just review the latest developments in the first. Radio measurements of the geosynchrotron radiation produced by high energy cosmic ray particles entering the atmosphere of the Earth as well as radio Čerenkov radiation coming from interactions in the Moon are another path; radio observations of interactions in ice at the horizon in Antarctica is a related attempt. Radio galaxy hot spots are prime candidates to produce the highest energy cosmic rays, and the corresponding shock waves in relativistic jets emanating from nearly all black holes observed. We will review the arguments and the way to verify the ensuing predictions. This involves the definition of reliable samples of active sources, such as black holes, and galaxies active in star formation. The AUGER array will probably decide within the next few years, where the highest energy cosmic rays come from, and so frame the next quests, on very high energy neutrinos and perhaps other particles.

No abstract received.

The KASCADE–Grande experiment measures extensive air showers induced by primary cosmic rays in the energy range of 10¹⁴ — 10¹⁸ eV. As extension of the original KASCADE experiment it allows the investigation of the knee and the possible second knee in the cosmic ray energy spectrum. An overview of the experimental setup and preliminary results are given.

Observations of Ultra-Heavy galactic cosmic rays (GCR) help to distinguish the possible origins of GCRs. The Trans-Iron Galactic Recorder (TIGER) is designed to measure the charge (Z) and energy of GCRs using a combination of scintillation counters, Cherenkov counters, and a scintillating fiber hodoscope. TIGER has accumulated data on two successful flights from McMurdo, Antarctica: the first launched in December of 2001 with a total flight duration of 31.8 days and the second in December of 2003 with a total flight duration of 18 days. The two flights of TIGER achieved sufficient statistics and charge resolution to resolve ~140 particles with Z > 30, and have provided the best measurements to date for Zn, Ga, Ge, and Se. We present a preliminary analysis of the combined data from both flights for Ultra-Heavy GCRs and discuss the results in the context of different GCR source models.

The Alpha Magnetic Spectrometer (AMS) to be installed on the International Space Station (ISS) will be equipped with a proximity focusing Ring Imaging Čerenkov detector (RICH). Reconstruction of the Čerenkov angle and the electric charge with RICH are discussed. A likelihood method for the Čerenkov angle reconstruction was applied leading to a velocity determination for protons with a resolution around 0.1%. The electric charge reconstruction is based on the counting of the number of photoelectrons and on an overall efficiency estimation on an event-by-event basis. The isotopic mass separation of helium and beryllium is presented.

Two multidirectional muon telescopes with EAS arrays are now under construction in Israel: one from 24 scintillators on Mt. Hermon (in combination with neutron monitor), and one from 96 scintillators as semi-underground (in the big bomb-shelter in Qazrin at a distance of about 1 nkm from the Central Laboratory of the Israel Cosmic Ray & Space Weather Center). The big one consists from 49 scintillation detectors inside the special constructed building with very light roof over the bomb-shelter and 49 scintillation detectors underground inside the bomb-shelter. This multidirectional telescope contain more than two thousand elementary telescopes directed at different zenith and az-imuthal angles and formed by double coincidences of any top scintillator with each bottom scintillator (the effective energy of primary CR from about 50 GeV for vertical direction to about 1–2 TeV for very inclined directions). It will give possibility to investigate global and other types of galactic CR modulations in the Heliosphere at very high energies, near the upper limit of CR energy on which magnetic fields frozen in solar wind may yet influence. Also we plane to obtain detailed information on the sidereal CR anisotropy in this range of energy.

We will measure also three types of EAS. Our estimations show that by EAS array we can continue measure high energy CR time variations in the broad range from about 1-2 TeV to about 10,000 TeV. By this experiment, we suppose to investigate with a high accuracy CR anisotropy in the Galaxy in dependence of particle energy and CR modulation in the Heliosphere at high-energy range.

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. In this paper we report our results on diffuse emission from the galactic plane and in particular the Cygnus region. Our observations show at least one new TeV source MGRO J2020+37 as well as correlations with the matter density in the region as would be expected from cosmic-ray proton interactions. However, the TeV gamma-ray flux from the Cygnus region (after excluding MGRO J2020+37) is roughly 5 times that expected from a conventional model of cosmic ray production and propagation.

