15 October 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 (1). The measurements at ESO will continue during the next years.
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.
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.
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.
Note:
(1) 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.
Commonwealth Scientific and Industrial Research Organization (CSIRO)
Australia
8 May 1998
And they have been pulling out all the stops in the last few days as they scramble to get data on this remarkable event, which could be the key to explaining the enigmatic 'gamma-ray bursters' -- the most powerful explosions in the Universe since the original Big Bang.
Telescopes of Mt Stromlo and Siding Spring Observatories, the Anglo-Australian Observatory and CSIRO's Australia Telescope near Narrabri are trained on a spectacular fireball in the nearby galaxy ESO 184-82.
The fireball looks like a supernova -- the gigantic explosion that ends the lives of stars many times bigger than our Sun.
Supernova are well studied. But there is a once-in-a-million chance of an exploding star being so massive that the explosion crushes its innards into a black hole.
Such a possibility was predicted more than a decade ago but nobody knew what it would look like when it happened. This may be it.
The first sign of the current event was a blast of gamma rays seen by the Italian/Dutch BeppoSAX satellite on 25 April.
BeppoSAX detects and locates the enigmatic 'gamma-ray bursters' -- enormous blasts of gamma rays that appear at random and usually last for only a few seconds.
Because the blast appeared in the southern sky, Southern Hemisphere telescopes in Australia and Chile swung into action, looking for light and radio waves from the explosion.
Such alerts are frequent but almost always end in disappointment, with nothing being detected. "We've done two or three in the last year, without success," says Dr Mark Wieringa, who is doing the current observations at the Australia Telescope.
On 26 April, Mt Stromlo Observatory near Canberra became the first to pick up light from the fireball.
On 2 May the Anglo-Australian Telescope near Coonabarabran got a spectrum of the light. This was used to work out the distance to the fireball -- 100 million light-years, "which is virtually in our backyard," says Dr Chris Tinney of the Anglo-Australian Observatory.
Pouring out light and radio waves, the object is rapidly getting brighter.
"This is only the third gamma-ray burster that anyone has been able to see radio waves from," says Professor Ron Ekers, Director of CSIRO's Australia Telescope. "And those first two were extremely faint and distant, which made them very hard to study."
"This one is already ten times stronger -- it's doubled in strength since last week and is still increasing."
This makes it totally different from any other 'gamma-ray burster' known, where any light that was seen has quickly faded away after the initial burst.
Controversy over the nature of gamma-ray bursters raged for almost three decades, until last year. Then the BeppoSAX satellite, the Keck II telescope in Hawaii, the Hubble Space Telescope, and the Very Large Array radio telescope in the USA all played a part in showing that the bursters were explosions of mind-boggling power in the very distant Universe.
On 6 May [1998] NASA announced that gamma ray burster GRB 971214, found in December 1997, had occurred 12 billion light-years away and was the most powerful explosion since the Big Bang that created the Universe.
Colliding neutron stars or black holes has been the most popular idea for the cause of such events.
The explosion now being studied (GRB980425) differs from all previous ones in happening extremely close to us, rather than in the distant Universe. It is by far the closest such explosion known, which means it can be studied in much more detail. And it appears to be behaving differently from previous gamma-ray bursters.
"Very few of these gamma-ray bursters give us light and radio waves. Every one like that which we find tells us something new," says Dr Brian Boyle, Director of the Anglo-Australian Observatory.
It's remotely possible that the astronomers are seeing two separate events: a gamma-ray burst and a nearby supernova. "Nature can be very cruel, and it has done this to me before," says Dr Dale Frail of the US National Radio Observatory, who is head of the team observing with the Australia Telescope.
But the chance of such a coincidence is less than one in a million, according to the University of Sydney's Dr Elaine Sadler.
"There's no doubt we're seeing a supernova," says Dr Sadler. "It could be the death of a really giant star, up to 100 times the mass of the Sun."
"There would be a handful of stars that size out of the hundred million in our own Galaxy."
As in the case of the giant distant explosion announced by NASA on 6 May, the intense burst of gamma rays in this event almost certainly came from the sudden collapse of matter into a black hole. But in this case the black hole has formed in the centre of a star.
An electronic picture is available at
http://www.csiro.au/news/blackhole.html
PHOTO CAPTION:
Credit: Mt Stromlo and Siding Spring Observatories of the Australian National University / Dr Mike Begam
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