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Gamma-Ray Bursts: Black Hole Birth Announcements

Gamma-ray bursts are the brightest, most violent explosions in the universe, but they can be surprisingly tricky to detect. Our eyes can’t see them because they are tuned to just a limited portion of the types of light that exist, but thanks to technology, we can see even the highest-energy form of light in the cosmos – gamma rays.

So how did we discover gamma-ray bursts?

Accidentally!

This image depicts a gamma-ray burst caused by the merger of two neutron stars. The merger creates gravitational waves (shown as pale arcs rippling outward) being created following the merger of two neutron stars, a near-light-speed jet that produced gamma rays (shown as brown cones and a rapidly traveling magenta glow erupting from the center of the collision), and a donut-shaped ring of expanding blue debris around the center of the explosion. A variety of colors represent the wavelengths of light produced by the kilonova, creating violet to blue-white to red bursts above and below the collision.

As neutron stars collide, some of the debris blasts away in particle jets moving at nearly the speed of light, producing a brief burst of gamma rays.

NASA's Goddard Space Flight Center/CI Lab

We didn’t actually develop gamma-ray detectors to peer at the universe – we were keeping an eye on our neighbors! During the Cold War, the United States and the former Soviet Union both signed the Nuclear Test Ban Treaty of 1963 that stated neither nation would test nuclear weapons in space. Just one week later, the US launched the first Vela satellite to ensure the treaty wasn’t being violated. What they saw instead were gamma-ray events happening out in the cosmos!

Things Going Bump in the Cosmos

Each of these gamma-ray events, dubbed “gamma-ray bursts” or GRBs, lasted such a short time that information was very difficult to gather. For decades, their origins, locations and causes remained a cosmic mystery, but in recent years we’ve been able to figure out a lot about GRBs. They come in two flavors: short (less than two seconds) and long (two seconds or more). Short and long bursts seem to be caused by different cosmic events, but the end result is thought to be the birth of a black hole.

This image shows the birth announcement of a black hole. At the top of the image, in a pink block, is the text: “A black hole is born … ” Below that, on the left-hand side, is a polaroid picture of a black hole – a blank spot against a field of stars. The polaroid is labeled: “Welcome to the cosmos!” On the middle right-hand side is the text: “BORN: millions of years ago / MASS: over 2x mass of the sun / HEIGHT: singularity.” At the bottom right of the image is another polaroid of two whitish-blue neutron stars. They are circling each other and sport cartoon illustrations of smiley faces and hands. A block of text below, with a pink background, reads: “Born to two proud neutron stars”

This black hole birth announcement declares the arrival of a new black hole, the result of two neutron stars that collided.

NASA's Goddard Space Flight Center

Most short GRBs are created by binary neutron star mergers. Neutron stars are the superdense, leftover cores of really massive stars that have gone supernova. When two of them crash together (long after they’ve gone supernova) the collision releases a spectacular amount of energy before producing a black hole. Astronomers suspect something similar may occur in a merger between a neutron star and an already-existing black hole.

In this stylized GIF, two neutron stars orbit each other, getting closer and closer until they collide against a blue background. The stars are depicted as nested circles of bluish-white. As they get close, blue clouds of debris are pulled off of each one. After they collide, a black hole forms in the center, surrounded by a horizontal oval cloud of blue debris. An orange cloud of debris shoots up from above and below the black hole. Pink cones of light extend above and below the black hole, extending beyond the orange clouds.

Astronomers suspect that most short-duration gamma-ray bursts, as shown in this animation, originate from merging systems containing neutron stars, objects more massive than the Sun but as small as a city.

NASA’s Goddard Space Flight Center

Long GRBs account for most of the bursts we see and can be created when an extremely massive star goes supernova and launches jets of material at nearly the speed of light (though not every supernova will produce a GRB). They can last just a few seconds or several minutes, though some extremely long GRBs have been known to last for hours!

A massive star, represented by concentric circles of blue with a wavy edge, sits against a blue background. Then the circles start to move inward until a black circle representing a black hole appears. Then plumes of white material move outward from the black hole through the star. Once they reach space, the white plumes turn magenta. They continue to move further and further from the star, which then begins to expand in an explosion that fills the frame with white.

When a massive star runs out of fuel, its core suddenly collapses and forms a black hole, as illustrated here. In some cases, matter swirling into the black hole produces two powerful jets that rush outward at almost the speed of light that cause a gamma-ray burst.

NASA’s Goddard Space Flight Center

A Gamma-Ray Burst a Day Sends Waves of Light Our Way!

NASA’s Fermi Gamma-ray Space Telescope detects a GRB nearly every day, but there are actually many more happening – we just can’t see them! In a GRB, the gamma rays are shot out in a narrow beam. We have to be lined up just right in order to detect them, because not all bursts are beamed toward us – when we see one it’s because we’re looking right down the throat of the gamma-ray jet. Scientists estimate that there are at least 50 times more GRBs happening each day than we detect!

In this illustration, an exploding star powers jets of material. The star is shown as an almost flower-like shape. The purple “petals” represent clouds of material created in the explosion. The bluish-white and yellow center shows where the newly formed black hole begins driving the jets. The core of the jet pointed toward us is whitish and the broader regions are magenta. In the distance, on the far side of the star, you can see the opposite side of the jet disappearing into space.

As a star explodes, narrow beams (white) of gamma rays are emitted first, followed by wider beams (magenta).

NASA/Swift/Cruz deWilde

So what’s left after a GRB – just a solitary black hole? Since GRBs usually last only a matter of seconds, it’s very difficult to study them in-depth. Fortunately, each one leaves an afterglow that can last for hours or even years in extreme cases. Afterglows are created when the GRB jets run into material surrounding the star. Because that material slows the jets down, we see lower-energy light, like X-rays and radio waves, that can take a while to fade. Afterglows are so important in helping us understand more about GRBs that NASA’s Neil Gehrels Swift Observatory was specifically designed to study them!

Cartoon with Earth and the Fermi Gamma-ray Telescope in space. A ripple passes by and a speech bubble from two spots on Earth say "did you hear that?" Then a bright flash in the sky appears, and a speech bubble from Fermi says, "no, but I sure saw it"

On Aug. 17, gravitational waves from merging neutron stars reached Earth. Just 1.7 seconds after that, NASA's Fermi saw a gamma-ray burst from the same event. Now that astronomers can combine what we can “see” (light) and what we can “hear” (gravitational waves) from the same event, our ability to understand these extreme cosmic phenomena is greatly enhanced.

NASA’s Goddard Space Flight Center

In 2017, from 130 million light-years away, Fermi witnessed a pair of neutron stars collide, creating a spectacular short GRB. What made this burst extra special was the fact that ground-based gravitational wave detectors LIGO and Virgo caught the same event, linking light and gravitational waves to the same source for the first time ever!