March 30, 2023

La Ronge Northerner

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Researchers crack the code behind the cosmic planes

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

Outflows of matter are observed as jets in astronomical systems with varying speeds, ranging from fast to slow. Jets that are in the form of flowing matter are commonly observed in astronomical systems with varying speeds, ranging from fast to slow. The fastest aircraft are highly relativistic and can reach speeds close to the speed of light. Although a widely observed phenomenon, the origin of these jets and many of their characteristics remain a mystery.

For a long time, experts were baffled by the bimodal distribution of aircraft speeds, some incredibly fast and others slow, and a remarkable absence of speeds in between. However, researchers at Bar-Ilan University have reconsidered the data and it appears that they have finally solved this perplexing mystery.

In many different galaxy systems and extragalaxies, the emission of matter is commonly observed in the form of jets. The speed at which this scene occurs varies greatly. Besides the 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 a few seconds in which a strong emission of gamma radiation appears. Then follows an “afterglow” that lasts much longer than hours, days, or even months. During this era, the emission later fades and is observed as lower wavelengths, X-rays, ultraviolet, optical, infrared, and radio frequencies at much later times.

Beyond the question of why the jets from these objects speed up, there is a mystery that seems unrelated to what happens during the intermediate period of hundreds to thousands of seconds, during which the emission fades away or remains constant. In some cases, after a few tens of seconds, the X-ray emission decays exponentially, as would be expected from a relativistic explosion colliding with the matter and radiation present in the interstellar space of the galaxy.

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

Researchers from Bar-Ilan University’s Department of Physics have now demonstrated that this permanent visible emission is a natural consequence of the jet’s velocity, which is much lower than was generally assumed and fills the gap between the velocities measured from various sources. In other words, the lower initial jet velocity could explain the lack of decay and the more pronounced and durable emission.

The researchers showed that the previous results, from which high speeds were deduced in these bodies, are not valid in these cases. In doing so, they changed the model and proved that planes form in nature at all speeds. The study has been published in the journal Nature Communications And the editor of the magazine chose him as one of the 50 of the most important recently published articles.

One of the main open questions in the study of gamma-ray bursts is why in such a large proportion of cases, X-rays, which can be seen for several days, do not fade for a long time. To answer this question, the researchers set about carefully mapping the data, which was numerous but scattered and “noisy.”

After an exhaustive literature search, they created a high-quality data sample. After examining explanations for the phenomenon in the existing literature, they found that all current models, without exception, make additional assumptions that are not supported by the data. What is more important is that none of the models provided a convincing explanation for the clean data. So, the researchers went back to the basic model and tried to understand which of the basic assumptions is incorrect.

They discovered that changing just one assumption, about the planes’ initial velocity, was enough to explain the data. The researchers went on and examined the data that led other astrophysicists to conclude that the jets must be highly relativistic (ie, traveling very close to the speed of light = extremely fast), and discovered, to their surprise and delight, that none of the jets should be highly relativistic. The arguments found were valid in the cases they studied. From there they quickly concluded that they were most likely in the right direction.

Professor Assaf Peer, who led the theoretical part of this research, describes himself as a theoretician who enjoys working with data.

“Astrophysical systems are generally very complex, and theoretical models, which are more simple in nature, often miss key points,” he explains. “In many cases, close examination of the data, as we performed here, shows that existing ideas simply don’t work. This is what prompted us to come up with new ideas. Sometimes the simplest, least complex idea is enough.”

Professor Beer’s partners in this research are the first author of the study, Dr Hussein Direli Beg, from the Bar-Ilan Research Group, and Professor Felix Reid, from KTH Royal Institute of Technology in Stockholm. While Beer focused on theory, his collaborators focused on analyzing the data that motivated and supported the theory he proposed.

“It took us some time to develop an understanding, and once I realized there was one standard in the whole that needed to change, the whole thing worked like a puzzle,” says Professor Beer. “So much so that from some point in time, every time we brought up a new potential problem, it was clear to me that the data would be in our favor, and in fact, it was.”

Astrophysical research is, by its very nature, fundamental research. If the researchers are indeed correct, the findings have far-reaching implications that could lead to a paradigm shift in the field, as well as in the understanding of the physical processes that produce jets. It is important to note that the origins of this phenomenon are still not fully known, but it is clearly related to the collapse of a star (or pair of stars) in[{” attribute=””>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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