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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…
Cosmic rays travelling through interstellar space have their propagation directions repeatedly scattered by fluctuating interstellar magnetic fields. The nature of this scattering is a major unsolved problem in astrophysics, one that has…
Galactic diffuse emission has provided us with evidence for cosmic ray acceleration throughout the Galaxy and the background for searches for physics beyond the Standard Model. However, only the very limited measurements of the diffuse flux…
The recent detection of pulsed $\gamma$-ray emission from the Vela pulsar in the $\sim10$ to 100 GeV range by H.E.S.S. promises an important potential to probe into the very high energy (VHE) radiation mechanisms of pulsars. The recent…
At TeV energies, the gamma-ray horizon of the universe is limited to redshifts z<<1, and, therefore, any observation of TeV radiation from a source located beyond z=1 would call for a revision of the standard paradigm. While robust…
$\gamma$-ray pulsar halos, most likely formed by inverse Compton scattering of electrons and positrons propagating in the pulsar-surrounding interstellar medium with background photons, serve as an ideal probe for Galactic cosmic-ray…
The unexpectedly high flux of cosmic ray positrons detected at Earth may originate from nearby astrophysical sources, dark matter, or unknown processes of cosmic-ray secondary production. We report the detection, using the HighAltitude…
We present a short overview of the TeV-Halos objects as a discovery and a relevant contribution of the High Altitude Water \v{C}erenkov (HAWC) observatory to TeV astrophysics. We discuss history, discovery, knowledge, and the next step…
Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying…
Since the era of the Fermi/LAT and atmospheric Cerenkov telescopes, pulsars are known to emit high and very high-energy photons, in the MeV-GeV range and sometimes up to TeV. To date, it is still unclear where and how these photons are…
The GeV-scale gamma-ray excess observed from the region surrounding the Galactic Center has been interpreted as either the products of annihilating dark matter particles, or as the emission from a large population of faint and…
High energy gamma-ray astronomy has recently made significant progresss through ground-based instruments like the {\it H.E.S.S.} array of imaging atmospheric Cherenkov telescopes. The unprecedented angular resolution and the large field of…
The field of TeV gamma-ray astronomy has produced many exciting results over the last decade. Both the source catalogue, and the range of astrophysical questions which can be addressed, continue to expand. This article presents a topical…
Context. Star-forming galaxies emit {\gamma}rays with relatively low luminosity, but the study of their emission is no less captivating. While it is known that their {\gamma}-ray luminosity in the GeV band is strongly linked to their star…
Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae (PWNe) are observed in the radio, optical, x-rays and, in some cases, also at…
Recent observations have revealed the existence of enormously energetic ~10^61 erg AGN outbursts in three relatively distant galaxy clusters. These outbursts have produced bubbles in the intra-cluster medium, apparently supported by…
The detection of diffuse radio emission associated with clusters of galaxies indicates populations of relativistic leptons infusing the intracluster medium. Those electrons and positrons are either injected into and accelerated directly in…
Various studies have implied the existence of a gaseous halo around the Galaxy extending out to 100 kpc. Galactic cosmic rays (CRs) that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale…
Cosmic rays are a sample of solar, galactic and extragalactic matter. Their origin and properties are one of the most intriguing question in modern astrophysics. The most energetic events and active objects in the Universe: supernovae…
Recent measurements of cosmic gamma ray intensities up to TeV energies have been used to estimate the spectral shape of the parent cosmic ray particles present in the interstellar medium. The case is made for the particle spectrum in the…