Black hole possibly caught in creation
By Robert Roy Britt
(SPACE.com) -- Startled astronomers peered through an apparent crack in the expanding bubble from an exploded star to glimpse what may be the youngest black hole ever detected.
It is the closest researchers have come to witnessing the birth of a black hole, from the explosion two decades ago to the recent emergence of a dense object amid the chaotic scene. The object may be a neutron star instead of a black hole, however. Scientists hope to figure that out with continuing observations, they said Thursday.
Other teams have recorded many stellar explosions, called supernovas. And they've discovered many black hole candidates presumed to be the result of previous supernovas. But no one has confirmed the connection so strongly.
"This is the first time we've seen it happen," said Michael Bietenholz of York University in Toronto, Ontario. "We've never seen a supernova leave behind a black hole, and the only supernovae we've seen that left behind neutron stars are several centuries or more old, and we only know them from historical records."
Bietenholz and his colleagues described the series of events as a textbook example of how things were theorized to go.
"No matter whether the central source is a black hole or neutron star, it would be by far the youngest of either ever observed," he said.
In fact this explosion occurred a long time ago, too. The star, SN 1986J, is about 30 million light-years away, so observations of the scene over the past 20 years represent light that took 30 million years to arrive. With that in mind, here's what happened, in terms of the observations:
Around 1983, the core of the star ran out of fuel and was no longer able to support itself against its own gravity. It began to collapse.
"This collapse is extremely fast, and the core collapses into a neutron star in about one second," Bietenholz explained. "It stops, at least momentarily at this point."
The outer layers of the star were then thrown outward with a bounce, generating a classic supernova explosion that was first spotted in 1986. Exactly what happened next is not known.
"If the core winds up with less than about 1.4 times the mass of our sun, it will remain stable as a neutron star," Bietenholz said. "If the core mass is larger, it will continue to collapse into a black hole, with this further collapse occurring in a fraction of a second."
The star's original mass is not known, so there's a roughly equal chance that the remaining central object is a neutron star or a black hole. Either dense object would generate intense magnetic fields, creating charged particles that would have allowed the researchers to detect it.
The outer layers of the star initially raced into space at more than 44 million mph (20,000 kilometers per second). The expansion continues but has slowed since.
The layers of material remain "pretty dense and we didn't yet expect to see through them right into the center to see the neutron star or black hole nebula," Bietenholz said. "The fact that we can suggests that, as they expand, they are also fragmenting, so we are seeing in through a crack that has developed in the shell."
The discovery required several radio telescopes: the National Science Foundation's Very Long Baseline Array, Robert C. Byrd Green Bank Telescope and Very Large Array; and telescopes from the European Very Long Baseline Interferometry Network.
More observations are planned.
"We'll be watching it over the coming years," said Michael Rupen of the National Radio Astronomy Observatory in Socorro, New Mexico. "First, we hope to find out whether it's a black hole or a neutron star. Next, whichever it is, it's going to give us a whole new view of how these things start and develop over time."
The discovery is detailed in Thursday's online version of the journal Science.
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