Hypernova

A hypernova is an exceptionally energetic type of stellar explosion. It represents a more powerful version of a standard supernova. These events are at least ten times more luminous than most supernovae. They also eject material with much higher kinetic energy. This energy is often an order of magnitude greater than typical core collapse supernovae. Scientists sometimes refer to them as superluminous supernovae. However, that term can also include other bright explosions with different origins. Hypernovae are vital for understanding the most massive stars in our universe.

The mechanism behind a hypernova often involves a process called the collapsar model. This model describes the collapse of a rapidly rotating star. It starts when a massive star undergoes core collapse. If the star's core is at least fifteen solar masses, it may collapse into a black hole. In some cases, the explosion energy is too low to expel the outer layers. However, if the star is rotating quickly, the collapse creates a black hole with an accretion disk. This disk is a swirling ring of matter around the black hole. The rotation drives twin astrophysical jets that shoot outward from the center. These relativistic jets plow through the stellar material at nearly the speed of light. This action creates powerful shock waves that detonate the hypernova explosion.

Hypernovae are usually classified as Type Ic supernovae. This means their spectra show no hydrogen and no clear helium features. Instead, they show strong silicon lines. A key feature is the presence of extremely broad spectral lines. These lines indicate that the ejected material has an extremely high expansion velocity. The velocity of this ejected material can reach up to 99% of the speed of light. These explosions are also one mechanism for producing long-duration gamma-ray bursts. These bursts are intense flashes of gamma rays that last from two seconds to over a minute.

Our understanding of hypernovae has changed significantly over time. In the 1980s, the term described theoretical events like pair-instability supernovae. It was also used for hypothetical explosions from hyperstars or black hole mergers. In February 1997, the BeppoSAX satellite traced a gamma-ray burst to a distant galaxy. Researchers Bloom et al. analyzed this data in 1998. They concluded that a hypernova was the likely cause. That same year, Polish astronomer Bohdan Paczyński hypothesized that hypernovae come from rapidly spinning stars. This helped refine the modern definition of the term.

The first observed hypernova was SN 1998bw. This event was 100 times more luminous than a standard Type Ib supernova. It was also the first to be associated with a gamma-ray burst, known as GRB 980425. The total mass ejected by SN 1998bw was about 10 solar masses. It also ejected about 0.4 solar masses of nickel. The radioactive decay of the ejected nickel, specifically 56Ni, makes the outburst much brighter. Hypernovae are also associated with unusually bright radio supernovae, sometimes called radio hypernovae.

Scientists study different models to explain how these stars lose their outer layers. One idea involves binary systems, where two stars orbit each other. A companion star might strip the outer envelope of a massive star. This leaves behind a bare carbon-oxygen core. This process helps create the specific conditions needed for a Type Ic explosion. Another theory involves induced gravitational collapse. This occurs when a neutron star collapses into a black hole because of a nearby companion star. These models help explain why some stars become the progenitors of hypernovae.

While hypernovae are distinct, they can be confused with other energetic events. In 2023, astronomers observed a very strong event named AT2021lwx. This event emitted energy from mid-infrared to X-ray wavelengths. However, it was not a hypernova. It was likely a huge gas cloud being absorbed by a massive black hole. The Zwicky Transient Facility gave it the random name ZTF20abrbeie. Because of its ferocity, the press nicknamed it "Scary Barbie." Studying these different high-energy events helps astronomers distinguish between various cosmic phenomena.