The most violent and energetic phenomena in the Universe include gamma-ray bursts, supernova explosions, black holes, neutron stars, and the as yet unidentified cosmic accelerators which produce the highest energy photons and cosmic rays.
Accretion of gas onto black holes can extract huge amounts of energy and lead to some of the most energetic phenomena in the Universe: active galactic nuclei, X-ray binaries, tidal disruption events or gamma-ray bursts.
TA scientists have been studying black hole accretion flows for many years using state-of-art general relativistic numerical codes which allow us to properly follow the black hole physics and evolution of magnetic fields leading to turbulence.
G2 is a cloud on a near-radial orbit about our galaxy's central black hole, Sagittarius A*. TA scientists are working on a number of observational and theoretical projects relating to this fascinating object.
Radio astronomers are not bothered by dust and have discovered a very bright, point-like object, called Sgr A*, that is probably a super-massive black-hole. At infrared wavelengths one can also see through the dust to the Center, and one sees thousands of stars. But, which of these stars, if any, corresponds to Sgr A*? The problem of identifying the infrared counterpart of Sgr A* exists because it is extremely difficult to align the infrared and radio reference frames to the high accuracy needed to sort out the complex field of stars seen in the infrared image.
Strong gravitational fields in the vicinity of black holes and neutron stars give rise to a variety of exotic high energy phenomena in X-ray binaries, active galactic nuclei and gamma-ray bursts. TA scientists use theoretical models and numerical magnetohydrodynamics simulations to investigate the physics of accretion flows in these compact objects, as well as the formation and acceleration of relativistic jets.
Nearly a century after their 1912 discovery by Victor Hess the origin of these energetic particle streaming to us from space remains controversial. Where in the universe is there an accelerator far more powerful than anything we can build on Earth? Supernova remnants, young stars, microquasars and even quasars have been suggested.
As a rapidly spinning young neutron star (a "pulsar") slows down, it deposits its enormous reservoir of rotational energy into its environment via a relativistic wind, producing an observable pulsar wind nebula (PWN). PWNe are a rich source of information. Most fundamentally, they provide a direct probe of the high-energy processes through which a neutron star's considerable reservoir of rotational energy is eventually deposited into its environment.
Little known Astronomy facts (secrets): When neutron stars were first discovered by astronomers, they were called 'LGM' - for Little Green Men. This was because they were found be be sending out very fast, very bright, periodic radio pulses, and different ones were pulsing at different speeds - they actually looked like they might be space beacons, similar to lighthouses, that space-faring LGM were using as navigational aids. We know now that they are a perfectly natural (not engineered) phenomenon, but they are still highly interesting and exotic.
The Milky Way contains about a hundred million black holes, which were formed by the collapse of very massive stars. Each of these stellar black holes weighs about 10 times as much as our Sun. A very few of these black holes are closely orbited by an ordinary star that is slowly bleeding matter onto the black hole. As this gas falls toward the black hole, it is heated by strong gravity and friction. Near the black hole, the gas reaches a typical temperature of 10 million degrees.
The most luminous beacons in the the universe are quasars. They are powered by the release of gravitational energy as matter falls onto supermassive black holes at the centers of galaxies. This energy release is far more efficient than the nuclear fusion reactions that power stars, so that quasars can be seen all the way across the observable universe.