As if they weren't considered beastly enough, black holes can dive into nearby stars and devour them from the inside out, scientists now suggest. Such invasions by such black holes could help explain the most powerful explosions in the universe, gamma-ray bursts, whose origins remain elusive.
The idea needs support from further theoretical work, and observations would help, too. Meanwhile, here's what spawned the notion:
Gamma-ray bursts are narrow beams of intense radiation that can unleash as much energy as our sun will during its entire 10-billion-year lifetime — all in anywhere from milliseconds to a minute or more. The processes that can generate that much energy in that short a time are among the biggest mysteries in astronomy today.
The majority of gamma-ray bursts last two seconds or more. These cosmic flashes, dubbed long gamma-ray bursts, are linked to jets of plasma from massive dying stars. Scientists currently suggest this plasma is heated up by the energy released from neutrinos as they meet and annihilate their antimatter counterparts. Both kinds of particles are emitted by the dense, hot disk of matter that accretes or builds up around a black hole as it rips apart a dying star.
Now researchers have come up with a different, radical explanation — the plasma jets come directly from black holes when they invade stars.
Powerful forces
Their concept is based on recent observations by the Swift satellite that indicates the central engine driving these plasma jets can operate for up to 10,000 seconds, much longer than the neutrino model can explain.
Scientists at the University of Leeds in England instead suggest the matter that falls into black holes can generate extremely powerful magnetic forces that focus and drive the plasma jets linked with long gamma-ray bursts. The matter has to whirl very rapidly, with the centrifugal forces caused by this spin opposing the powerful gravitational pull of the black hole, for the prolonged blast seen in long gamma-ray bursts.
The researchers found one way such whirling matter could result is if a black hole plunged into a star and began eating it from the inside. As the black hole ripped the star apart, its remains could twirl apart in precisely the right way needed for a long gamma-ray burst.
"This 'invader variant' provides a natural explanation of the very fast rotation," researcher Serguei Komissarov, a mathematician and astrophysicist at the University of Leeds in England, told.
Other ideas
Another way such rapid spinning might have occurred is if the dying star was initially born rotating very quickly and retained this rate of spin during its entire life. Also, the dying star in question might have orbited very close to another star, and the resulting tidal forces — the tug of one object that distorts the shape of another, just as the sun and moon cause tides in Earth's ocean and even in its rocky crust — could have spun it up, Komissarov explained.
"The magnetic model has been proposed by other scientists, say 10 years ago or so, but was never popular," Komissarov said. "During the last few years we have been studying the true potential of this model and now we argue that some observational data, including the latest data from Swift, speak in favor of it."
Komissarov did caution that no direct observations linked with long gamma-ray bursts have revealed the extremely strong magnetic fields required by their model so far.
"Further research, both theoretical and observational, is needed to clarify this issue," he said.
Komissarov and his colleague Maxim Barkov detailed their findings in the Monthly Notices of the Royal Astronomical Society.
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