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The European Southern Observatory
Astronomers witness the birth of a black hole
University of Amsterdam, Oct. 15, 1998. An international team of astronomers have identified the gamma-ray burst of April 25, 1998 with a rare type of supernova. This supernova signalled the formation of a black hole.
During the 1970's American military satellites detected the existence of brief outbursts of gamma rays. Gamma rays are very energetc radiation (similar in nature but more energetic than light or radio waves), which, for instance, is emitted in nuclear explosions. The gamma-ray bursts (as they were called) did not come from bombs but from mysterious sources somewhere in the Universe. To detect such gamma-ray bursts and accurately locate them on the sky Dr John Heise of the Dutch Space Research Organisation SRON built the Wide Field Camera, which is now circling around the Earth on board of the Italian-Dutch satellite BeppoSAX. With this camera the position of a gamma-ray burst can be determined with an accuracy of a few arcminutes across a field of view of 40 by 40 degrees. [note: the diameter of the full moon is 30 arcminutes]. Thanks to these accurate positions a team lead by astrophysicist Dr Jan van Paradijs (University of Amsterdam and University of Alabama in Huntsville) was able , in February 1997, for the first time to identify a gamma-ray burst with a weak source of optical radiation. Since then optical and radio telescopes have detected such 'afterglows' for another seven gamma-ray bursts. It turned out that the sources of these gamma-ray bursts are located billions of light years away from us, in faint galaxies. In several cases it was possible to determine their distances from the redshift of the optical light, which is caused by the expansion of the Universe as a whole.
On April 25 1998 BeppoSAX registered another gamma-ray burst. University of Amsterdam graduate students Titus Galama and Paul Vreeswijk alerted optical observatories in Chile and Australia, and several hours later the first exposures were made of the gamma-ray burst location on the sky. From a comparison with images made during the next several nights Galama and Vreeswijk found that in that same direction a supernova had occurred, at about the same time as the gamma-ray burst. This was a big surprise, since a relation between gamma-ray bursts and supernovae had never been found before. The explosion had taken place not in the far recesses of the universe, but in a relatively nearby galaxy, at a distance of about 120 million light years, cosmologically speaking just around the corner. A gamma-ray burst from such a small distance blurred the picture that astrophysicists had just obtained on the basis of the afterglow observations of gamma-ray bursts which showed them to be located some hundred times farther away, and to be intrinsically more than ten thousand times as powerful than the event of April 25. The probability that the gamma-ray burst and the supernova would occur in the same direction within a day of each other just by chance is less than one in ten thousand according to a conservative estimate. Apparently nature can produce gamma-ray bursts in two very different ways.
The second surprise is the supernova itself. In the first place the optical spectrum looks abnormal. It shows no signs for the presence of hydrogen and helium, by far the most abundant elements in the Universe. This implies that the supernova was caused by the collapse of a very massive star that consisted predominantly of carbon and oxygen. In the second place American astronomers had found that the supernova is also a strong source of radio emission.
Japanese astronomers of the University of Tokyo have made extensive computer simulations of the supernova, on the basis of the optical observations during the weeks and months following April 25. They come to the conclusion that in the collapse of the very massive star which caused the supernova, at least three times the mass of the Sun was left behind in the firm of a compact star. According to out present knowledge this is too massive for a neutron star, hence the conclusion that in this supernova a black hole was formed.
The research described in this note was done in collaboration with dr John Heise and dr Jean in 't Zand of the Space Research Laboratory (Utrecht, the Netherlands), astronomers of the Marshall Space Flight Center, Huntsville Alabama (Dr Chryssa Kouveliotou and collaborators) and the European Southern Observatory (Dr Thomas Augusteijn and collaborators), and the group of Dr Filippo Frontera of the Bologna Observatory. The research in Amsterdam is partially funded by the Dutch ASTRON foundation of the Netherlands Organisation for Research.
A Strange Supernova with a Gamma-Ray Burst:
Important Observations with La Silla Telescopes
European Southern Observatory, Oct. 15, 1998. Several articles appear today in the scientific journal Nature about the strange supernova SN 1998bw that exploded earlier this year in the spiral galaxy ESO184 G-82. These studies indicate that this event was linked to a Gamma-Ray Burst and may thus provide new insights into this elusive phenomenon.
