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The development of techniques whereby gamma rays of energy 100 GeV and above can be studied from the ground, using indirect, but sensitive, techniques has opened up a new area of high energy photon astronomy. The most exciting result that has come from these is the detection of highly variable fluxes of TeV gamma rays from the relativistic jets in nearby AGN. The recent detection of signals from a starburst galaxy and from a radio galaxy opens the possibility that the extragalactic emission of TeV gamma rays is a ubiquitous phenomenon. Here we attempt to summarize the properties of the sources detected so far. [Copyright &y& Elsevier]

Over the past eight years, observations of BL Lacertae type objects have been an important part of the observing program at the Whipple Observatory 10 m γ-ray telescope. During this time, TeV emission was detected from five such objects: Markarian 421, Markarian 501, 1ES2344+514, H1426+428 and 1ES1959+650. Upper limits were placed on the TeV emission from 29 other BL Lacertae objects. Among these objects are two unconfirmed TeV sources, 3C66A and BL Lacertae. An overview of the results of these observations of BL Lacertae objects, between 1995 and 2000, at the Whipple Observatory is presented here. [Copyright &y& Elsevier]

We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e + e − pair-creation regime. We use data collected at a gamma-ray beam of known polarisation. The TPC provides two orthogonal projections ( x, z ) and ( y, z ) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified. We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam. We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy. This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime. [ABSTRACT FROM AUTHOR]

Current γ -ray telescopes based on photon conversions to electron-positron pairs, such as Fermi, use tungsten converters. They suffer of limited angular resolution at low energies, and their sensitivity drops below 1 GeV. The low multiple scattering in a gaseous detector gives access to higher angular resolution in the MeV–GeV range, and to the linear polarisation of the photons through the azimuthal angle of the electron-positron pair. HARPO is an R&D programme to characterise the operation of a TPC (Time Projection Chamber) as a high angular-resolution and sensitivity telescope and polarimeter for γ rays from cosmic sources. It represents a first step towards a future space instrument. A 30 cm cubic TPC demonstrator was built, and filled with 2 bar argon-based gas. It was put in a polarised γ -ray beam at the NewSUBARU accelerator in Japan in November 2014. Data were taken at different photon energies from 1.7 MeV to 74 MeV, and with different polarisation configurations. The electronics setup is described, with an emphasis on the trigger system. The event reconstruction algorithm is quickly described, and preliminary measurements of the polarisation of 11 MeV photons are shown. [ABSTRACT FROM AUTHOR]

Abstract: We describe a dedicated cosmic-ray telescope that explores a new method for detecting Cherenkov radiation from high-energy primary cosmic rays and the large particle air shower they induce upon entering the atmosphere. Using a camera comprising 16 multi-anode photomultiplier tubes for a total of 256 pixels, the Track Imaging Cherenkov Experiment (TrICE) resolves substructures in particle air showers with 0.086° resolution. Cherenkov radiation is imaged using a novel two-part optical system in which a Fresnel lens provides a wide-field optical trigger and a mirror system collects delayed light with four times the magnification. TrICE records well-resolved cosmic-ray air showers at rates ranging between 0.01Hz and 0.1Hz. [Copyright &y& Elsevier]

Abstract: The short gamma-ray front air-cherenkov experiment (SGARFACE) uses the Whipple 10m telescope to search for bursts of γ-rays. SGARFACE is sensitive to bursts with duration from a few ns to ∼20μs and with γ-ray energy above 100MeV. SGARFACE began operating in March 2003 and has collected 2.2 million events during an exposure time of 2267h. A search for bursts of γ-rays from explosions of primordial black holes (PBH) was carried out. A Hagedorn-type PBH explosion is predicted to be visible within 60pc of Earth. Background events were caused by cosmic rays and by atmospheric phenomena and their rejection was accomplished to a large extent using the time-resolved images. No unambiguous detection of bursts of γ-rays could be made as the remaining background events mimic the expected shape and time-development of bursts. Upper limits on the PBH explosion rate were derived from the SGARFACE data and are compared to previous and future experiments. We note that a future array of large wide-field air-Cherenkov telescopes equipped with a SGARFACE-like trigger would be able to operate background-free with a 20–30 times higher sensitivity for PBH explosions. [Copyright &y& Elsevier]