By Heather Rock Woods
Looking for bright x-ray bursts in space, a research team headed by
SLAC, the E.O. Hulburt Center for Space Research at the Naval Research
Laboratory and the NASA Ames Research Center has nailed down a distinctive
quality of black holes.
Derek Tournear (l) and
Elliott Bloom (both EK), with Erica Raffaut, a Stanford graduate
student who worked on the project.
(Photo by Diana Rogers)
Black holes have fascinated astronomers since Einstein predicted them
in his General Relativity theory. Since then, scientists have accumulated
limited evidence for their existence. But until now, astronomers still
took it on theoretical faith that black holes have ‘event horizons’ (the
point of no return for matter falling into a black hole) but no solid
The research team greatly strengthened the evidence that black hole
candidates indeed do not have surfaces by looking for telltale signs of a
surface—huge thermonuclear x-ray bursts created on the surfaces of other
large, gravity-intense objects like neutron stars. A methodical search of
celestial objects revealed that black hole candidates do not display these
type I bursts, indicating the lack of a surface. The results appeared in
the October 1 issue of The Astrophysical Journal.
"We have strong evidence that black hole candidates do not burst the
way neutron stars do and that black hole candidates are indeed black
holes," said Derek Tournear (EK), a Stanford graduate student in physics
and the lead author of the paper.
One of the main ways astronomers distinguish neutron stars from black
holes has been the presence or absence of type I x-ray bursts observed
from the object. This classification method had never been systematically
tested. To check the method’s validity, the researchers classified their
objects using other approaches, including mass and differences between how
neutron stars and black hole candidates have been observed to release
x-ray radiation (non type I). Neutron stars emit radiation that has more
rapid variability than black hole candidates do. Only black hole
candidates with a measured mass were used in the study. Their minimum mass
is greater than the mass of neutron stars.
The team then collected data on bursts from two space-based x-ray
timing telescopes, the Unconventional Stellar Aspect (USA) Experiment—
built jointly by the Naval Research Laboratory and SLAC—and the NASA Rossi
X-ray Timing Explorer (RXTE). Both telescopes devoted a large fraction of
their lifetimes to observing black hole candidates and neutron stars.
If stellar-mass black holes (roughly 10 times the mass of our sun) had
surfaces, calculations published last year by Ramesh Narayan and Jeremy
Heyl of the Harvard University Center for Astrophysics show they would
also emit type I x-ray bursts at about the same frequency as neutron
stars. Tournear and his colleagues couldn't find any bursts coming from
black hole candidates, although they saw many coming from neutron stars.
In addition to backing up black hole theory, this confirmed that bursts
are a useful way to differentiate neutron stars from black hole
The researchers can't completely rule out bursts from black hole
candidates because they can't constantly observe the objects. However,
their results show with 95 percent confidence that they would have seen
bursts, even if black hole candidates burst at only 1/20th the rate of the
neutron stars they observed. "We set good limits on whether black hole
candidates have surfaces and we gave some teeth to the assumption that
they do not," said co-author Elliott Bloom (EK).
These new observations are an important step forward in characterizing
the real nature of black holes and confirming that black hole candidates
are indeed black holes.
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