6 Apr. 2011

 

The imprint of dark supernovae

 

We are used to thinking of supernovae as among the most luminous phenomena in the Universe, stellar explosions visible even from billions of light years away. But incredibly, it is estimated that about 20% of supernovae are completely dark, not managing to emit the light from the explosion. This is due to the speed of the stellar collapse and associated formation of the black hole, which consumes everything that made up the progenitor star, including the light of the supernova explosion.
Therefore, if the estimates are correct, 1 supernova in 5 is completely invisible, and if we were to observe it at the instant of its collapse we would simply see it disappear from the sky.
There may however be a way, not even so indirect, to detect such a "failed supernova". One of the products of supernova explosions are neutrinos, and even if to date we have only one detection of neutrinos from a supernova, SN 1987A in the Large Magellanic Cloud, models predict that all supernovae produce them, and more massive progenitors result in more neutrinos.
Neutrinos produced in the early stages of the collapse would be able to escape the gravity of the forming black hole, and therefore signal to the external Universe the stellar collapse.
This possibility has been taken into consideration by Lili Yang (Arizona State University) and Cecilia Lunardini (Brookhaven National Laboratory) in a recently published article. The two researchers calculate, that if today neutrino detectors are capable of detecting only 1-3 neutrino events from supernovae in the Galaxy and Magellanic Clouds per century, those of the next generation, about 1000 times larger according to plans, will be able to detect neutrinos from SN explosions as distant as 13 million light years, including not only M31 in Andromeda but also various galaxies with high rates of star formation, and therefore high rates of supernovae.
The increased volume of Universe that will become "observable" will therefore lead to the detection of a certain number of the stellar explosions, or rather, implosions, associated with dark supernovae, perhaps one every 10 years. The energy of their neutrinos (around 56 MeV, compared to 33 MeV for visible supernovae) and the very brief duration of the emission (about 1 second compared to 10 seconds for the other 80% of events) should allow them to be distinguished from the more common luminous supernovae.

 

by Michele Ferrara & Marcel Clemens

credit: ASU, BNL, chandra.harvard.edu