Scientists crack the code behind cosmic jets

Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with initial Lorentz factors on the order of a few hundred. Many GRBs show an early X-ray light curve plateau, which was not theoretically expected and therefore confused the community for many years.

Matter outflows in the form of jets are observed in astronomical systems at varying speeds, ranging from fast to slow. Jets in the form of matter outflows are often observed in astronomical systems with varying speeds, from fast to slow. The fastest jets are highly relativistic and can reach speeds close to the speed of light. Despite being a widely observed phenomenon, the origin and many characteristics of these jets remain a mystery.

For a long time, experts have been puzzled by the bimodal distribution of jet speeds, with some incredibly fast and others slow, and a noticeable absence of speeds in between. However, researchers at Bar-Ilan University have looked at the data and seem to have finally solved this perplexing puzzle.

In many different galactic and extragalactic systems, emissions of matter are often observed in the form of jets. The rate at which this play occurs varies greatly. Alongside relatively slow jets associated with neutron stars or binary star systems, very fast, relativistic jets are seen with speeds very close to the speed of light. The fastest known jets are associated with a phenomenon known as “gamma-ray bursts”.

This phenomenon is characterized by an initial flash of gamma rays, which lasts for a few seconds, during which a strong emission of gamma radiation is visible. It is then followed by an “afterglow” that lasts for much longer hours, days and even months. During this epoch, the emission then fades and is observed as lower wavelength, X-ray, ultraviolet, optical, infrared and radio frequencies at very late times.

Beyond the question of why jets from these objects are so fast is a seemingly unrelated mystery about what happens in the intervening period of hundreds to thousands of seconds, where the emission either fades or remains constant. In some cases, after a few tens of seconds, the X-ray emission decreases considerably, as can be expected from a relativistic burst colliding with the matter and radiation found in the interstellar space of a galaxy.

However, in approximately 60% of observed cases, the visible emission does not fade, but rather remains constant. This observation has long been a source of confusion for scientists, and no convincing explanation has been found for it since this phenomenon was discovered about 18 years ago.

Researchers from the Department of Physics at Bar-Ilan University have now proven that this visible, perpetual emission is a natural consequence of the jet’s speed, which is significantly lower than what was commonly assumed and fills the gap between speeds measured from different sources. In other words, a lower initial jet velocity could explain the lack of decay and more visible and persistent emission.

The researchers showed that previous results, from which high velocities were derived in these objects, are not valid in these cases. In doing so, they changed a paradigm and proved that jets form in nature at all speeds. The study was published in the journal Nature communication and selected by the journal’s editor as one of the 50 most important articles recently published.

One of the most important open questions in the study of gamma-ray bursts is why X-rays, which are visible for up to several days, in a significant percentage of cases do not fade for a long time. To answer this question, the researchers began a careful survey of the data, which is abundant but scattered and “noisy”.

After thorough literature research, they created a selection of high-quality data. After a survey of explanations for the phenomenon in the existing literature, they found that all existing models, without exception, make additional assumptions that are not supported by the data. More significantly, neither model provided a convincing explanation for the pure data. Therefore, the researchers returned to the basic model and tried to understand which of the basic assumptions are not valid.

They discovered that changing just one assumption, about the takeoff speed of the jets, was sufficient to explain the data. The researchers went on to examine the data leading other astrophysicists to conclude that the jets must be highly relativistic (that is, traveling very close to the speed of light = extremely fast), and discovered, to their surprise and delight, that none of the existing arguments were valid in the cases they studied. From there, they quickly concluded that they were most likely headed in the right direction.

Prof. Asaf Pe’er, who led the theoretical part of this research, describes himself as a theoretician who likes to work with data.

“Astrophysical systems in general are characterized by great complexity, and often theoretical models, inherently more simplified, can miss key points,” he explains. “In many cases, careful examination of the data, as we performed here, shows that existing ideas simply do not work. This is what led us to come up with new ideas. Sometimes the simplest, least complex idea is sufficient.”

Prof. Pe’er’s partners in this research are the study’s first author, Dr. Hüsne Déréli-Begue, from the Bar-Ilan research group, and Prof. Felix Ryde, from KTH Royal Institute of Technology in Stockholm. While Pe’er focused on theory, his collaborators focused on analyzing the data that stimulated and supported the theory he proposed.

“It took us a while to develop the understanding, and once I realized that one parameter had to be totally changed, everything worked like a puzzle,” says Professor Pe’er. “So much so that from a certain point, every time we brought up a new potential problem, it was clear to me that the data would be in our favor, and indeed it was.”

Astrophysical research is by its very nature basic research. If the researchers are indeed correct, the results have far-reaching implications that could lead to a paradigm shift in the field, as well as for understanding the physical processes that produce jets. It is important to note that the origin of the phenomenon is still not fully known, but it is clearly related to the collapse of a star (or a pair of stars) into a

Reference: “A wind environment and Lorentz factors of tens explain gamma-ray burst X-ray plateau” by Hüsne Dereli-Bégué, Asaf Pe’er, Felix Ryde, Samantha R. Oates, Bing Zhang and Maria G. Dainotti, 24 September 2022,