Ground-based Gamma Ray Astronomy

This paper is the write-up of a rapporteur talk given by the author at the 33rd International Cosmic Ray Conference in Rio de Janeiro, Brazil, in 2013. It attempts to summarize results and developments in ground-based gamma-ray observations and instrumentation from among the $\sim300$ submissions to the gamma-ray sessions of the meeting. Satellite observations and theoretical developments were covered by a companion rapporteur. Any review of this nature is unavoidably subjective, and incomplete. Nevertheless, the article should provide a useful status report for those seeking an overview of this exciting and fast-moving field

VERITAS Observations of a "Forbidden Velocity Wing"

The H.E.S.S. extended Galactic plane survey revealed the presence of a new extended TeV gamma-ray source, HESSJ1503-582, with no obvious counterpart at other wavelengths. The source is, however, coincident with an HI structure with a velocity significantly different from that of galactic rotation - a so-called "Forbidden Velocity Wing". These structures have been suggested as the fast moving shells and filaments associated with the oldest supernova remnants in our galaxy. The detection of TeV gamma-ray emission from these structures might indicate that supernova remnants remain efficient particle accelerators for much longer than is commonly believed. Here we report on recent VERITAS observations of one of these structures, FVW 190.2+1.1, which shows a clear shell-like morphology in the HI maps.Comment: Proc. 31st ICRC, Lodz, Poland, 200

Exploiting VERITAS Timing Information

The 499 pixel photomultiplier cameras of the VERITAS gamma ray telescopes are instrumented with 500MHz sampling Flash ADCs. This paper describes a preliminary investigation of the best methods by which to exploit this information so as to optimize the signal-to-noise ratio for the detection of Cherenkov light pulses. The FADCs also provide unprecedented resolution for the study of the timing characteristics of Cherenkov images of cosmic-ray and gamma-ray air showers. This capability is discussed, together with the implications for gamma-hadron separation.Comment: 4 pages. To appear in the Proceedings of the 29th ICRC, Pun

Likelihood Functions for Galaxy Cluster Surveys

Galaxy cluster surveys offer great promise for measuring cosmological parameters, but survey analysis methods have not been widely studied. Using methods developed decades ago for galaxy clustering studies, it is shown that nearly exact likelihood functions can be written down for galaxy cluster surveys. The sparse sampling of the density field by galaxy clusters allows simplifications that are not possible for galaxy surveys. An application to counts in cells is explicitly tested using cluster catalogs from numerical simulations and it is found that the calculated probability distributions are very accurate at masses above several times 10^{14}h^{-1} solar masses at z=0 and lower masses at higher redshift.Comment: 6 pages, 3 figures, updated key referenc

The unusual smoothness of the extragalactic unresolved radio background

If the radio background is coming from cosmological sources, there should be some amount of clustering due to the large scale structure in the universe. Simple models for the expected clustering combined with the recent measurement by ARCADE-2 of the mean extragalactic temperature lead to predicted clustering levels that are substantially above upper limits from searches for anisotropy on arcminute scales using ATCA and the VLA. The rms temperature variations in the cosmic radio background appear to be more than a factor of 10 smaller (in temperature) than the fluctuations in the cosmic infrared background. It is therefore extremely unlikely that this background comes from galaxies, galaxy clusters, or any sources that trace dark matter halos at z<5, unless typical sources are smooth on arcminute scales, requiring typical sizes of several Mpc.Comment: 4 pages, 1 figur

Whipple observations of 1ES1959+650: an Update

Strong flares of TeV gamma-ray emission up to a level of ~5 Crab were detected by the Whipple 10 m atmospheric Cerenkov telescope from the BL Lacertae object 1ES1959+650 during May - July 2002. We report here the results of follow up observations during 2002 - 2003.Comment: 4 pages to be published in the Proceedings of the 28th International Cosmic Ray Conference (Tsukuba, Japan 2003

VERITAS: HAWC's Neighbour to the North

This paper summarizes a presentation given on the occasion of the inauguration of the High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory in Puebla, Mexico in March 2015. The inauguration of a new facility for the study of astrophysical gamma-rays provides an excellent opportunity to review the technical evolution and the scientific achievements of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) since its own inauguration in 2007. HAWC and VERITAS are separated by only 14 degrees in longitude, and so can view much of the same sky at the same time. In combination with other ground-based facilities, and with the instruments onboard the Fermi Gamma-ray Space Telescope, VERITAS and HAWC will give an unprecedented view of the gamma-ray sky. We provide an overview of VERITAS, and discuss the complementarity of the two observatories for future gamma-ray observations.Comment: Presented at the HAWC Inauguration workshop in Puebla, Mexico in March 201

Hydrodynamic Coulomb drag and bounds on diffusion

We study Coulomb drag between an active layer with a clean electron liquid and a passive layer with a pinned electron lattice in the regime of fast intralayer equilibration. Such a two-fluid system offers an experimentally realizable way to disentangle the fast rate of intralayer electron-electron interactions from the much slower rate of momentum transfer between both layers. We identify an intermediate temperature range above the Fermi energy of the electron fluid but below the Debye energy of the electronic crystal where the hydrodynamic drag resistivity is directly proportional to a fast electron-electron scattering rate. The results are compatible with the conjectured scenario for strong electron-electron interactions which poses that a linear temperature dependence of resistivity originates from a "Planckian" electron relaxation time $\tau_{eq}\sim \hbar/k_BT$. We compare this to the better known semiclassical case, where the diffusion constant is found to be not proportional to the microscopic timescale.Comment: 8 pages, 2 figures, supplementary materia

VERITAS Observations of LS I +61 303 in the Fermi Era

The high-mass X-ray binary system LS I +61 303 is well known as a rare example of a variable Galactic GeV and TeV gamma-ray emitter. Despite years of study, many aspects of the system remain unclear; the nature of the compact object, the particle acceleration mechanisms and the gamma-ray emission and absorption processes can all be modelled in a variety of different scenarios. Here we report on a deep exposure of LS I +61 303 made with the VERITAS array during the 2008-2009 observing season. These are the first TeV observations made with contemporaneous coverage at lower energies by the LAT onboard Fermi, and as such provide a new set of constraints for system models.Comment: Proc 31st ICRC, Lodz, Poland, 200

Galactic Binary Systems

The population of binary systems known to emit in the GeV and TeV bands consists of only a few firmly identified Galactic sources. These rare objects constitute extreme particle accelerators operating under varying, but regularly repeating, conditions. As such, they provide access to a unique laboratory in which to study particle acceleration, and the nature of gamma-ray production, emission and absorption processes near compact objects. Here we review the current observational status of the field, and discuss some of the recent interpretations of the results.Comment: Proceedings of the 2009 Fermi Symposium. eConf Proceedings C09112