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 SPACE.com.
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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