March 1, 2024

Gathering knowledge of the most powerful explosions in the universe

This article has been reviewed in accordance with Science X’s editorial process and policies. The editors have highlighted the following attributes, ensuring the credibility of the content:

checked

trusted source

review


Two neutron stars begin to merge in this artist’s concept, launching jets of particles at high speeds. Collision events like this create small bursts of gamma rays. Credit: NASA Goddard Space Flight Center/ A. Simonnet, Sonoma State University

× to close


Two neutron stars begin to merge in this artist’s concept, launching jets of particles at high speeds. Collision events like this create small bursts of gamma rays. Credit: NASA Goddard Space Flight Center/ A. Simonnet, Sonoma State University

The most powerful events in the known universe – gamma ray bursts (GRBs) – are short-lived bursts of the highest energy light. They can erupt with a quintillion (10 followed by 18 zeros) times the luminosity of our sun. Now thought to herald the birth of new black holes, they were discovered by accident.

The story takes us to 1963, when the US Air Force launched Vela satellites to detect gamma rays coming from banned nuclear weapons tests. The United States had just signed a treaty with the United Kingdom and the Soviet Union to prohibit testing in the Earth’s atmosphere, and the Vela satellites ensured compliance by all parties. Instead, the satellites found 16 gamma-ray events.

In 1973, scientists were able to rule out that either the Earth or the Sun were the sources of these bright eruptions. That’s when astronomers at Los Alamos National Laboratory published the first paper announcing that these explosions originate outside our solar system.

Scientists at NASA’s Goddard Space Flight Center quickly confirmed the results using an X-ray detector on the IMP 6 satellite. It would take another two decades and contributions from the Italian Space Agency’s BeppoSax and NASA’s Compton Gamma-ray Observatory to show that these explosions occur far beyond our galaxy, the Milky Way, are evenly distributed across the sky and are extraordinarily powerful. The closest GRB ever recorded occurred more than 100 million light years away.

Although discovered by chance, GRBs have proven invaluable to today’s researchers. These flashes of light are rich in information about phenomena such as the end of life of very massive stars or the formation of black holes in distant galaxies.

Still, there are still many scientific gems to discover. In 2017, GRBs were associated for the first time with gravitational waves – ripples in the fabric of space-time – leading us to a better understanding of how these events work.

The long and short term of GRBs

Astronomers separate GRBs into two main classes: short events (where the initial burst of gamma rays lasts less than two seconds) and long events (lasting two seconds or more).

Shorter bursts also produce fewer gamma rays overall, leading researchers to hypothesize that the two classes originated from different progenitor systems.

Astronomers now associate short bursts with the collision of two neutron stars or a neutron star and a black hole, resulting in a black hole and a short burst. Short GRBs are sometimes followed by kilonovae, light produced by the radioactive decay of chemical elements. This decay generates even heavier elements, such as gold, silver and platinum.

Long bursts are linked to the explosive death of massive stars. When a massive star runs out of nuclear fuel, its core collapses and then ricochets, driving a shock wave through the star. Astronomers see this explosion as a supernova. The nucleus can form a neutron star or a black hole.

In both classes, the newborn black hole emits jets in opposite directions. The jets, made of particles accelerated to nearly the speed of light, pierce and eventually interact with the surrounding material, emitting gamma rays as they do so.


As a massive star explodes in this artist’s concept, it produces a jet of high-energy particles. We see GRBs when these jets point almost directly at Earth. Credit: NASA/Swift/Cruz deWilde

× to close


As a massive star explodes in this artist’s concept, it produces a jet of high-energy particles. We see GRBs when these jets point almost directly at Earth. Credit: NASA/Swift/Cruz deWilde

This general outline is not the last word, however. The more astronomers study GRBs, the more likely they are to find events that challenge current classifications.

In August 2020, NASA’s Fermi Gamma-ray Space Telescope located a second burst called GRB 200826A, more than 6 billion light-years away. It should have fallen into the short-burst class, triggered by mergers of compact objects.

However, other features of this event – ​​such as the supernova it created – suggested that it originated from the collapse of a massive star. Astronomers think this explosion may have fizzled out before reaching the duration typical of long explosions.

