Researchers from Bar-Ilan University in Ramat Gan figured out why some matter jets in astronomical systems don’t all travel at the same speed.
Matter outflows in the form of jets at fast, medium and slow speeds, when observed in astronomical systems, with the fastest ones traveling very close to the speed of light. Their origin, as well as many of their properties are uncertain. One of the puzzles that has long challenged experts is the fact that jet velocities seem to have a bi-modal distribution, with some being very fast and others slow – and a gap in between.
The researchers published their findings in the journal Nature Communications under the title “A wind environment and Lorentz factors of tens explain gamma-ray bursts X-ray plateau,” a study chosen by the journal’s editor as one of the 50 most important articles recently published.
Emission of matter
The emission of matter in many galactic and extragalactic systems is commonly observed in the form of jets of greatly varying speed. Alongside relatively slow jets associated with neutron stars or binary star systems, very fast jets are observed at speeds very close to the speed of light.
The fastest known jets are associated with a phenomenon known as gamma-ray bursts, characterized by an initial flash of gamma rays that lasts for a few seconds, in which a strong emission of gamma radiation is visible. It is then followed by an afterglow lasting for hours, days and even months. During this period, the emission subsequently fades and is observed as lower wavelengths, X-rays, ultraviolet, optical, infrared and radio frequencies at much later times.
Besides the question of why jets from these objects are so rapid, a seemingly unrelated mystery is 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 dozen 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 of a galaxy. However, in about 60% of 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 the phenomenon was discovered 18 years ago.
Researchers from the BIU physics department 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 also fills the gap between velocities measured from different sources. Meaning, 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.
ONE OF the main open questions in the study of gamma-ray bursts is why X-rays, which are visible for up to several days, do not fade for a long time in a significant percentage of cases. 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. After examining 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 even more significant is that none of the models offered a convincing explanation for the clean data.
The researchers returned to the basic model to understand which of the basic assumptions aren’t valid. They discovered that changing just one assumption, about the initial speed of the jets, was enough to explain the data. They then examined the data that led other astrophysicists to conclude that the jets must be traveling very close to the speed of light 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.
“Astrophysical systems in general are characterized by great complexity, and often theoretical models, inherently more simplistic, may miss key points,” according to Prof. Asaf Pe’er, who led the theoretical part of this research.
“In many cases, careful examination of the data, as we performed here, shows that existing ideas simply don’t work,” he said. “This is what led us to come up with new ideas. Sometimes the simplest, least complex idea is sufficient.”