Curved jet model solves pulse reversibility
Gamma-ray bursts – some of the brightest and most energetic phenomena observed by astronomers. In just a few seconds, such a burst can release an amount of energy exceeding the radiation of our Sun over billions of years of existence. Scientists suggest that such colossal power of gamma-ray bursts is due to catastrophic events in the Universe – supernova explosions, mergers of neutron stars or radiation from young black holes.
However, the exact reasons and triggering mechanisms for these mysterious phenomena still remain unsolved mysteries of modern astrophysics. In an attempt to shed light on the nature of gamma-ray bursts, a team of researchers from the University of Alabama in Huntsville, led by John Hakkila, studied changes in their luminosity over time.
«Despite the fact that the mechanisms by which gamma-ray bursts occur have been studied for more than fifty years, they still remain a mystery. Understanding gamma-ray bursts helps us understand some of the fastest and most powerful processes. The bursts are so bright that they can be detected anywhere in the Universe, and because the radiation travels at a finite speed, they allow us to peer into the earliest epochs of the existence of stars, — notes Hakkila.
The main difficulty in studying gamma-ray bursts is that modern theoretical models cannot explain the behavior of their light curves — graphs showing changes in radiation intensity over time. Moreover, no two light curves are identical, and the duration of the bursts themselves can vary from fractions of a second to tens of minutes.
Hakkila's team modeled gamma-ray bursts as a series of energy pulses, considering them the basic «building blocks» these outbreaks. «They indicate the moments when a gamma-ray burst becomes brighter and then fades,— explains the scientist, — During the emission of a gamma-ray burst pulse, it undergoes changes in brightness, which can sometimes occur on very short time scales. The strange thing about these variations is that they are reversible, just like palindromic words.
Hakkila adds that such reversibility seems extremely mysterious, since, unlike the letters in a word, time flows strictly in one direction. The gamma-ray burst pulse mechanism somehow creates a pattern of brightness, and then reproduces that same pattern in reverse. This is quite strange and makes gamma-ray bursts unique phenomena», — he explains.
Researchers have focused on simulating gamma-ray bursts, which occur when jets of particles moving at near-light, or “relativistic”, speeds are ejected from forming black holes. «In these models, the core of a dying massive star collapses to form a black hole, and the material falling into it is redirected outward along two opposing jets. Material moving in our direction is ejected almost at the speed of light», — describes Hakkila.
The results of new research bring scientists closer to understanding the fundamental processes underlying gamma-ray bursts.
«Because a gamma-ray burst is a relatively short-lived event, it was traditionally believed that the jet remained strictly aimed at the observer throughout the entire event. However, the features of time-reversible pulses are extremely difficult to explain if they arise inside a stationary jet», — Hakkila continues.
To resolve the mystery of the palindromic nature of gamma-ray burst light curves, scientists have proposed a model in which relativistic jets escaping from young black holes undergo sideways motion.
«The idea of transverse jet motion provides a simple solution to explain the time-reversible pulse structure of gamma-ray bursts. When the jet crosses the line of observation, we will first see light from one edge, then from the center, and finally from the opposite edge. The jet will become brighter when its center crosses the line of sight, and then dim, and the radially symmetric structure around the core will be observed in the opposite order», — explains the researcher.
According to Hakkila, within this model, relativistic jets of black holes scatter matter in the same way as water is sprayed from a fire hose. Because the jets behave like a liquid rather than a solid, an observer will see them as curved rather than straight lines.
«Motion causes radiation from different parts of the jet to reach us at different times, and this can be used to better understand the mechanism of radiation generation, and also as a "laboratory" to study the effects of special relativity», — concludes Hakkila.
Thus, by introducing the lateral motion of relativistic jets into their models, scientists are closer to solving one of the biggest mysteries of gamma-ray bursts — their time-reversible nature. Further research should shed light on the fundamental processes underlying these cosmic cataclysms.