Lecture28: Gamma-Ray Bursts

The First Gamma-Ray Burst

• The testing of nuclear weapons was banned in 1963.

• The Americans didn't trust the Soviet Union and placed the Vela satellite in orbit to watch for nuclear explosions.

• In a nuclear explosion, gamma-rays are emitted, so the Vela satellite was designed to look for gamma-rays.

• Vela discovered gamma-rays from the sky, instead of the Earth.

• The gamma-rays came in short bursts, so they were called gamma-ray bursts.

Animation of a Gamma-Ray Burst

• This animation is based on data from the Compton Gamma-Ray satellite.

Distribution of Gamma-Ray Bursts

• Gamma-ray bursts are seen in every direction of the sky.

• We use the word isotropic to describe a distribution that doesn't depend on direction.

• There are two possible explanations for the isotropy:
  1. The Earth is surrounded by a spherical cloud of unseen objects that emit gamma-ray bursts.
  2. The gamma-ray bursts occur in far away galaxies, since galaxies are distributed approximately isotropically.

Optical Afterglows from Gamma-Ray Bursts

• After a gamma-ray burst occurs, a glow of visible and infrared light can be seen for many days afterwards.

• Observation of the "afterglow" with an optical telescope allows the spectrum and a redshift to be found.

• The redshift of the light for this gamma-ray burst was found to be z = 1.6

• Using Hubble's law, the source of the burst is far away and very luminous.

Host Galaxy of a Gamma-Ray Burst

• The Hubble Space Telescope imaged the region around a gamma-ray burst.

• We can see a faint galaxy in the same place where the burst occured.

The Spectrum of a Gamma-ray Burst

• Optical observations of a gamma-ray burst allow us to see how its spectrum changes with time.

• In this graph, the black curves show the spectrum of the gamma-ray burst afterglow at different numbers of days after the burst.

• The spectrum of a type II supernova is shown in red below the burst spectrum.

• The similarity between the spectra suggests that gamma-ray bursts are a very energetic type of supernova explosion.

Hypernovae?

• The energy released in a Gamma-Ray Burst is at least 10 times larger than a supernova explosion.

• One possible theory states that Gamma-Ray bursts are what happens when an ultra-massive, rapidly rotating star goes supernova.

• The supernova forms a rapidly rotating black hole.

• This type of supernova has been named a hypernova.

• This is still quite controversial and has yet to be proved conclusively.