The brightest objects in the Universe are… black holes!

Illustration: Artistic visualization by Bing Image Creator of a black hole shooting lightning from its poles. In reality, there would be so many of these “lightning” that they would blend into a continuous jet. The following text explains why the light emitted by gamma-ray bursts may exhibit similar behaviour. (Image source: Bing/AI)

These objects are incredibly bright, with their afterglows remaining visible for up to several dozen days. As a result, the total energy they emit turns out to be greater than the energy our Sun will release over its entire lifetime. It is believed that these events originate from the explosions of very massive stars that, in the final stages of their evolution, run out of fuel. We know that radiation pressure prevents typical stars from collapsing, but once a star loses the ability to shine due to the shutdown of nuclear fusion in its core, it begins to collapse. In the case of the most massive stars, this collapse quickly leads to the formation of a black hole in the star's centre. The initial phase of such an implosion results in extremely bright flashes lasting up to several minutes. However, we observe light from these events even months after the star's death. So where does this long-lasting afterglow come from?

The progenitor star was spinning before its collapse. All stars in the Universe rotate at different speeds. Therefore, when a black hole forms, it must inherit the angular momentum of the star it originated from. Additionally, when the size of a rotating object decreases, its rotation speed increases. The resulting black hole must, therefore, spin very rapidly. At the same time, another physical quantity behaves similarly — the magnetic field. Just like the rotational speed, it doesn’t disappear but instead increases dramatically as the object shrinks. This magnetic field becomes trapped around the black hole by the remnants of the star’s infalling matter, coiled into the space surrounding it. It turns out that, much like a bicycle dynamo, this magnetic field can extract some energy from the black hole and transfer it to particles like electrons, allowing them to emit light.

Lightning from Black Holes

When electrons are accelerated, they emit light, a phenomenon we commonly encounter in everyday life. Antennas operate on this principle: electrons in them are moved using alternating voltage, which causes them to emit radio waves (photons of low energy). In gamma-ray bursts, electrons are accelerated by an enormous magnetic field. As a result, they can emit photons of extremely high energy. The highest energy photon ever observed from a gamma-ray burst was 18 TeV. While a soccer ball kicked by Robert Lewandowski contains a billion times more energy, a gamma-ray burst emits an enormous number of such photons. In fact, the total energy of a single gamma-ray burst is equivalent to the energy of 10⁴² soccer balls kicked by Lewandowski.

But that's not all — such a photon is very unstable and can produce an electron-positron pair. These particles are then accelerated again in the magnetic field and emit more high-energy photons. This cycle repeats continuously, creating an avalanche of photons. One can imagine this phenomenon as lightning bolts erupting from a black hole. It’s certainly not something you’d want to be in the path of. In fact, there are even theories that a jet from a gamma-ray burst may have caused a mass extinction event on Earth in the past (see: How deadly would a nearby Gamma Ray Burst be?).

Light Curves

As it shines, the black hole loses its rotational kinetic energy. As a result, it spins slower and shines more faintly over time. We, therefore, observe a very characteristic decline in brightness, as shown in the figure below.

The explanation that a black hole is responsible for this particular shape of the light curve is one of the most significant conclusions of the new study. These findings will help us better understand the origins of black holes observed via gravitational waves. Moreover, by analysing the detailed shapes of these light curves, we can study how accretion changes over time. This, in turn, gives us insights into the internal structure of the most massive stars in the Universe right before their death. Once again, gamma-ray bursts prove to be an excellent tool for studying the fundamental laws governing our Universe.