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02.02.2023 | יא שבט התשפג

In Time for Israel Space Week

Researchers from the Department of Physics at Bar-Ilan University shift our understanding of a well-known astrophysical phenomenon



Matter outflows in the form of jets are observed in astronomical systems at fast, medium and slow speeds. The fastest jets are highly relativistic, namely travel very close to the speed of light. The origin, as well as many properties of the jets, is uncertain. One of the puzzles, that jet velocities seem to have a bi-modal distribution – some very fast and others slow, with a gap in velocities in between, has long challenged experts. Bar-Ilan University researchers re-examined the data and have now seemingly solved the 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, lower initial jet speed can explain 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, travelling 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."