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AAO Press Releases

8 May 1998

AUSSIES FIRST TO SEE LIKELY BLACK HOLE BIRTH

Australian astronomers may be witnessing what has never been seen before - a black hole being born as the core of a super-massive star collapses in on itself.

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 ESO184-82.

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 a gamma ray burster 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 di fferently 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 po ssible 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.

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© Anglo-Australian Observatory 2004, PO Box 296, Epping NSW 1710 Australia

Roger Bell rb@aaoepp.aao.gov.au
01 Jan 1998