The missing link of stellar explosions

Observations in the radio band have identified a sort of intermediate supernova that is halfway between an ordinary supernova and a gamma-ray burst. This is an important result which provides new clues about the mechanism that lies behind these phenomena violent and explosive and that determines their physical properties. The results were published in the Astrophysical Journal.
Thanks to a series of observations made with the Very Large Array (VLA), the Giant Meterwave Radio Telescope (GMRT) and the Interplanetary Network (IPN), a group of astronomers led by Sayan Chakraborti the Harvard-Smithsonian Center for Astrophysics ( CfA) has found a kind of “missing link”, which is hunted for a long time, which would represent the connection between the ordinary supernovae, i.e. where there is no high energy emission in the form of gamma rays, and those which instead produce gamma rays (Gamma-Ray Burst, GRB). The object in question, observed in 2012, shows a series of characteristics that are compatible with the presence of a mechanism that generates gamma rays, even if such an event has not yet occurred.


Credit: Bill Saxton, NRAO/AUI/NSF

“It is a surprising result that provides new clues about the mechanism that lies behind these phenomena violent and explosive,” says Chakraborti. “This object appears to be midway between GRBs and other types of supernovae, which implies the existence of a wide variety of phenomena that characterize these stellar explosions.”
Named with the initials SN 2012ap, it is a sort of “intermediate supernova” produced by the gravitational collapse of its nucleus. This catastrophic event occurs when the nuclear fusion reactions in the core of a very massive star are no longer able to provide the power necessary to counterbalance the pressure of the outermost layers of the star. Therefore, the core collapses or becoming a super dense neutron star or a black hole. The rest of the material is blown into the interstellar medium as a result of the explosion the star that gives rise to the supernova.
In the most common case of stellar explosion, the material is ejected in space according to an envelope, a kind of “bubble” with a nearly spherical symmetry, which expands rapidly albeit with a much lower speed than that of light. This event does not give rise to the issuance of gamma rays. There is, however, a small percentage of cases in which the material that collapses ends to form an accretion disk, of short duration, which orbits the residual stellar explosion (neutron star or black hole). This structure generates two jets of matter that propagate outward in a direction perpendicular to the disk and with speeds near that of light. In this case, the simultaneous presence of an accretion disk and two jets, called “central engine”, causes the production of gamma rays.
Therefore, this study shows that not all objects where there is the “central engine” produce gamma rays. “This supernova appeared initially two relativistic jets that were then slowed rapidly, like those we see in the gamma-ray bursts,” says Alicia Soderberg of CfA and co-author of the study. In fact, another supernova, observed in 2009 (SN 2009bb), it still had its relativistic jets that subsequently propagated freely in space, without being slowed down as in the case of objects that emit gamma rays.
The free expansion observed in supernova SN 2009bb is similar to what is observed in stellar explosions which do not show the “central engine” and this indicates, very likely, that its jets had to contain a high percentage of heavy particles, and unlike the jets characterizing bursts where instead particles are lighter. In fact, the heavy particles make their way more easily through the material surrounding the star.
“What we see is that there is a wide variety of mechanisms in this type of stellar explosions. Those which are characterized by unique ‘central motors’ and light particles emit gamma rays and, on the contrary, those where there are ‘central motors’ weak and heavy particles do not generate gamma rays, “concludes Chakraborti.
In short, the study of this peculiar supernova SN 2012ap suggests that the nature of the “central engine” plays a vital role in determining the physical properties of stellar explosions.

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