Magnetar behind bright supernova and an extreme burst of gamma radiation
Researchers have observed a super-bright supernova with a very unusual long lasting gamma-ray burst and the reason behind this ultra-long duration GRB according to them it was a magnetar that drove the two and produced the otherwise unusual association.
Astronomers say that it is very rare to observe gamma-ray bursts in connection with supernovae, which are the deaths of massive stars and they usually only last a few minutes. The latest observed bursts which lasted for 30 minutes are rare and were associated with a supernova which was extremely bright itself – more than three times as bright as the supernovae previously associated with gamma-ray bursts.
Gamma-ray bursts are powerful bursts of gamma radiation, which is ejected out into space in connection with massive stars that die in a violent supernova explosion. A gamma-ray burst is typically short and only lasts a few minutes. However, in recent years researchers have observed several long gamma-ray bursts lasting over a half an hour, but they had not yet been able to connect them with a supernova.
One such ultra-long duration GRB was picked up by the on December 9, 2011 and was subsequently named GRB 111209A. It was both one of the longest and brightest GRBs ever observed. As the afterglow from this burst faded it was studied using both the GROND instrument on the MPG/ESO 2.2-metre telescope at La Silla and also with the X-shooter instrument on the Very Large Telescope at Paranal.
The clear signature of a supernova, later named SN 2011kl, was found. This is the first time that a supernova has been found to be associated with an ultra-long GRB, astronomers say.
The lead author of the new paper, Jochen Greiner from the Max-Planck-Institut für extraterrestrische Physik , Garching, Germany explains that since such long-duration gamma-ray burst is produced only once every 10,000-100,000 supernovae, the star that exploded must be somehow special.
It has long been assumed that such GRBs came from very massive stars — about 50 times the mass of the Sun — and that they signalled the formation of a black hole. But now our new observations of the supernova SN 2011kl, found after the GRB 111209A, are changing this paradigm for ultra-long duration GRBs, Greiner added.
“We have now observed an ultra-long gamma-ray burst in excess of a half an hour and for the first time we have managed to connect it with a supernova,” explains Johan Fynbo, a professor at the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.
15 times as bright
Giorgos Leloudas, a postdoc at the Dark Cosmology Centre at Niels Bohr Institute and the Weizmann Institute, Israel said that when they observed the light with the X-shooter on the Very Large Telescope in Chile, their analysis of the spectra showed that a very bright supernova is associated with the explosion.
However, the spectra look different than usual as the radiating matter has a very high outflow velocity. They therefore concluded that there was a very powerful explosion and that the supernova is about 15 times as bright as the supernovae we usually observe with the death of massive stars.
The combination of extreme brightness and a low content of heavy elements could indicate a massive star that releases extra energy in the death process. When the massive star dies, the core collapses into a neutron star that revolves very quickly and forms an extremely intense magnetic field. Such objects are called magnetars – a class of neutron stars that were first discovered by Chryssa Kouveliotou, who is a close collaborator with the gamma-ray burst group at the Dark Cosmology Centre at Niels Bohr Institute, which she frequently visits.
“A magnetar has a magnetic field that is in the realm of a billiard times stronger than the Earth’s magnetic field (a 1 followed by 15 zeros). At the same time, these bizarre magnetars rotate several times per second and this is a gigantic reservoir of energy, which can facilitate a huge explosion, resulting in a particularly bright supernova and an extreme burst of gamma radiation, which is what we observe,” explains Johan Fynbo.
The light from the super-bright supernova has travelled 6.4 billion years before it arrived at the Earth, so the incident took place about 7.3 billion years after the Big Bang.