Gamma-ray bursts are the most powerful explosions in the universe, usually caused by collisions of collapsed stars or compact stellar remnants. But a recent discovery has challenged that understanding, as it doesn’t fit into any of these categories. Astronomers at the Niels Bohr Institute played an important role in this work, which has the potential to revise existing theories about these powerful events.

Daniele Bjorn Malesani conducted a planned follow-up of the gamma-ray burst GRB 211211A with the Northern Optical Telescope on La Palma Canary Island. The standard procedure after the text message is received is automatically triggered by the Neil Gehrels Swift Observatory spacecraft, which monitors the sky for gamma-ray bursts.

But something was wrong…

Malesani is an astronomer at Radboud University in the Netherlands and a visiting researcher at the Center for the Cosmic Dawn in Copenhagen. He specializes in gamma-ray bursts, the most energetic explosions in the universe. But to understand what went wrong, let’s first look at what a “gamma ray burst” is:

As bright as the universe itself

Gamma-ray bursts are short, ultra-bright bursts of gamma rays, the most energetic form of light. They are mostly found in the very distant universe and fall into two categories thought to originate from two different physical scenarios:

“Long” flashes usually last from a few seconds to a few minutes, but are usually accompanied by a longer afterglow of less energetic light. They occur in most star-forming regions of galaxies and are thought to be the result of a massive star collapsing into a compact neutron star or black hole and ejecting its outer parts in a massive supernova-like explosion.

“Short” bursts pass even faster, with typical durations of 1/10 to 1 second. They can often be seen offset from galactic centers or even outside galaxies. The prevailing theory is that they are the result of two massive stars orbiting each other in a “binary” system. At some point they explode as supernovae and blast them out of their host galaxy. Eventually, however, the two objects will merge into a spiral, resulting in a gamma-ray burst.

Both emit incredible energy: at their peaks, they can shine as brightly as all the stars in the observable universe combined (assuming they emit light equally in all directions; in fact, they’re probably slightly less bright), but they radiate most of their energy. light in the narrow beams we are in that direction).

Mysterious gamma-ray bursts

Gamma-ray bursts were first detected in 1967 by the Vela satellite, which was designed to monitor the sky for possible nuclear weapons tests that would violate the 1963 Nuclear Test Ban Treaty. In the 1990s, more sensitive space observatories, which first believed they came from nearby sources in our galaxy, showed that they must be far beyond the Milky Way, which is spread throughout the universe.

The short-lived nature of the flares has made them difficult to study, but since the late 1990s astronomers have also detected lower-energy afterglows, from X-rays to optical light to infrared, helping to form a theory for their origin. .

Gamma-ray bursts come in two variants, “short” and “long”, which are believed to result from two different physical mechanisms, respectively, the merging of two compact objects and the collapse of a massive star. . Thanks to new observations, this theory is now being questioned.

mixed signals

So what was the problem with the Malesani explosion GRB 211211A? Well, it didn’t seem to fit either, or maybe both, of these categories. “Observations showed that the explosion originated from outside the galaxy, which is typical for hosting short bursts. But this beast lasted for almost a minute instead of a millisecond or a few seconds,” says Malesani.

A strange event prompted an international group of astronomers led by Gillian Rastinejad of Northwestern University (USA) to launch an intense campaign to study this strange object. These efforts led to the completely unexpected discovery of the so-called kilonova, which is striking evidence of the collision of two neutron stars or a neutron star and a black hole. Binary neutron star mergers are believed to be the ancestors of short gamma-ray bursts. Why did it follow a long burst instead of baffling astronomers?