Nature has again thrown astronomers for a loop. Just when they thought they understood how gamma-ray bursts formed, they have uncovered what appears to be evidence for a new kind of cosmic explosion. These seem to arise when a newly born black hole swallows most of the matter from its doomed parent star.
Gamma-ray bursts (GRBs), the most powerful explosions in the Universe, signal the formation of a new black hole and come in two flavours, long and short ones. In recent years, international efforts have shown that long gamma-ray bursts are linked with the explosive deaths of massive stars (hypernovae; see e.g. ESO PR 16/03).
Last year, observations by different teams - including the GRACE and MISTICI collaborations that use ESO's telescopes - of the afterglows of two short gamma-ray bursts provided the first conclusive evidence that this class of objects most likely originates from the collision of compact objects: neutron stars or black holes (see ESO PR 26/05 and ESO PR 32/05).
The newly found gamma-ray bursts, however, do not fit the picture. They instead seem to share the properties of both the long and short classes.
“Some unknown process must be at play, about which we have presently no clue,” said Massimo Della Valle of the Osservatorio Astrofisico di Arcetri in Firenze, Italy, lead author of one of the reports published in this week’s issue of the journal Nature. “Either it is a new kind of merger which is able to produce long bursts, or a new kind of stellar explosion in which matter can’t escape the black hole.”
One of the mysterious events went bang on 14 June 2006, hence its name, GRB 060614. The gamma-ray burst lasted 102 seconds and belongs clearly to the category of long GRBs. As it happened in a relatively close-by galaxy, located only 1.6 billion light-years away in the constellation Indus, astronomers worldwide eagerly pointed their telescopes toward it to capture the supernova, watching and waiting as if for a jack-in-the-box to spring open.
The MISTICI collaboration used ESO’s Very Large Telescope to follow the burst for 50 days. “Despite our deep monitoring, no rebrightening due to a supernova was seen,” said Gianpiero Tagliaferri from the Observatory of Brera, Italy and member of the team. “If a supernova is present, if should at least be 100 times fainter than any other supernova usually associated with a long burst.”
The burst exploded in a dwarf galaxy that shows moderate signs of star formation. Thus young, massive stars are present and, at the end of its life one of them could have uttered this long, agonising cry before vanishing into a black hole. “Why did it do so in a dark way, with no sign of a supernova?” asked Guido Chincarini, from the University of Milano-Bicocca, Italy, also member of the team. “A possibility is that a massive black hole formed that did not allow any matter to escape. All the material that is usually ejected in a supernova explosion would then fall back and be swallowed.”
The same conclusion was previously reached by another team, who monitored both GRB 060614 and another burst, GRB 060505 (5 May 2006) for 5 and 12 weeks, respectively. For this, they used the ESO VLT and the 1.54-m Danish telescope at La Silla.
GRB 060505 was a faint burst with a duration of 4 seconds, and as such also belongs to the category of long bursts.
For GRB 060505, the astronomers could only see the burst in visible light for one night and then it faded away, while for GRB 060614, they could only follow it for four nights after the burst. Thus, if supernovae were associated with these long-bursts, as one would have expected, they must have been about a hundred times fainter than a normal supernova.
“Although both bursts are long, the remarkable conclusion from our monitoring is that there were no supernovae associated with them,” said Johan Fynbo from the DARK Cosmology Centre at the Niels Bohr Institute of the Copenhagen University in Denmark, who led the study. “It is a bit like not hearing the thunder from a nearby storm when one could see a very long lasting flash.”
For the May burst, the team has obtained deep images in very good observing conditions allowing the exact localisation of the burst in its host galaxy. The host galaxy turns out to be a small spiral galaxy, and the burst occurred in a compact star-forming region in one of the spiral arms of the galaxy. This is strong evidence that the star that made the GRB was massive.
“For the 5 May event, we have evidence that it was due to a massive star that died without making a supernova,” said Fynbo. “We now have to find out what is the fraction of massive stars that die without us noticing, that is, without producing either a gamma-ray burst or a supernova.”
“Whatever the solution to the problem is, it is clear that these new results challenge the commonly accepted scenario, in which long bursts are associated with a bright supernova,” said Daniele Malesani, from the International School for Advanced Studies in Trieste, and now also at the DARK Cosmology Centre. “Our hope is to be able to find more of these unconventional bursts. The chase is on!”
Henri Boffin | alfa
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
28.04.2017 | Event News
20.04.2017 | Event News
18.04.2017 | Event News
28.04.2017 | Medical Engineering
28.04.2017 | Earth Sciences
28.04.2017 | Life Sciences