Exploring Quantum Gravity with Very-High-Energy Gamma-Ray Instruments - Prospects and Limitations

Some models for quantum gravity (QG) violate Lorentz invariance and predict an energy dependence of the speed of light, leading to a dispersion of high-energy gamma-ray signals that travel over cosmological distances. Limits on the dispersion from short-duration substructures observed in gamma-rays emitted by gamma-ray bursts (GRBs) at cosmological distances have provided interesting bounds on Lorentz invariance violation (LIV). Recent observations of unprecedentedly fast flares in the very-high energy gamma-ray emission of the active galactic nuclei (AGNs) Mkn 501 in 2005 and PKS 2155-304 in 2006 resulted in the most constraining limits on LIV from light-travel observations, approaching the Planck mass scale, at which QG effects are assumed to become important. I review the current status of LIV searches using GRBs and AGN flare events, and discuss limitations of light-travel time analyses and prospects for future instruments in the gamma-ray domain.Comment: 11 pages, 4 figures, proceedings of "Science with the New Generation of High Energy Gamma-Ray Experiments", Euganean Spa Area, Padova: October 8-10, 200

Lorentz Symmetry breaking studies with photons from astrophysical observations

Lorentz Invariance Violation (LIV) may be a good observational window on Quantum Gravity physics. Within last few years, all major Gamma-ray experiments have published results from the search for LIV with variable astrophysical sources: gamma-ray bursts with detectors on-board satellites and Active Galactic Nuclei with ground-based experiments. In this paper, the recent time-of-flight studies with unpolarized photons published from the space and ground based observations are reviewed. Various methods used in the time delay searches are described, and their performance discussed. Since no significant time-lag value was found within experimental precision of the measurements, the present results consist of 95% confidence cevel limits on the Quantum Gravity scale on the linear and quadratic terms in the standard photon dispersion relations.Comment: 22 pages, 9 figures. V2 match the published version. Invited review talk to the 2nd International Colloquium "Scientific and Fundamental Aspects of the Galileo Programme", 14-16 october 2009, Padua, Ital

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

High-Energy Gamma-Ray Astronomy

This volume celebrates the 30th anniversary of the first very-high energy (VHE) gamma-ray Source detection: the Crab Nebula, observed by the pioneering ground-based Cherenkov telescope Whipple, at teraelectronvolts (TeV) energies, in 1989. As we entered a new era in TeV astronomy, with the imminent start of operations of the Cherenkov Telescope Array (CTA) and new facilities such as LHAASO and the proposed Southern Wide-Field Gamma-ray Observatory (SWGO), we conceived of this volume as a broad reflection on how far we have evolved in the astrophysics topics that dominated the field of TeV astronomy for much of recent history.In the past two decades, H.E.S.S., MAGIC and VERITAS pushed the field of TeV astronomy, consolidating the field of TeV astrophysics, from few to hundreds of TeV emitters. Today, this is a mature field, covering almost every topic of modern astrophysics. TeV astrophysics is also at the center of the multi-messenger astrophysics revolution, as the extreme photon energies involved provide an effective probe in cosmic-ray acceleration, propagation and interaction, in dark matter and exotic physics searches. The improvement that CTA will carry forward and the fact that CTA will operate as the first open observatory in the field, mean that gamma-ray astronomy is about to enter a new precision and productive era.This book aims to serve as an introduction to the field and its state of the art, presenting a series of authoritative reviews on a broad range of topics in which TeV astronomy provided essential contributions, and where some of the most relevant questions for future research lie

Gamma-ray summary report

ManuscriptThis paper reviews the field of gamma-ray astronomy and describes future experiments and prospects for advances in fundamental physics and high-energy astrophysics through gamma-ray measurements. We concentrate on recent progress in the understanding of active galaxies, and the use of these sources as probes of intergalactic space. We also describe prospects for future experiments in a number of areas of fundamental physics, including: searches for an annihilation line from neutralino dark matter, understanding the energetics of supermassive black holes, using AGNs as cosmological probes of the primordial radiation fields, constraints on quantum gravity, detection of a new spectral component from GRBs, and the prospects for detecting primordial black holes

Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS

Blazars are active galactic nuclei with a relativistic jet directed towards the observer's line of sight. Characterization of the non-thermal continuum emission originating from the blazar jet is currently an essential question in high-energy astrophysics. A blazar spectral energy distribution (SED) has a typical double-peaked shape in the flux vs. energy representation. The low-energy component of the SED is well-studied and thought to be due to synchrotron emission from relativistic electrons. The high-energy component, on the other hand, is still not completely understood and the emission in this part of the blazar spectrum can extend to energies as high as tera electron volts in some objects. This portion of the electromagnetic spectrum is referred to as the very-high-energy (VHE or TeV, E > 0.1 TeV) regime. At the time of this writing, more than half a hundred blazars have been detected to emit TeV gamma rays, representing the high energy extreme of these objects and constituting a population of its own. Most of these TeV blazars have also been detected in the high-energy (HE or GeV, 0.1 GeV < E < 0.1 TeV) gamma-ray range. In this work, we report on our discovery of the TeV emission from the blazar RBS 0413 and perform a detailed data analysis on this source, including contemporaneous multi-wavelength observations to characterize the broad-band SED and test various emission models for the high-energy component. Further, we extend our focus on the high-energy component to all archival TeV-detected blazars and study their spectral properties in the framework of GeV and TeV gamma-ray observations. To do this, we assemble for the first time the GeV and TeV spectra of a complete sample of TeV-detected blazars available in the archive to date. In the Appendix we present an analysis method for improved observations of large zenith angle targets with VERITAS