Free Astronomy Magazine September-October 2015

SPACE CHRONICLES The clear signature of a supernova, later named SN 2011kl, was found. This is the first time that a super- nova has been found to be associat- ed with an ultra-long GRB. The link between supernovae and (normal) long-duration GRBs was established initially in 1998, mainly by observa- tions at ESO observatories of the su- pernova SN 1998bw, and confirmed in 2003 with GRB 030329. The lead author of a new study, Jo- chen Greiner from the Max-Planck- Institut für extraterrestrische Physik, Garching, Germany explains: “Since a long-duration gamma-ray burst is produced only once every 10,000– 100,000 supernovae, the star that exploded must be somehow special. Astronomers had assumed that these 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 chang- ing this paradigm for ultra-long du- ration GRBs.” In the favoured scenario of a mas- sive star collapse (sometimes known as a collapsar) the week-long burst of optical/infrared emission from the supernova is expected to come from the decay of radioactive nick- el-56 formed in the explosion. But in the case of GRB 111209A the combined GROND and VLT observa- tions showed unambiguously for the first time that this could not be the case. The amount of nickel-56 mea- sured in the supernova with the GROND instrument is much too large to be compatible with the strong ultraviolet emission as seen with the X-shooter instrument. Other suggested sources of energy to explain superluminous superno- vae were shock interactions with the surrounding material — possibly linked to stellar shells ejected be- fore the explosion — or a blue su- pergiant progenitor star. In the case of SN 2011kl the observations clear- ly exclude both of these options. The only explanation that fitted the observations of the supernova fol- lowing GRB 111209A was that it was being powered by a mag- netar — a tiny neutron star spin- ning hundreds of times per second and possessing a magnetic field much stronger than normal neu- tron stars, which are also known as radio pulsars. Magnetars are thought to be the most strongly magnetised objects in the known Universe. They are thought to develop mag- netic field strengths that are 100 to 1000 times greater than those seen in pulsars. This is the first time that such an unambiguous connection between a supernova and a mag- netar has been possible. Paolo Mazzali, co-author of the study, reflects on the significance of the new findings: “The new results Biggest explosions in the universe powered by strongest magnets by ESO G amma-ray bursts (GRBs) are one of the outcomes associ- ated with the biggest explo- sions to have taken place since the Big Bang. They are detected by or- biting telescopes that are sensitive to this type of high-energy radia- tion, which cannot penetrate the Earth’s atmosphere, and then ob- served at longer wavelengths by other telescopes both in space and on the ground. GRBs usually only last a few sec- onds, but in very rare cases the gam- ma rays continue for hours. (Normal long-duration GRBs last between 2 and 2000 seconds. There are now four GRBs known with durations between 10 000–25 000 seconds — these are called ultra-long GRBs. There is also a distinct class of short- er-duration GRBs that are believed to be created by a different mecha- nism.) One such ultra-long duration GRB was picked up by the Swift satellite on 9 December 2011 and 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 Tele- scope (VLT) at Paranal.