Important observations of SN 1998bw have been made with several astronomical telescopes at the ESO La Silla Observatory by some of the co-authors of the Nature articles . The measurements at ESO will continue during the next years.
The early observations
On April 25, the BeppoSAX satellite detected a Gamma-Ray Burst from the direction of the constellation Telescopium, deep in the southern sky. Although there is now general consensus that they originate in very distant galaxies, the underlying physical causes of these events that release great amounts of energy within seconds are still puzzling astronomers.
Immediately after reports about the April 25 Burst had been received, astronomers at La Silla took some images of the sky region where the gamma-rays were observed as a "Target of Opportunity" (ToO) programme. The aim was to check if the visual light of one of the objects in the field had perhaps brightened when compared to exposures made earlier. This would then provide a strong indication of the location of the Gamma-Ray Burst.
The digital exposures were transferred to the Italian/Dutch group around BeppoSax that had requested these ToO observations. Astronomers of this group quickly noticed a new, comparatively bright star, right on the arm of a small spiral galaxy. This galaxy was first catalogued in the 1970's during the ESO/Uppsala Survey of the Southern Sky and received the designation ESO184-G82. It is located at a distance of about 140 million light-years.
The ESO astronomers at La Silla decided to continue observations of the new star-like object and set up a comprehensive programme with several telescopes at that observatory. During the subsequent weeks and months, they obtained images through various filtres to determine the brightness in different colours, as well as detailed spectra. These observations soon showed the object to be a supernova. This is a heavy star that explodes during a late and fatal evolutionary stage. The new supernova now received the official designation SN 1998bw.
From a careful study based on these observations, it has been concluded that SN 1998bw underwent an exceptionally powerful explosion, more violent than most other supernovae observed so far. It was also unusual in the sense that very strong radio emission was observed within a few days after the explosion - normally this only happens after several weeks. In fact, at radio wavelengths, SN 1998bw was the brightest supernova ever observed.
The origin of the Gamma-Ray Burst
SN 1998bw is obviously an unusual supernova. It is therefore of particular significance that a Gamma-Ray Burst was observed from the same sky region just before it was discovered in optical light. It is very unlikely that these two very rare events would happen in the same region of the sky without being somehow related. Most astronomers therefore tend to believe that the gamma-rays do indeed originate in the supernova explosion.
But can a single supernova be sufficiently energetic to produce a powerful Gamma-Ray Burst? New theoretical calculations, also published today in Nature, indicate that this may be so. Moreover, if the Gamma-Ray Burst observed on April 25 did originate in this supernova that is located in a relatively nearby galaxy, it was intrinsically much fainter than some of the other Gamma-Ray Bursts that are known to have taken place in extremely distant galaxies.
The main idea is that while the centres of most other supernovae collapse into neutron stars at the moment of explosion, a black hole was created in a very massive star consisting mostly of carbon and oxygen. If so, a very strong shockwave may be produced that is capable of generating the observed gamma rays.
A comparison of synthetic spectra from such a supernova model, based on a new spectrum-modelling technique developed by Leon Lucy at the ESA/ESO Space Telescope/European Coordinating Facility (ST/ECF), with the spectra of SN 1998bw observed at La Silla, show good agreement, thus lending credibility to the new models.
Much data has already been collected at ESO on the strange supernova SN 1998bw. More observations will be obtained by the astronomers at the ESO observatories in the future during a long-term monitoring programme of SN 1998bw. There is a good chance that this effort will ultimately provide fundamental information on the explosion mechanism and the nature of the progenitor star of this exceptional object.
This supernova's connection with a Gamma-Ray Burst will significantly enhance our understanding of the nature of these powerful and enigmatic events. In view of the range in emitted energy, it now seems likely that there may be more than one class of Gamma-Ray Burst.
According to some models for Gamma-Ray Bursts that include beaming (emission of the radiation in one prefered direction), it is possible that these events are only detected if they have a favourable angle with respect to the line of sight. In the case of SN 1998bw this is probably not the case, however, and it was only detected in gamma-rays, because it is so relatively nearby. The question of differences in intrinsic brightness and possible different classes of objects is far from settled yet.
 The ESO astronomers involved in this work are Thomas Augusteijn, Hermann Boehnhardt, James Brewer, Vanessa Doublier, Jean-Francois Gonzalez, Olivier Hainaut, Bruno Leibundgut, Christopher Lidman and Fernando Patat.
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