11:32 am - Monday May 28, 2012

Gamma Ray Bursts

gamma ray burstGamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical burst lasts 20–40 seconds. The initial burst is usually followed by a longer-lived “afterglow” emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).

Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process, possibly the merger of binary neutron stars.

The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years). All observed GRBs have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeater flares, are associated with magnetars within the Milky Way. It has been hypothesized that a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.

Causes of Gamma Ray Bursts

The immense distances of most gamma-ray burst sources from Earth has made pinning down the nature of the system that produces these explosions extremely difficult. The currently favored model for the origin of most observed GRBs is the collapsar model, in which the core of an extremely massive, low-metallicity, rapidly-rotating star collapses into a black hole, and the infall of material from the star onto the black hole powers an extremely energetic jet that blasts outward through the stellar envelope. When the jet reaches the stellar surface, a gamma-ray burst is produced.

While the collapsar model has enjoyed a great deal of success, many other models exist that are still seriously considered. Winds from highly magnetized, newly-formed neutron stars (protomagnetars), accretion-induced collapse of older neutron stars, and the mergers of binary neutron stars have all been proposed as alternative models. The different models are not mutually exclusive, and it is possible that different bursts have different progenitors. For example, there is now good evidence that some short gamma-ray bursts (GRBs with a duration of less than about two seconds) occur in galaxies without massive stars, providing strong evidence that this subset of events are associated with a different progenitor population from longer bursts - for example, merging neutron stars. However, in 2007 the detection of 39 short gamma-ray bursts could not be associated with gravitational waves which are thought of as observables of such compact mergers.

Effects of Gamma Ray Bursts

Cosmic gamma-ray bursts (GRBs) are bursts of energetic gamma radiation produced by the violent explosions of distant stars. There is a huge amount of interesting astrophysics involved that is far beyond the scope of this document. From the perspective of the VLF group at Stanford, GRBs are interesting because of their effects on the Earth’s atmosphere and the near-Earth space environment.

The burst of gamma-rays from a GRB interacts with the upper atmosphere and lower ionosphere, depositing its energy and ionizing atoms and molecules. This ionization is in addition to natural ionization due to ultraviolet light from the sun and cosmic rays and acts to change the conductivity of the upper atmosphere. The change in conductivity is visible as a change in the way VLF radio waves reflect from the ionosphere and thus can be detected by monitoring transmitted VLF signals from long distances.

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