A missing link between different types of supernovae

On the left, a common Supernova with no engine, on the right a Supernova with a strong engine that emits a gamma-ray burst, in the middle an intermediate case such as SN 2012ap (Image Bill Saxton, NRAO/AUI/NSF)
On the left, a common Supernova with no engine, on the right a Supernova with a strong engine that emits a gamma-ray burst, in the middle an intermediate case such as SN 2012ap (Image Bill Saxton, NRAO/AUI/NSF)

An article published in “Astrophysical Journal” describes the diskovery made by a group of astronomers led by Sayan Chakraborti of the Harvard-Smithsonian Center for Astrophysics (CfA). They studied a supernova called 2012ap (SN 2012ap) which is a missing link between ordinary ones and the ones that cause the emission of a gamma-ray burst.

The astronomers used the Very Large Array (VLA) but also data collected from the Meterwave Giant Radio Telescope (GMRT) and space probes that are part of the InterPlanetary Network (IPN) to study the supernova 2012ap. It’s the type that is caused by the core collapse of a massive star when nuclear fusion can no longer provide the necessary energy to counteract the weight of its outer layers.

The most common consequence of this type of supernova is the expulsion of materials of the star outward in an almost spherical bubble that expands rapidly but at speeds much slower than that of light. This type of supernova doesn’t produce gamma-ray bursts.

There’s a small percentage of cases in which the materials which collapse form for a short time an accretion disk surrounding what remains of the star’s core, which can be a neutron star or a black hole. This accretion disk produces jets of materials which move outwards from the disk’s poles at speeds approaching that of light. The combination of the disk and of the jets is called an “engine” and is the one that generates the gamma-ray bursts.

The supernova 2012ap is an intermediate case because it had jets moving at nearly the speed of light and then they quickly slowed down, but it produced no gamma-ray burst. This shows that the engine isn’t a sufficient element to cause that kind of supernova.

The researchers also studied a previous supernova called SN 2009bb. It presented jets at speeds approaching that of light which propagated in space instead of being slowed down as in the cases in which gamma-ray bursts are emitted. It’s a phenomenon similar to that of supernovae with no engine and most likely its jets contained a high proportion of heavy particles. In the case of gamma-ray bursts emission, jets contain light particles instead, which can’t easily make their way through the material surrounding the star.

This research shows a wide variety of mechanisms in supernovae. The combination of a powerful engine and light particles seems to be the one which determines the gamma-ray burst. The fact that only a small percentage of supernovae generate a gamma-ray burst is positive because its energy emission is enormous and can devastate a planet that would be hit even over very long distances.



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