spaceplasma: Relativistic jets and the Collapsar…


This animation shows the most common type of gamma-ray burst, thought to occur when a massive star collapses, forms a black hole, and blasts particle jets outward at nearly the speed of light.

spaceplasma:

Relativistic jets and the Collapsar model

Gamma-ray bursts (GRBs) were first detected by American nuclear detection surveillance satellites in the late 1960s. The Vela spacecraft series were designed to monitor world-wide compliance with the 1963 Nuclear Test Ban Treaty. The satellites detected no clandestine nuclear explosions, but they discovered something far more interesting: powerful bursts of gamma rays emanating from random directions in space. By analyzing the different arrival times of the bursts as detected by different satellites, scientists concluded that the sources of the bursts were cosmic and not terrestrial or solar. The discovery was declassified and published in 1973 as an Astrophysical Journal article entitled “Observations of Gamma-Ray Bursts of Cosmic Origin”. This alerted the astronomical community to the existence of gamma-ray bursts (GRBs), now recognized as the most violent events in the universe.

To this day GRBs remain one of the greatest mysteries of modern astronomy. We know that GRBs lasting less than 2 seconds (short GRBs) may originate from a variety of processes. There are several theories that explain how the energy from a gamma-ray burst progenitor is turned into radiation. One hypothesis describing how long gamma-ray bursts originate is called the “collapsar” model: gamma-rays are generated when massive, spinning stars collapse to form black holes and spew out powerful jets of plasma at nearly the speed of light.These stellar collapses (collapsars) are thought to be similar to supernovae, except that a jet is produced by the accretion of stellar material onto a compact object formed at the center of the collapsing star.

These jets are called ’relativistic jets’ and they can transport the energy from the collapsed core to large distances. Inside the jet, the uneven distribution of temperature, density and pressure create internal shock waves that move inward and outward as faster regions within the jet collide with slower ones. The collisions between the fast-moving gas and its surroundings, as well as within the jet itself, create gamma rays. When the jet hits the surrounding interstellar medium it produces another shock wave. This causes particles to rapidly lose energy (fast cooling), due to the strong magnetic field in the GRB emission region, through a process known as synchrotron radiation. This phenomenon is observed as long gamma-ray bursts and it’s followed by a so-called “afterglow”, a slowly fading emission that can be seen at all wavelengths; starting with X-rays, followed by ultraviolet, visible and infrared light, and eventually radio waves.The afterglow can last for days or even weeks.

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Credit: NASA’s Goddard Space Flight Center

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