GAMMA RADIATION FROM RAPIDLY ROTATING BLACK HOLES

Image: This artist's impression illustrates how high-speed jets from supermassive black holes would look. These outflows of plasma are the result of the extraction of energy from a supermassive black hole’s rotation as it consumes the disc of swirling material that surrounds it. Credit: NASA, ESA, M. Chiaberge (STScI)

The radiation from an active galactic nuclei (AGN) is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy.

In a recent paper (Hirotani et al., 2016 ApJ) the authors demonstrate that electrons and positrons created by photon collisions near a rotating black hole are accelerated in the opposite direction by the electric field present near the event horizon. This electric field acts as a particle accelerator (or gap) and dissipates a part of the hole’s rotational energy. The resultant gamma-ray luminosity increases with decreasing plasma accretion from the surroundings.

The authors show that in the case of extremely rotating supermassive black-holes, the gap can reproduce the very high energy (VHE) gamma ray flux observed is some active galactic nuclei.

▪ Hirotani & Pu, 2016 ApJ - Energetic gamma radiation from rapidly rotating black holes (arXiv)

▪ Supermassive black holes - HST

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GAMMA-RAY EMISSION FROM THE SNR HB3

Image: At a distance of about 20,000 light years, G292.0+1.8 is one of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements (that is, everything other than hydrogen and helium) necessary to form planets and people. The X-ray image from Chandra shows a rapidly expanding, intricately structured, debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded. Credit: NASA/CXC/SAO The processes of particles acceleration to very high energies from the supernova shock region and diffusion in the interstellar medium of such particles has not been well understood so far. Gamma-ray observations in the GeV regime are a powerful probe of these mechanisms

THE HITCHCHIKER'S GUIDE TO THE LOCAL SUPERCLUSTER

Image: Virgo Supercluster. Credit: Andrew Z. Colvin The Virgo Supercluster is a region with a diameter of 33 megaparsecs (~1000 times larger the Milky Way's diameter) containing at least 100 galaxy groups and clusters.

ORBITAL PERIODS OF THE PLANETS

For orbital period generally we refer to the sidereal period, that is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference (365,25 days for the earth).

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