What causes gamma ray bursts? (Intermediate)

In recent news about Gamma Ray Bursts I have heard that the current thinking is that they are caused by super massive stars going supernova but the core collapsing into black hole instead of a neutron star. How massive a star do they theroize would be required for this to happen? 10 Solar Masses? 20sm ? Larger?

In the last few years, conclusive evidence has been uncovered that at least some gamma ray bursts are associated with supernovae. Some gamma ray bursts are observed to have "afterglows" in longer wavelengths--that is, first you get gamma rays, then x-rays, then UV light, then optical light. Supernovae are generally identified by their optical "light curves", or the way the light changes over time. The conclusive evidence came when astronomers recorded the afterglow of a few gamma ray bursts in the optical--and saw the clear signatures of a supernova in the optical light curve. This was a big event, since there were a other theories that sounded quite plausible. Of course, this does not mean that all gamma ray bursts are due to supernovae--gamma ray bursts are infamous for the variability of their own light curves. Some even seem to turn off and then turn back on in a weird way.

The supernovae that are blamed for gamma ray bursts are a special type. They are known as hypernovae, and occur at the death of "Wolf-Rayet" stars. W-R stars are very hot and massive, and tend occasionally to toss off their outer layers. At birth, a W-R star is roughly 20-30 solar masses, but by death it will be down closer to 10 solar masses. What distinguishes a hypernova from a plain old supernova is the extra kick it gets. The core collapses and forms a black hole, which then sends out jets of material (that's a whole other mystery--but the jets need to be there to carry off angular momentum and keep the black hole from spinning too fast). These jets then slam into the outer material of the star, creating extremely high temperatures and sending off gamma rays. As the jets get further from the black hole, they encounter increasingly less dense material, and thus the radiation they send out is less energetic, or longer wavelength--hence the afterglow I referred to earlier.

Gamma ray bursts can also come from merging neutron stars. Neutron stars are tiny and super dense (about the size of a city, but having the mass of the Sun), so you can imagine that two neutron stars running into each other would release a lot of energy! Neutron star mergers have been predicted as a source for short GRBs (GRBs with a duration of less than two seconds), but it wasn't until 2018 that LIGO detected gravitational waves from two neutron stars merging at the same time as the Fermi satellite detected a gamma ray burst, confirming that neutron star mergers can produce short gamma-ray bursts.

This page was last updated January 28, 2018.

About the Author

Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.