- 4190
- BLACKHOLES -
proving that they spin?
Astronomers are hoping the Event Horizon Telescope saw pulsars near the
Milky Way's supermassive black hole.
Millisecond pulsars are amazing astronomical tools. They are
fast-rotating neutron stars that sweep beams of radio energy from their
magnetic poles, and when they are aligned just right we see them as rapidly
flashing radio beacons. They flash with such regularity that we can treat them
as cosmic clocks.
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--------------------- 4190 - BLACKHOLES - proving that they spin?
- Astronomers are hoping the Event Horizon
Telescope saw pulsars near the Milky Way's supermassive black hole. Millisecond pulsars are amazing astronomical
tools. They are fast-rotating neutron stars that sweep beams of radio energy
from their magnetic poles, and when they are aligned just right we see them as
rapidly flashing radio beacons. They flash with such regularity that we can
treat them as cosmic clocks.
-
- Any change in their motion can be measured
with extreme precision. Astronomers have used millisecond pulsars to measure
their orbital decay due to gravitational waves and to observe the background
gravitational rumblings of the universe. They have even been proposed as a
method of celestial navigation. They may soon also be able to test the most
fundamental nature of gravity.
-
- Since pulsars are the remnants of massive
stars, our galaxy is likely to be filled with them. Although we have only
observed about 2,000 pulsars thus far, it’s estimated that nearly a billion
pulsars could exist in the Milky Way. Right now they are just too faint for us
to see, either because they are shrouded in dust, or are on the other side of
the galaxy.
-
- But this means that there should be several
pulsars in the central region of the galaxy, and a few of them could orbit our
supermassive black hole, Sag A*. If we can observe millisecond pulsars closely
orbiting Sag A*, we could test Einstein’s theory of general relativity in ways
not currently possible.
-
The center of our galaxy is
shrouded in gas and dust, but thanks to radio astronomy we can peer through the
veil to see the region. We have long been able to see several stars orbiting
Sag A*. By observing their
motions over decades we have been able to confirm that general relativity holds
true even in the strong gravitational fields near a black hole.
-
- But our measurements aren’t precise enough
to distinguish between the predictions of general relativity and rival
gravitational theories. Millisecond
pulsars would allow astronomers to measure orbital dynamics near Sag A*
precisely, giving us a detailed view of how strong gravitational fields
interact with mass.
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- Astronomers used three detection methods
based on Fourier analysis, which is a mathematical technique that can detect
patterns within data. Since pulsars emit regular pulses, they would tend to
stand out against random noise. Unfortunately, the team didn’t find evidence
for any new, previously unknown pulsars.
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- Millisecond pulsars are almost certainly
orbiting the Milky Way’s supermassive black holes, just like the stars we can
currently see. It is only a matter of time before we find them.
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- When you think of a black hole, you might
think its defining feature is its event horizon. That point of no return not
even light can escape. While it’s true that all black holes have an event
horizon, a more critical feature is the disk of hot gas and dust circling it,
known as the accretion disk.
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- According to Newton, if you drop an object
from rest near a planet or star, the object will fall straight down, tracing a
linear path until it strikes the planet or star. Einstein says something
slightly different. That straight path is only possible if the planet or star
isn’t rotating. If it is rotating, then space near the planet or star is
twisted. It’s an effect known as “frame dragging”.
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- Frame Dragging means our object will be
pulled around an object as it falls. We have measured frame dragging on
satellites near Earth, so we know it is a real effect. Near fast-rotating black
holes the frame-dragging effect can be immense. This means as gas and dust
start to fall toward the black hole it’s swept out into a disk around the
equatorial plane of the black hole.
-
- All the gas and dust are superheated, which
builds up tremendous pressure. An accretion disk can generate strong magnetic
fields, emit powerful X-rays, and even power jets of gas that stream away from
the black hole at nearly the speed of light.
-
- Most of the black holes we’ve identified
in the Universe have been through the high-energy effects of their accretion
disks. But the physics of black hole accretion disks are complex, and we don’t
yet fully understand their dynamics or even have a precise gauge of their size.
