Science & Technology

Astrophysics Mystery Solved: Researchers Crack the Code Behind Cosmic Jets

Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with initial Lorentz factors in the order of a few hundreds. Many GRBs display an early X-ray light-curve plateau, which was not theoretically expected and therefore puzzled 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 commonly observed in astronomical systems at varying speeds, ranging from fast to slow. The quickest jets are highly relativistic and can reach velocities that are close to the speed of light. Despite being a widely observed phenomenon, the origin and many properties of these jets remain a mystery.

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

In many different galactic and extragalactic systems, emission of matter is commonly observed in the form of jets. The speed at which this spectacle occurs greatly varies.  Alongside relatively slow jets associated with neutron stars or binary star systems, very fast, relativistic jets are seen at 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, lasting for a few seconds in which a strong emission of gamma radiation is visible. It is then followed by an “afterglow” lasting a much longer time of hours, days, and even months. During this epoch, the emission subsequently fades and is observed as lower wavelengths, X-rays, ultraviolet, optical, infrared, and radio frequencies at very late times.

Beyond the question of why jets from these objects are so rapid, is a seemingly unrelated mystery as to what happens during the intermediate period of hundreds to thousands of seconds, in which the emission either fades or remains constant. In some cases, after a few tens of seconds, X-ray emission decays considerably, as would be expected from a relativistic burst colliding with the matter and radiation that exist in the space between the stellar systems in a galaxy.

However, in about 60% of the observed cases, the visible emission doesn’t fade but rather remains constant. This observation has long been a source of confusion to researchers, and no convincing explanation has been found for it since this phenomenon was discovered approximately 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 jet velocity, which is significantly lower than what was commonly assumed and fills the gap between velocities measured from different sources. In other words, a lower initial jet speed can explain the lack of decay and more visible and perpetual emission.

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

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

After thorough literature research, they created a sample of high-quality data. Following an examination of explanations for the phenomenon in existing literature, they found that all existing models, without exception, make additional assumptions that are not supported by the data. What is more significant is that none of the models offered a convincing explanation for the clean data. Therefore, the researchers returned to the basic model and tried to understand which of the basic assumptions isn’t valid.

They discovered that changing just one assumption, about the initial speed of the jets, was sufficient to explain the data. The researchers continued and examined the data that led 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 was valid in the cases they studied. From there they quickly concluded they were most likely in the right direction.

Prof. Asaf Pe’er, who led the theoretical part of this research, describes himself as a theorist who enjoys working with data.

“Astrophysical systems in general are characterized by great complexity, and often theoretical models, inherently more simplistic, may miss key points,” he explains. “In many cases, careful examination of the data, as we performed here, shows that existing ideas simply don’t 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 in total needed to be changed, everything worked out just like a puzzle,” Prof. Pe’er says. “So much so that from some 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, they were.”

Astrophysics research by its very nature is basic research. If, indeed, the researchers are correct, the results have far-reaching implications that can lead to a paradigm shift in the field, as well as in understanding the physical processes that produce jets. It is important to note that the origins of the phenomenon still aren’t fully known, but it is clearly related to the collapse of a star (or pair of stars) into a black hole. The research results are very important in understanding these mechanisms, as well as the type of stars that end their lives in a way that produces strong gamma radiation.

“Scientific research is fascinating. New ideas are constantly born and tested. Since the data are often inconclusive, people often publish their ideas and move on,” says Prof. Pe’er. “Here was a unique case, in which, after examining many ideas, I suddenly realized that the explanation could be very simple. After I proposed the explanation, we checked it again and again against the existing data, and it passed test after test. So sometimes, the simplest explanation is also the most successful one.”

Reference: “A wind environment and Lorentz factors of tens explain gamma-ray bursts X-ray plateau” by Hüsne Dereli-Bégué, Asaf Pe’er, Felix Ryde, Samantha R. Oates, Bing Zhang and Maria G. Dainotti, 24 September 2022, Nature Communications.
DOI: 10.1038/s41467-022-32881-1

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