The Variable Jets of Gamma-Ray Bursts

What drives rapid flickering in the jets that are produced in some powerful, high-energy explosions? Recent research explores the role of magnetic fields.

Mapping an Explosion

Gamma-ray bursts — brief flashes of high-energy emission from beyond our galaxy — have been detected since the 1960s. Though we’ve collected many observations of this explosions through the decades, it’s only recently that new evidence has clarified what causes some gamma-ray bursts.

In 2017, the merger of two neutron stars was observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO), just before the detection of a short (less than ~2 seconds) gamma-ray burst from the same location. These observations support the following picture for short gamma-ray bursts:

1. A neutron-star–neutron-star binary or a neutron-star–black-hole binary merges, generating a potentially observable gravitational-wave signal.
2. The merger either immediately produces a black hole, or it produces a hypermassive neutron star that collapses into a black hole shortly thereafter.
3. The remnant material surrounding the newly formed black hole then rapidly accretes, leading to the production of powerful jets along the black-hole rotation axis.

Jetted Mysteries

This picture, while seemingly straightforward, is loaded with uncertainties. In particular, the jets launched in the third step are not well understood. We’re not sure what drives the production of these jets in the first place, and we also don’t know what collimates the jets, causing them to become tightly beamed as they travel, rather than spraying out in all directions.

What’s more, we observe rapid variability within the gamma-ray-burst jets; for short gamma-ray bursts, the timescales for variability are just ten-thousandths to hundredths of a second! What drives this rapid flickering within the jets?

In a recent study, two researchers at the Center for Theoretical Physics of the Polish Academy of Sciences, Konstantinos Sapountzis and Agnieszka Janiuk, explore the role that magnetic fields might have in the launching and properties of short-gamma-ray-burst jets.

Magnetic Fields at Work

Sapountzis and Janiuk perform a series of general-relativistic, magnetohydrodynamic simulations of a black hole surrounded by a torus of accreting material.

The authors use these simulations to explore how the magnetic field piles up as hot, ionized gas spirals inward and falls onto the black hole. The building field eventually forms a magnetic barrier that halts the inward flow of gas, leading to the formation of jets along twisting field lines that extend down the black-hole rotation axis.

But the role of the magnetic fields isn’t over with the launch of the jet. In the authors’ simulations, they observe a magnetic instability in the accreting plasma — known as the magnetorotational instability — operating on similar timescales to the variability in gamma-ray-burst jets. This suggests a link between the activity of magnetic fields at the base of the jet and the flickering we observe in the brief gamma-ray-burst jets.

We still have a lot to learn about gamma-ray bursts — and we can hope that future observations, especially now that LIGO is back online, will shed more light on these explosions! It certainly seems clear, however, that magnetic fields have an important role to play.

Citation

“The MRI Imprint on the Short-GRB Jets,” Konstantinos Sapountzis and Agnieszka Janiuk 2019 ApJ 873 12. doi:10.3847/1538-4357/ab0107