Gamma-ray bursts (GRBs) are the most powerful and violent explosions in the universe. These short-lived bursts of high-energy light can release more energy in a few seconds than the Sun will in its entire lifetime. GRBs are immensely energetic events occurring in distant galaxies. They can erupt with a quintillion times the luminosity of our Sun.

Gamma-Ray Bursts

How much energy is released?

GRBs release immense amounts of energy. Some GRBs have a bolometric flux comparable to a bright star in our galaxy, despite being billions of light-years away. The energy output of some GRBs could be within a factor of two of the rest-mass energy of the Sun if the explosion was spherical. However, GRBs are thought to be highly focused explosions with most of their energy collimated into a narrow jet. This focusing means that when a GRB is pointed towards Earth, it appears much brighter than if its energy was emitted spherically. The total energy of a typical GRB has been estimated at 3 x 10^44 J, which is larger than the total energy of ordinary supernovae.

  • An average long GRB has a bolometric flux comparable to a bright star of our galaxy despite a distance of billions of light years.
  • The energy output of GRB 080319B would be within a factor of two of the rest-mass energy of the Sun, if the gamma-ray explosion was spherical.
  • The total energy of typical gamma-ray bursts has been estimated at 3 × 10^44 J.

What are the sources of this immense energy?

GRBs are generally classified into two main categories: short and long.

  • Long GRBs (lasting two seconds or longer) are associated with the explosive deaths of massive stars, when the core of the star collapses to form a black hole. The collapse of a massive star leads to a supernova, and a black hole may form. In both classes, the newly formed black hole sends out jets of particles accelerated to near the speed of light, which interact with surrounding material to emit gamma rays.
  • Short GRBs (lasting less than two seconds) are caused by the merger of two neutron stars or a neutron star and a black hole.

GRBs are thought to be highly focused explosions with most of their energy collimated into a narrow jet. These jets are ultrarelativistic. The jets of gamma-ray bursts are the most relativistic jets in the universe. Because the energy is strongly focused, the gamma rays emitted by most bursts are expected to miss the Earth and never be detected. The approximate angular width of the jet can be estimated by observing the achromatic “jet breaks” in afterglow light curves.

What about the afterglow?

After the initial burst of gamma rays, a longer-lived afterglow is emitted. This afterglow is usually in the longer wavelengths of X-ray, ultraviolet, optical, infrared, microwave, or radio frequencies. Any energy released by the explosion that is not radiated away in the burst itself takes the form of matter or energy moving outward at nearly the speed of light. This matter collides with the surrounding interstellar gas, creating a relativistic shock wave that radiates as synchrotron emission across most of the electromagnetic spectrum.

Why are they important?

Despite being discovered by chance, GRBs have proven invaluable for researchers. These flashes of light provide insights into phenomena such as the end of life of very massive stars or the formation of black holes in distant galaxies. In 2017, GRBs were linked to gravitational waves, which has led to a better understanding of these events. The study of GRBs continues to reveal the workings of the universe and its most energetic phenomena.

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