-
- One of the things we’ve noticed with
quasars is that they can fluctuate in brightness. Quasars are supermassive
black holes with a radio-bright accretion disk. Given the finite speed of
light, the rate of fluctuations gives us an upper bound on the size of the
accretion disk.
-
- If a quasar fluctuates on the scale of a
year, we know the accretion disk can’t be larger than about a light-year
across. The most accurately measured fluctuating quasar is “3C 273”, and we
know its accretion disk is about 1.5 light-years across, or about 100,000
Astronomical Units.
-
- But this is only an upper bound, and the
accretion disk could be smaller. Rather
than using brightness fluctuations, astronomers have measured the emission
lines of a supermassive black hole at the center of a galaxy known as “III Zw
002”. Using the Gemini North telescope, they were able to study a particularly
bright emission line of hydrogen and one of oxygen.
-
- Both of these spectra presented a double
peak feature. This double peak is caused by the rotation of the accretion disk.
As the disk rotates, light from the portion of the disk rotating toward us is
shifted toward the blue spectrum, while light on the portion of the disk
rotating away from us is redshifted. The effect is most significant on the
outer edges of the disk, causing the appearance of a double peak.
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- From this spectral data, the team determined
that the black hole is about 400 – 900 million solar masses, and its axis of
rotation is tilted about 18 degrees relative to our line of sight. The peaks of
the hydrogen line are about 16.8 light-days from the black hole, and the peaks
of the oxygen line are about 18.9 light-days from the black hole. That means
the accretion disk is around 40 light-days across.
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- The supermassive black hole M87*, which rose
to fame in 2019 when it became the first void to be imaged and revealed a fuzzy
orange donut, is now confirmed to be spinning.
-
- A network of radio telescopes have been
eyeing the black hole, which resides in the heart of the Messier 87 (M87)
galaxy about 55 million light-years away from Earth in the constellation Virgo.
These instruments have been especially intrigued by a powerful jet of radiation
and particles blasting from the black hole's poles, and according to new
results, that relativistic jet appears to be swinging like a pendulum on a
11-year cycle.
-
- Scientists say this is because of
gravitational interactions between the spinning black hole, which is thought to
be some 6.5 billion times more massive than the sun, and the disk of material
around it, providing "unequivocal evidence" for the black hole's
spin.
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- The jet changes its directions by roughly
10 degrees once every 11 years. The results are also consistent with
theoretical supercomputer simulations and will help shed light on how black
holes form and evolve into the monstrous beasts we see them as all across the
universe.
-
- In 2019, astronomers had spotted wobbling
jets escaping from a black hole much closer to us, about 8,000 light-years from
Earth. Those jets swung over time periods of just a few minutes, which, to
date, marks the most rapid oscillations of this kind observed by astronomers.
-
- Comparatively, the latest findings show M87's
black hole jets follow a much longer time frame. However, they are still
consistent with theoretical predictions made by Einstein in his landmark theory
of general relativity.
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- According to this theory, the spinning
black hole is so massive that it pulls the surrounding fabric of space and time
inward in what's called “frame-dragging”.
Because the spin axis of a black hole is not perfectly aligned with the
rotation axis of the surrounding accretion disk from which the black hole sucks
stellar material. This triggers the black hole's jets to wobble ever so
slightly, which was what was measured.
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- The specific processes that cause black
holes to spin are not very well understood. A leading theory suggests smaller
black holes form by feeding on star matter through an accretion disk, which
causes them to spin rapidly. Over eons, they are thought to collide and
eventually merge to form supermassive black holes.
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- These second generation black holes are
expected to spin slower compared to their younger counterparts. To really
confirm the hypothesis, researchers need to study spin rates of black holes
sporting different sizes, and the latest study could be a step in that
direction.
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October 18, 2023
BLACKHOLES - proving that they spin? 4190
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--------------------- --- Wednesday, October 18,
2023
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