Fermi and NASA’s Neil Gehrels Swift Observatory captured its opposite number, GRB 211211A, in December 2021. Located a billion light-years away, the explosion lasted about a minute. Although this makes it a long GRB, it was followed by a kilonova, which suggests it was triggered by a merger. Some researchers attribute the strangeness of this explosion to the merger of a neutron star with a partner black hole.

As astronomers discover more explosions that last several hours, there may yet be a new class in the making: ultralong GRBs. The energy created by the death of a massive star probably won’t be able to sustain an explosion for that long, so scientists must look for different sources.

Some think that ultralong bursts occur in newborn magnetars – neutron stars with fast rotation rates and magnetic fields a thousand times stronger than average. Others say this new class requires the power of the universe’s largest stellar residents, the blue supergiants. Researchers continue to explore ultralong GRBs.

Afterglows shedding new light

Although gamma rays are the most energetic form of light, they are certainly not the easiest to detect. Our eyes only see a narrow band of the electromagnetic spectrum. The study of any light outside this range, such as gamma rays, relies heavily on the instruments that our scientists and engineers develop. This need for technology, coupled with the already fleeting nature of GRBs, made the bursts more difficult to study in the early years.


The Hubble Space Telescope’s Wide Field Camera 3 revealed the infrared glow (circled) of GRB 221009A and its host galaxy, seen almost edge-on as a band of light extending to the upper left corner of the burst. Credit: NASA, ESA, CSA, STScI, A. Levan (Radboud University); Image processing: Gladys Kober

× to close


The Hubble Space Telescope’s Wide Field Camera 3 revealed the infrared glow (circled) of GRB 221009A and its host galaxy, seen almost edge-on as a band of light extending to the upper left corner of the burst. Credit: NASA, ESA, CSA, STScI, A. Levan (Radboud University); Image processing: Gladys Kober

GRB afterglow occurs when material in the jets interacts with the surrounding gas.

Afterglows emit radio, infrared, optical, UV, X-rays, as well as gamma rays, which provide more data about the original explosion. Remnant glows also last hours or days (or even years) longer than their initial burst, creating more opportunities for discovery.

The study of residual glows has become fundamental to deducing the driving forces behind the different explosions. In long bursts, as the afterglow fades, scientists eventually see the source glow again as the underlying supernova becomes detectable.

Although light is the fastest traveler in the universe, it cannot reach us instantly. By the time we detect an explosion, millions to billions of years may have passed, allowing us to probe part of the early Universe through distant glows.

Full of discovery

Despite the extensive research carried out to date, our understanding of GRBs is far from complete. Each new discovery adds new facets to scientists’ gamma-ray burst models.

Fermi and Swift discovered one such revolutionary event in 2022 with GRB 221009A, a burst so bright it temporarily blinded most space-based gamma-ray instruments. A GRB of this magnitude is predicted to occur once every 10,000 years, making it likely the highest luminosity event witnessed by human civilization. Consequently, astronomers dubbed it the brightest of all time – or the BOAT.

This is one of the closest long bursts ever observed at the time of its discovery, offering scientists a more detailed look at the inner workings of not only GRBs, but also the structure of the Milky Way. By observing BOAT, they discovered radio waves missing in other models and traced X-ray reflections to map our galaxy’s hidden dust clouds.


NASA’s Neil Gehrels Swift Observatory detected X-rays from GRB 221009A’s initial flash for weeks as dust in our galaxy scattered light back to us, shown here in arbitrary colors. Credit: NASA/Swift/A. Beardmore (University of Leicester)

GRBs also connect us to one of the most sought-after messengers in the universe. Gravitational waves are invisible distortions of spacetime, born from cataclysmic events like neutron star collisions. Think of spacetime as the all-encompassing blanket of the universe, with gravitational waves like ripples floating through the material.

In 2017, Fermi detected the gamma-ray flash from a neutron star merger just 1.7 seconds after gravitational waves were detected from the same source. After traveling 130 million light years, the gravitational waves reached Earth just before the gamma rays, proving that gravitational waves travel at the speed of light.

Scientists had never detected the joint journey of light and gravitational waves to Earth. These combined messengers paint a more vivid picture of neutron star mergers.

With continued research, our ever-evolving knowledge of GRBs could unravel the invisible structure of our universe. But the actual explosion is just the tip of the iceberg. A plethora of information rises just beneath the surface, ready for the harvest.

Leave a Reply

Your email address will not be published. Required fields are marked *