by Scientists may be one step closer to finding out how gamma-ray bursts form some of the most powerful explosions in the known universe.
For context, a single gamma-ray burst, or GRB, can produce more energy in seconds than the sun emits in billions of years. Because of this power, scientists theorize that GRBs are created by some of the most violent events in the universe. This includes things like supernova explosions that mark the death of massive stars and the collision and merger of two neutron stars, which are “dead” stars made up of the densest matter we know, as well as explosions from baby black holes.
However, aspects of these bursts are shrouded in mystery, including the exact mechanism that triggers a GRB and what exactly causes a “long” GRB lasting more than 2 seconds versus a “short” GRB which will last less time.
One team of scientists from the University of Alabama in Huntsville, for example, is studying the light anomalies of GRBs and how they change over time in order to better model these bursts and eventually break their secrets.
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“Despite being studied for more than fifty years, the mechanisms by which GRBs produce light remain unknown, a great mystery of modern astrophysics,” said team leader Jon Hakkila, a scientist at the University of Alabama in Huntsville, in statement. “Understanding GRBs helps us understand some of the fastest and most powerful light-producing mechanisms used by Nature.
“GRBs are so bright, they can be seen across the universe, and – because light travels at a finite velocity – they allow us to look back to the earliest times when stars existed.”
Shining light on GRBs
One of the main reasons GRBs have remained so difficult to understand is that theoretical models to describe them have not been able to explain the behavior of their light curves, which are graphs that show how an object’s light intensity changes over time.
Further complicating matters is that no two GRB light curves are exactly alike, and the duration of bursts can last from milliseconds to tens of minutes.
Hakkila and her colleagues modeled GRBs as a series of energetic pulses, considering these pulses to be the basic units of GRB emission. “They show times when a GRB brightens and then fades away. During the time that a GRB pulse it emits, under brightness changes that can sometimes occur on very short time scales,” said Hakkila. “The strange thing about these variations is that they are equally reversible [palindrome] words like ‘rotator’ or ‘kayak’ are reversible.”
The scientist also said that it is very difficult to understand how this reversibility can be so because, unlike the letters in a word, time can only be read in one direction.
“The mechanism that produces light in a GRB pulse somehow produces a pattern of brightness, then it generates the same pattern in reverse order,” he said. “That’s quite strange, and it makes GRBs unique.”
The team focused on models of GRBs produced when black holes (formed recently when a massive star died) exploded jets of particles traveling at speeds close to light, or “relativistic” speeds.
“In these models, the core of a dying massive star collapses to form a black hole, and material falling into the black hole is torn apart and redirected out along two other beams or jets,” explains Hakkila. “The jet of material directed towards us is ejected at almost the speed of light.
“Since the GRB is relatively short, it is always assumed that the jet remains focused on us during the event. But it has been very difficult to explain whether they originate within a jet that is not moving.”
To explain the palindrome nature of GRB light curves, the team added lateral, or lateral, motion to relativistic jets blasting out from infant black holes.
“The idea of a laterally moving jet provides a simple solution by which to explain the reverse GRB pulse structure,” continued Hakkila. “As the jet crosses the line of sight, an observer will see the light produced first from one side of the jet, then the center of the jet, and finally the other side of the jet.
“The jet will brighten and then weaken as the center of the jet crosses the line of sight, and a radially symmetric structure will appear around the center of the jet in reverse order as the jet becomes weaker.”
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Hakkila added that in this “ballistic model of GRBs”, relativistic jets from black holes spray material in a manner similar to a fire hose spraying water. Because the jets act as a fluid rather than a solid, he said an observer would see that the entire jet was curved and not straight.
“The motion of the nozzle causes light from different parts of the jet to reach us at different times, and this can be used to better understand the mechanism by which the jet produces light, as well as a laboratory to study on the effects of special relativity,” he said.
The team’s research was published on April 22 in The Astrophysical Journal.
