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Gamma-ray bursts

Article curated by Ed Trollope

Gamma-ray bursts are short, powerful blasts of gamma-ray radiation, which originate in deep space. Certainly, there origins are amongst the most destructive events in the universe. Some are believed to be caused by an exploding star (a supernova), while others may be due to neutron stars colliding. Gamma-ray bursts are often created by events very far away, it may have taken the burst so long to reach the Earth that it occurred billions of years ago in the early universe. In fact, there's a lot about that time that we don't yet know, and Gamma-ray bursts are one of our best tools for investigating it. Likewise, Gamma-ray bursts can help us learn more about how the star formation rate has changed throughout the ages.

NASA
In a binary star system, two stars rotate around each other. If these orbits decay, and the stars will collide and merge. Could the merger of binary neutron stars be the cause of short duration gamma-ray bursts? Image credit: Public domain

Most Gamma-ray bursts last just a matter of seconds, but some last far longer. In general, these "long" GRBs are thought to be formed when a massive star reaches the end of its life. Accompanying these we expect to see a supernova - assuming the GRBs isn't too distant (the supernova part is much fainter). However, there are some apparently long GRBs which, despite being nearby, do not seem to have an accompanying supernova (e.g. GRB 060614)[1] - why is unknown.

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Short Gamma-ray bursts are thought to be formed when two neutron stars in a binary system merge[2]. We would expect gravitational waves to be emitted as this happens - but LIGO and the other gravitational wave observatories have yet to detect them.

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There are also some Gamma-ray bursts (with associated supernovae) which show unusual curvature in their X-ray spectra, which can be modelled with a blackbody component. This was first clearly seen in GRB 060218[3] and, at the time, was thought to be the shock-breakout of the supernova. However, the energetics are not completely understood, mainly because the numbers are so high. In fact, that's why we believe the systems contain jets; if the energy were emitted equally in all directions, it would mean so much energy was released that we couldn't explain it at all!

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Cosmic Jets

ESO/M. Kornmesser
Among the oldest and brightest entities in the universe, quasars eject jets of very bright light that can be seen from lightyears away. It was initially believed that different events were being seen when quasars were observed, but it was later established that our line of sight affected the appearance of the quasar, for example a blazar is a quasar with jets that are pointing towards Earth. Image credit: ESO/M. Kornmesser 

Cosmic jets are seen in all kinds of objects (including GRBs, active galactic nuclei, quasars, and radio galaxies), but how they're formed is not yet understood. The phenomenon is defined as streams of matter being emitted along the axis of rotation of a compact object, and are a staple component of most artists' impressions of such objects.

Although researchers are not clear as to exactly what causes these jets, their focus tends to be on the central body - such as a black hole - or the surrounding accretion disc.
Delve deeper into The Impossible Quasar at the Dawn of the Universe.

 Lightbulb Institutes Delving Deeper
List of Institutes Researching Cosmic Jets
Things We Don’t Know (1 researchers)

This article was written by the Things We Don’t Know editorial team, with contributions from Kim Page.

References
why don’t all references have links?

[1] Mangano, V., et al., (2007) Swift observations of GRB 060614: an anomalous burst with a well behaved afterglow Astronomy & Astrophysics 470.1:105-118 DOI: 10.1051/0004-6361:20077232
[2] Gehrels, N., et al., (2005) A short gamma-ray burst apparently asssociated with an elliptical galaxy at redshift z=0.225 Nature 437:851-854 DOI: 10.1038/nature04142
[3] Campana, S., et al., (2006) The shock break-out of GRB 060218/SN 2006aj Nature 442:1008-1010 DOI: 10.1038/nature04892

Blog posts about gamma-ray bursts

Science blog article.
A Swift glance at red dwarfs
Thursday 20th of November 2014


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