During its first cycle of observations, the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope has observed very high energy γ-rays from five galactic objects: the Crab Nebula, the SNRs HESS J1813-178 and HESS J1834-087, the Galactic Center and the γ-ray binary LS I +61 303. After a short introduction to the MAGIC telescope and the data analysis procedure, the results of these five sources are reviewed.

MAGIC is currently the largest single dish ground–based imaging air Cherenkov telescope in operation. During its first cycle of observations more than 20 extragalactic objects have been observed, and very high energy γ-ray signals have been detected in several of them. The results of this observations are presented, together with a discussion of the spectral characteristics and the flux variability of the detected sources.

VERITAS (Very Energetic Radiation Imaging Telescope Array System), an array of ground-based gamma-ray telescopes in southern Arizona, USA, has been taking data in hardware stereo mode since March, 2006. The April–May 2006 dark run provided a large set of data from two telescopes on the known blazar Markarian (Mrk) 421. An initial analysis produced a light curve and preliminary cuts showing the two telescope array's angular resolution to be 0.19°. The remaining two VERITAS telescopes will be brought online by January, 2007.

The Gamma-ray Large Area Space Telescope (GLAST) is a collaboration of several countries: France, Germany, Italy, Japan, Sweden and United States of America. GLAST is a satellite-based observatory that will study the Cosmos in the Energy Range 10 keV - 300 GeV and consists of two different instruments: the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The two instruments are ready and now are being tested and integrated with the spacecraft: the launch is scheduled for October 2007. GLAST will improve the knowledge about several astrophysical sources (AGNs, Pulsars, GRBs, etc.). In this paper we will consider the scientific case of gamma-ray emitting Super-novae Remnants (SNRs).

The Antarctic Muon And Neutrino Detector Array (AMANDA) has been taking data since 2000 and its data acquisition system was upgraded in January 2003 to read out the complete digitized waveforms from the buried Photo-multipliers (PMTs) using Transient Waveform Recorders (TWR). This system currently runs in parallel with the standard AMANDA data acquisition system. Once AMANDA is incorporated into the 1 km³ detector IceCube, only the TWR system will be kept. We report results from a first atmospheric neutrino analysis on data collected in 2003 with TWR. Good agreement in event rate and angular distribution verify the performance of the TWR system. A search of the northern hemisphere for localized event clusters shows no statistically significant excess, thus a flux limit is calculated, which is in full agreement with previous results based on the standard AMANDA data acquisition system. We also update the status of a search for diffusely distributed neutrinos with ultra high energy (UHE) using data collected by the TWR system.

The status of the project is described: the activity on long term characterization of water optical and oceanographic parameters at the Capo Passero site candidate for the Mediterranean km³ neutrino telescope; the feasibility study; the physics performances and underwater technology for the km³; the activity on NEMO Phase 1, a technological demonstrator that is going to be deployed at 2000 m depth 25 km offshore Catania; the realisation of an underwater infrastructure at 3500 m depth at the candidate site (NEMO Phase 2).

The ANtarctic Impulse Transient Antenna (ANITA) is the first long-duration balloon experiment designed to search and measure the flux of Greisen-Zapsepin-Kuzmin (GZK) neutrinos. We present new limits on neutrinos fluxes of astronomical origin from data collected with the successful launch of a 2-antenna prototype instrument, called ANITA-lite, that circled the Antarctic continent for 18.4 days in January 2004. We performed a search for Ultra-High-Energy (UHE) neutrinos with energies above 3 × 10¹⁸ eV. No excess events above the background expectation were observed and a neutrino flux following E⁻² spectrum for all neutrino flavors, is limited to for 10^18.5 eV < E_ν < 10^23.5 eV at 90% confidence level. The launch of ANITA is scheduled for December 2006. Looking beyond ANITA, we describe a new idea, called ARIANNA (Antarctic Ross Iceshelf ANtenna Neutrino Array), to increase the sensitivity for GZK neutrinos by one order of magnitude better than ANITA.

LIST OF PARTICIPANTS