Simulating Gamma Ray Bursts

A black hole eating a star.
Figure 1. Artist’s conception of a black hole eating a star. (Source, Wikipedia.)

It was the mid 1960s. The United States and the Soviet Union had recently signed the Partial Nuclear Test Ban Treaty, which forbid the detonation of nuclear weapons except underground. Since neither nation trusted the other, each was carefully monitoring the other for non-compliance. In particular, the United States feared that the soviets might be, I kid you not, testing bombs behind the moon.

Vela

The United States solved this problem with the Vela satellites. When a nuclear bomb goes off, it emits a short burst of gamma rays, which are rays of extremely high energy light. The Vela satellites were gamma ray detectors in space, orbiting the Earth 65,000 miles above the surface. Figure 2 shows one of these satellites in a clean room.

A Vela satellite
Figure 2. One of the Vela satellites in a clean room. (Source: NASA and Los Alamos)

The Vela satellites did detect gamma rays all right, but they didn’t come from nuclear weapons… they didn’t even come from the solar system. The satellites repeatedly detected short, very intense bursts of gamma radiation that nevertheless took too long to be from nuclear weapons blasts.

Gamma Ray Bursts

For a long time, we didn’t know anything about these events or what caused them. So we gave them the enigmatic name gamma ray bursts, and made up many models for what could cause them.

This changed in the late nineties, when we were able to measure X-rays and visible light emitted from the same source after the burst, which we call an afterglow. We now know that there are many causes for gamma ray bursts. Some bursts take a relatively long time and we’ve linked them to supernovae in distant galaxies.

The relatively shorter gamma ray bursts (creatively called short gamma ray bursts) are less common and less extensively studied. And we therefore know a lot less about them. One popular theory is that they’re caused by the merger of a black hole and a neutron star.

A Quick Aside on Neutron Stars

Neutron stars are the densest stars we know of. Figure 3 shows that a neutron star twice as massive as our sun might have a radius smaller than Manhattan. Indeed, the only thing preventing a neutron star from forming a black hole is the Pauli exclusion principle.

A neutron star superposed on Manhattan Island
Figure 3. A neutron star superposed on top of Manhattan island. In case you’re wondering, if this actually happened, the star would destroy the Earth. (Image source.)

Ordinary matter is made up of mostly empty space. The radius of an atomic nucleus is about a picometer, while the radius of an atom is about an angstrom. This means that, on average, 99.9999 % of matter is empty space. Not so with a neutron star. A neutron star is made up of neutrons packed as tightly as possible, like spheres. This means that in a neutron star, only about 25 % of a neutron star is empty space. (Obviously take this analogy with a grain of salt. The properties of a neutron star depend heavily on quantum mechanics and nuclear physics… so the neutrons aren’t actually packed like spheres. They’re waves.)

Anyway, neutron stars are incredible.

Black Hole-Neutron Star Mergers

When a neutron star gets too close to a black hole, the black hole can eat it. But as I’ve discussed before, black holes are messy eaters. The matter in the neutron star gets distorted and forms an accretion disk around the black hole, which glows incredibly brightly.

As the accreting matter falls the black hole, that matter can be accelerated to incredible velocities and launched out the poles, forming an ultrarelativistic “jet.” These jets are common in many circumstances, but we believe that the jet from a black hole-neutron star merger might be the source of short gamma ray bursts.

There’s a long and beautiful history of studies of accretion disks and the jets they drive. And we’ve known that black-hole neutron star mergers produce accretion disks of the right type. But there’s still a lot we don’t understand about jets, accretion physics, and neutron stars.

The Jet Emerges: A Piece of the Puzzle

Recently, Vasileios Paschalidis, Milton Ruiz, and Stuart L. Shapiro, of the University of Illinois at Urbana-Champaign numerical relativity group, helped add a bit to our understanding. For the first time, they simulated a black hole-neutron star merger, watched as the accretion disk formed, and the relativistic jet emerged. This provides additional evidence that black-hole neutron star mergers might be the progenitors of short gamma ray bursts. Figure 4 shows snapshots of the simulation as the black hole disrupts the star, accretes the matter, and finally drives the jet.

a black hole eating a neutron star
Figure 4. Snapshots of the simulation as the black hole disrupts the neutron star, accretes the matter, and finally drives the jet. The white lines are the magnetic field lines, which drive the jet of matter. Image source.

Now, studies like this have been attempted before. Researchers routinely run simulations of black hole-neutron star mergers to make predictions about gravitational waves. And many groups around the world have run simulations of the jets driven by black holes. However, no previous simulation has successfully observed a jet after merger. All the previous jet simulations started with an accretion disk already in place.

Paschalidis, Ruiz, and Shapiro got their jet to emerge by correctly configuring the magnetic field of the neutron star before merger. Previously, all magnetic fields were assumed to be confined only within the star, and not exist outside it.┬áPaschalidis, Ruiz, and Shapiro argue that this isn’t particularly realistic and, by including the exterior magnetic field, the jet emerges naturally.

This is a pretty cool piece of science!

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