Streams of
gas fall to their fates, plunging into black holes, locked away from the
universe forever. In their final instants, these gassy shreds send out one last
flare of light, some of the brightest emissions in the universe.
These
death dives are too far away to be seen directly, but astronomers have devised
a new technique for detecting their panicked cries for help. They're using the
method to test our knowledge of gravity in the most extreme environments in the
universe.
In a new
research, physicists looked at specific features of that light to figure out the
closest you can get to a black hole without having to work hard to prevent
disaster — a threshold called the innermost stable circular orbit or ISCO. The
researchers found their method could work with more sensitive X-ray telescopes
coming online.
The event
horizon of a black hole is the invisible line-in-the-sand across which you can
never return. Once anything passes through the event horizon, even light
itself, it can no longer return to the universe. The black hole's gravity is
just too strong within that region.
Outside a
black hole, however, everything is just dandy. A particular black hole will
have a certain mass (anywhere from a few times the mass of the sun for the
smaller ones in the galaxy up to billions of times heavier for the true
monsters roaming the cosmos), and orbiting the black hole is just like orbiting
anything else of identical mass. Gravity is just gravity, and orbits are
orbits.
Indeed,
lots of stuff in the universe finds itself orbiting around black holes. Once these
foolhardy adventurers get caught in the black hole's gravitational embrace,
they begin the journey toward the end. As material falls toward the black hole,
it tends to get squeezed into a razor-thin band known as an accretion disk.
That disk spins and spins, with heat, friction, and magnetic and electric
forces energizing it, causing the material to glow brightly.
In the
case of the most massive black holes, the accretion disks around them glow so
intensely that they get a new name: active galactic nuclei (AGN), capable of
outshining millions of individual galaxies.
In the
accretion disk, individual bits of material rub up against other bits, draining
them of rotational energy and driving them ever-inward to the gaping maw of the
black hole's event horizon. But still, if it weren't for those frictional
forces, the material would be able to orbit around the black hole in
perpetuity, the same way that the planets can orbit around the sun for billions
of years.
As you get
closer to the black hole's center, though, you reach a certain point where all
hopes of stability are dashed against the rocks of gravity. Just outside the
black hole, but before reaching the event horizon, the gravitational forces are
so extreme that stable orbits become impossible. Once you reach this region,
you cannot remain in placid orbit. You have only two choices: if you have
rockets or some other source of energy, you can propel yourself away to safety.
But if you're a hapless chunk of gas, you're doomed to fall freely toward the
waiting dark nightmare below.
This
boundary, the innermost stable circular orbit (or ISCO for the lovers of
astronomical jargon), is a firm prediction of Einstein's general theory of
relativity, the same theory that predicts the existence of black holes in the
first place.
Despite
the success of general relativity in predicting and explaining phenomena across
the universe, and our sure knowledge that black holes are real, we've never
been able to verify the existence of the ISCO and whether it conforms to the
predictions of general relativity.
But the
gas that falls to its doom may provide a way for us to verify that existence.
A team of
astronomers recently published an article in the journal Monthly Notices of the
Royal Astronomical Society, which also was uploaded to the preprint journal
arXiv, describing how to take advantage of that dying light to study the ISCO.
Their technique relies on an astronomical trick known as reverberation mapping,
which takes advantage of the fact that different regions around the black hole
light up in different ways.
When gas
flows from the accretion disk, past the ISCO — the innermost part of the
accretion disk — and into the black hole itself, it becomes so hot that it
emits a broad swath of high-energy X-ray radiation. That X-ray light shines in
all directions away from the black hole. We can see this emission all the way
from Earth, but the details of the accretion disk structure get lost in the
blaze of X-ray glory. (Understanding more about the accretion disk will help
astrophysicists get a handle on the ISCO, as well.)
That same
X-ray light also illuminates regions well outside the accretion disk, regions
dominated by clumps of cold gas. The cold gas becomes energized by the X-rays
and begins to emit its own light, in a process called fluorescence. We can
detect this emission too, separately from the X-ray blaze emanating from the
regions closest to the black hole.
It takes
time for light to travel outward from the ISCO and outer part of the accretion
disk to the cold gas; if we watch carefully, we can observe at first the
central regions (the ISCO and innermost parts of the accretion disk) flare,
shortly followed by the "reverberation" light-up of the layers
outside the ISCO and the immediately surrounding accretion disk.
The timing
and details of the reverberated light depend on the structure of the accretion
disk, which astronomers have previously used to estimate the mass of black
holes. In this most recent study, researchers used sophisticated computer
simulations to see how the movement of gas within the ISCO — how the gas dies
as it finally falls toward the black hole event horizon — affects the emission
of X-rays both nearby and in the outer gas.
They found
that while we currently don't have the sensitivity to measure the doomed gas,
the next generation of X-ray telescopes should be able to, allowing us to
confirm the existence of the ICSO and test whether it agrees with the
predictions of general relativity, in perhaps the most gravitationally extreme
regions of the entire universe.
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