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- Apathetic Retiree
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HEAPOW: Over African Skies (2021 Oct 04)
MPG: Galactic science with 15 years of H.E.S.S. data
H.E.S.S. Phase-I Observations of the Plane of the Milky Way
Some sources in the Milky Way are so powerful that they produce extremely high energy radiation. This radiation, traveling from the depths of the cosmos to earth, can interact with the earth's atmosphere, producing high-energy particles that move faster than light in the atmosphere
. These ultra-fast moving particles produce their own peculiar blue glow
, and this secondary radiation can be detected by sensitive telescopes on the ground. Since early 2003, the High Energy Stereoscopic System (otherwise known as H.E.S.S. - an acronym chosen in honor of Victor Hess
, the discoverer of cosmic rays) has been studying this secondary radiation produced by some of the highest energy sources in the Universe. The H.E.S.S. observatory is an array of optical telescopes located in the Khomas Highland of Namibia
, and operated by an international collaboration of about 40 scientific institutions in Africa, Europe, Australia and Japan. The image above shows a deep view of the very high energy gamma-ray emission (in orange) from sources in the plane of the Milky Way, obtained over 15 years of observing. The H.E.S.S. Galactic Plane Survey image is superimposed on an optical image of the Namibian sky, with one of the H.E.S.S. telescopes in the foreground. This deep H.E.S.S. survey reveals never-before-understood information
about the shocking nature of the high-energy radiation produced by neutron stars, black holes, stars, and other powerful, destructive sources which lurk within the disk of our home galaxy.
- Astronomy & Astrophysics 612 (Apr 2018) Special Issue
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
- Vacationer at Tralfamadore
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<<High Energy Stereoscopic System (H.E.S.S.) is a system of Imaging Atmospheric Cherenkov Telescopes (IACT) for the investigation of cosmic gamma rays in the photon energy range of 0.03 to 100 TeV. H.E.S.S. is located [at an altitude of 1,800 m] on the Cranz family farm, in Namibia, near the Gamsberg, an area well known for its excellent optical quality. All four were operational in December 2003.>>
<<Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. (Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.) Observation of gamma rays first became possible in the 1960s. Their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare, even a "bright" source needing an observation time of several minutes before it is even detected, and because gamma rays are difficult to focus, resulting in a very low resolution. The most recent generation of gamma-ray telescopes (2000s) have a resolution of the order of 6 arc minutes in the GeV range (seeing the Crab Nebula as a single "pixel"), and about 1.5 arc minutes in the high energy X-ray (100 keV) range seen by High-Energy Focusing Telescope (2005).
[The Chandra X-ray Observatory (1999) has a resolution of 0.5 arc seconds in the low energy X-ray (1 keV).]
The mechanisms emitting gamma rays are diverse, mostly identical with those emitting X-rays but at higher energies, including electron–positron annihilation, the inverse Compton effect, and in some cases also the decay of radioactive material in space reflecting extreme events such as supernovae and hypernovae, and the behaviour of matter under extreme conditions, as in pulsars and blazars. In a 18 May 2021 press release, China's Large High Altitude Air Shower Observatory (LHAASO) reported the detection of a dozen ultra-high-energy gamma rays with energies exceeding 1 peta-electron-volt, including one at 1.4 PeV [1,400 TeV], the highest energy photon ever observed. The authors of the report have named the sources of these PeV gamma rays PeVatrons. Gamma-ray astronomy observations are still limited by non-gamma-ray backgrounds at lower energies, and, at higher energy, by the number of photons that can be detected. Larger area detectors and better background suppression are essential for progress in the field.
Very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground-based experiments. The extremely low photon fluxes at such high energies require detector effective areas that are impractically large for current space-based instruments. Such high-energy photons produce extensive showers of secondary particles in the atmosphere that can be observed on the ground, both directly by radiation counters and optically via the Cherenkov light which the ultra-relativistic shower particles emit. Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Fred Lawrence Whipple Observatory at Mt. Hopkins, in Arizona in the USA. Modern Cherenkov telescope experiments like H.E.S.S., VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes. The most energetic photons (up to 16 TeV) observed from an extragalactic object originate from the blazar, Markarian 501 (Mrk 501). Markarian 501 (or Mrk 501) is a blazar or BL Lac object, which is an active galactic nucleus with a jet that is shooting towards the Earth. The gamma rays from Mrk 501 are extremely variable, undergoing violent outbursts. (The galaxy is also variable in visible light between magnitude 14.5 and 13.6.) The gamma ray spectrum of Mrk 501 shows two humps. One is below 1 keV and can be considered to be X rays and the other is above 1 TeV. During flares and outbursts the peaks increase in power and frequency. Flares lasting 20 minutes with rise times of 1 minute have been measured by MAGIC. In these flares the higher energy gamma rays (of 1.2 Tev) were delayed 4 minutes over the 0.25 TeV gamma rays.>>