We think that gamma-ray bursts, the brightest cosmic explosions, are created by two types of sources, each of them producing flashes with different durations, long or short. In December 2021, a long merger-driven burst was detected for the first time, emitting a flux of gamma-rays also never observed before. Both of these mysteries have a common protagonist: the coincident detection of a kilonova.
Gamma-ray bursts (GRBs in short) are the most luminous known objects in the entire Universe, emitting the amount of energy that our Sun radiates in its entire life, within just a few seconds. They are created by cosmic explosions, which generate a huge amount of gamma-rays in the blink of an eye.
As with all explosions, fuel is required to trigger them and after that there will be some debris. Very few objects can provide the ingredients needed to “ignite” a GRB: the fusion between two neutron stars orbiting around each other, or the collapse of massive stars that reach the final stages of their lives. Interestingly, regardless of which astrophysical system triggers the blast (that we define as “progenitor”), there will be a common leftover, most likely a black hole.
This newly born black hole will launch away the debris along its rotation axis at velocities close to the speed of light, creating what is called a “jet”. This collimated flow of matter is what generates the huge quantity of gamma-rays that we observe as a GRB.
After the explosion, the “central engine” that produces the light we observe is the same regardless of the progenitor. For that reason it is very difficult to distinguish what gives rise to a gamma-ray burst. The only way we have is by measuring the total duration of the burst. In fact, neutron star merger-driven GRBs can shine only for fractions of seconds, usually not more than 2 seconds, while collapsing stars can provide bursts longer than 2 seconds, up to several minutes. The former are called “short” GRBs, while the latter are “long” GRBs.
On December 11, 2021, we observed what may be the most bizarre GRB ever spotted, named GRB211211A. It was quite luminous, particularly close-by and most importantly it was followed by a kilonova.
Kilonovae are brief emissions produced after the collision of two neutron stars, like short GRBs. Indeed, in the past, kilonovae and short GRBs were observed together, as produced by the same neutron star merger through two distinct phenomena. But, unlike GRBs, the material that powers a kilonova is different, it is not very energetic and produces a fainter emission of optical light. The coincident detection of both GRB211211A and its kilonova leaves no doubt about their neutron star origin.
What left the astronomers astonished though, was that GRB211211A was not short at all, it was lasting around one minute! What appeared to be triggered by the death of a star because of its duration, was instead the first merger-driven long GRB observed. Furthermore, GRBs are so bright that they are able to produce gamma-rays with very different energies.
Thanks to the Large Area Telescope on board the Fermi satellite, our group detected an unusually large amount of high energy gamma-rays coming from this source. Typically, we detect high energy gamma-rays in GRBs only a long time after the explosion, in the so-called “afterglow”, an emission phase following the main burst characterised by a rapid luminosity decrease.
This time however, the high energy gamma-ray luminosity remained constant for hours after the main burst instead of rapidly dropping. State-of-the-art theories could not account for this steady luminosity, meaning that we were observing a new physical process generating high energy gamma-rays in addition to the standard “afterglow” emission. The possible solution to this puzzle arises from the main peculiarity of this long GRB: the presence of a kilonova. The extra gamma-rays are generated by an interaction between the jet and the kilonova, which we have witnessed for the first time. The photons produced by the kilonova can “bump” into the material (mainly electrons) accelerated in the jet, stealing some of their energy thanks to a process called “inverse Compton scattering”. This transforms most of the kilonova light into high energy gamma-rays.
This special source broke our understanding of GRB progenitors, suggesting that also two neutron stars can power a long burst. How this can happen is still an open question. Nonetheless, it was proved that either being short or long GRBs, did not necessarily constrain their origin. In our work, we showed that in this merger-driven GRB, a jet-kilonova interaction was not only possible, but also pretty visible. This interaction opens the window towards observation of new phenomena that can occur inside a GRB, telling us a lot about the structure of the jet, the presence of a kilonova and the object that creates the said GRB. All the latter discoveries were possible simply because we found a kilonova where we did not expect it: in a long burst!
Mei, A. et al. Gigaelectronvolt emission from a compact binary merger. Nature. doi:10.1038/s41586-022-05404-7 (2022)