What is a collapsar? How does the collapsar model account for the existence of long-duration gamma-ray bursters?

Long-duration gamma-ray bursts (GRBs) are among the most energetic events in the universe, fascinating astronomers and posing questions about their origins. These cosmic phenomena last from two seconds to several minutes and are characterized by the release of massive amounts of energy. They are detected by satellites in orbit around Earth that are equipped with instruments sensitive to gamma rays.

According to the collapsar model, long-duration GRBs are the result of the catastrophic collapse of massive stars. When a star many times the mass of our Sun exhausts its nuclear fuel, it may collapse under its own gravity. This forms a black hole and an accompanying intense burst of gamma rays, observed as a long-duration GRB. These bursts are not only fascinating for their sheer power but also because they serve as beacons to study the early universe.

Stellar collapse is a dramatic process signifying the end of a star's life cycle, occurring when a star exhausts its nuclear fuel and can no longer support itself against gravitational forces. For massive stars, typically more than 40 times the mass of the Sun, this collapse is particularly catastrophic. The core of the star implodes, causing the outer layers to either be expelled into space or to spiral inward. The inward spiraling material often forms an accretion disk around the newly forming black hole, while the core's implosion drives the creation of a black hole – the invisible monster lurking at the heart of a collapsar.

Understanding stellar collapse is crucial as it sets the stage for a string of cosmic events, including the formation of neutron stars, black holes, and the trigger of supernova explosions.

The formation of a black hole represents nature's limit of compression, where matter is squeezed into an infinitely dense point known as a singularity. During stellar collapse, if the mass of the core is sufficient, gravitational forces overwhelmingly crush the core, overcoming all known forces that usually keep atoms apart. In the collapsar model, the star's core becomes a black hole – an object of infinite density with a gravitational pull so strong that not even light can escape it.

Black holes formed through this process often anchor the material spiraling into them, creating an accretion disk which is a key feature in producing the incredible energy seen in gamma-ray bursts.

An accretion disk in the context of a collapsar is a swirling, donut-shaped ring of matter that orbits a new-born black hole. When the progenitor star collapses, angular momentum causes the in-falling material to flatten into a disk as it spirals inward. This accretion disk becomes incredibly hot and dense as particles rub against each other, converting their kinetic energy into heat.

Friction within the disk heats the material to such extremes that it radiates energy across various wavelengths, particularly X-rays. It is important to note that the accretion disk does not just passively orbit the black hole; the intense gravitational and thermal forces at play can result in the ejection of material in the form of spectacular jets.

High-energy jets are narrow beams of particles ejected at relativistic speeds from the poles of the accretion disk around a black hole. These jets are a key component in the collapsar model for creating long-duration gamma-ray bursts. As the black hole forms, magnetic fields near the event horizon can, under the right conditions, channel some of the infalling matter into these high-energy jets.

Not every stellar collapse results in significant jet formation. It is believed that rapid rotation and suitable magnetic fields are necessary. When these high-energy jets pierce the stellar envelope, they interact with the circumstellar material, producing shocks that give rise to the observed gamma-ray bursts. Understanding these jets is critical because they are one of the few ways we have to study the environments immediately surrounding black holes.