The Very High Energy Sky from ~ 20 GeV to Hundreds of TeV - Selected Highlights

Abstract

After nearly a decade of operation, the three major arrays of atmospheric Cherenkov telescopes have revolutionized our view of the Very High Energy Universe, unveiling more than 100 sources of various types. MAGIC, consisting of two 17 m diameter telescopes on the Canary island of La Palma, and VERITAS, with four 12 m telescopes installed in southern Arizona, USA, have primarily explored the extragalactic sky, where the majority of the sources are active galactic nuclei (AGN), with γ-ray emission originating in their relativistic jets. In July 2014 MAGIC discovered the most distant source of very high energy γ rays, the gravitationally lensed blazar S3 0218 residing at the redshift of z = 0.944. The flat spectrum radio quasar PKS 1441+25 (z = 0.939), observed by MAGIC & VERITAS in 2015, showed a strong flaring activity over a time span of several weeks. Rapid variability from various BL Lacertae objects, down to minute timescales, has been observed by the three experiments, and measurement of their high-energy spectra allows the level of extragalactic background light to be constrained. Since the commissioning of the fifth, large (28 m diameter) telescope in December 2012, H.E.S.S.-II is the only array operating telescopes of different sizes together ("hybrid array "). The largest Cerenkov telescope ever built, CT5, provides an energy threshold of ∼ 20 GeV. With its broad energy range, H.E.S.S. explored the Galactic Plane with unprecedented sensitivity. The legacy release of the H.E.S.S. Galactic Plane Survey, consisting of 2800 hours of observations of the Galactic disk, reveals major new results. This is the first high-resolution (∼ 0.1 deg) sensitive (∼ 2% Crab Nebula point-source sensitivity) survey of the Milky Way in VHE γ rays. The Milky Way harbors a large variety of high energy sources of various types. In recent years, deep observations of several key Galactic regions of utmost importance for this field have been conducted by the three experiments. Among them are the Galactic Center region and its halo (particularly relevant for dark matter searches), the Cygnus region and its mysterious Milagro sources, the Crab Nebula and pulsar (surprisingly showing pulsed emission up to above 1 TeV), the iconic γ-ray supernova remnant RX J1713.7-3946 and other SNRs such as Tycho, the Vela pulsar and several binary systems such as LS 5039, PSR B1259-63, HESS J0632+057 and LS I +61 • 303. Joint observations on several of these objects proved to be the most efficient way to understand their nature. A deep observation of the Large Magellanic Cloud revealed, for the first time, spectacular and powerful accelerators of stellar origin outside our own galaxy. Highlights of these observations with H.E.S.S., MAGIC and VERITAS have been presented and discussed at the conference.

Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons

“…They produce synchrotron radiation that corresponds to the low-energy hump of the spectrum and rescatter it by the inverse 3 We do not consider in this paper loop corrections to the dispersion relations that can induce a logarithmic running of the coefficients in (1) with momentum. 4 While the CP T symmetry follows from LI, the converse is not true: a theory can be CP T invariant and Lorentz violating at the same time.…”

“…Since the first ground-breaking detection of several multi-TeV gamma-ray events from Crab nebula in 1989 [3], it has evolved into a well-established technique for highquality astronomical observations. More than hundreds of TeV gamma-ray sources have been discovered and studies of their spectra and variability have made valuable contribution to our understanding of the internal processes in these objects [4]. The propagation of VHE photons is affected by the interstellar medium, in particular, the photon background and magnetic fields.…”

High-energy emission properties of pulsars

“…Panel (b): Disappearance of the Vela pulsar's first peak with energy as seen by Fermi [3] and H.E.S.S. [51]. Panel (c): Updated plot of L γ vs.Ė rot for old and young pulsars, exibiting two distinct trends: L γ ∝Ė 1/2 rot for young pulsars (orange and red dots), while L γ ∝Ė rot for MSPs (green dots) [62].…”

“…However, some pulsar light curves (about one third) show the reverse behaviour [33,110]. In addition, the main peak positions seem to remain more or less the same as the energy increases (but not the third peak of Vela, which migrates in phase with an increase in energy [3]), while the pulse widths become narrower [13,51]. 4.…”

Modelling energy-dependent pulsar light curves due to curvature radiation

“…Data from ground-based Cherenkov telescopes such as MAGIC, VERITAS, and H.E.S.S.-II that detected pulsed emission from the Crab and Vela pulsars in the very-high-energy (VHE) regime (> 100 GeV) also exhibit such light curve evolution. MAGIC recently detected pulsations from the Crab pulsar at energies up to 1 TeV [1], and H.E.S.S.-II detected pulsed emission from the Vela pulsar above 100 GeV, making this only the second pulsar to be detected at these high energies [2]. Notably, as E γ is increased, the main peaks of Crab and Vela seem to remain at the same normalised phase, the intensity ratio of the first to second peak decreases, and the peak widths decrease [3].…”