- 4175 - PULSARS - used to discover blackholes? 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.
--------------------- 4175 - PULSARS - used to discover blackholes?
- Astronomers are
hoping the Event Horizon Telescope saw pulsars near the Milky Way's
supermassive black hole.
-
- 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. Although modified gravity
models such as A QUAdratic Lagrangian (AQUAL) and Tensor–vector–scalar gravity
(TeVeS) aren’t popular, they do agree with the stellar observations we have
near our supermassive black holes.
-
- 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. It could provide experimental
tests precise enough to distinguish between general relativity and other
models. So a large team of astronomers has started to look for nearby
millisecond pulsars in the data from the Event Horizon Telescope (EHT).
-
- Although the EHT collaboration
didn’t release the first image of Sag A* until 2022, it has been gathering data
on our supermassive black hole since 2017. The observations don’t just contain
the data for an image, they also contain observations of the surrounding area
and things such as polarization of the radio light. If there are millisecond
pulsars in the region, evidence for them could be buried in the EHT
observations. However, because of the surrounding dust and the sensitivity
limits of our observations, the signals would be very faint.
-
- For this study,
they 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.
-
- 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.
-
- 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. And a team of astronomers have
made the first direct measure of one.
-
- 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”, and it
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.
-
- We do have a basic
gauge of the size of accretion disks. 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 AU.
-
- But this is only an
upper bound, and the accretion disk could be smaller. Without a direct measure
of an accretion disk, we rely on computer simulations to estimate its size. But
this recent work has measured the accretion disk of a supermassive black hole
directly, which gives us a step up in understanding black holes.
-
- To achieve this,
the team used a different approach. Rather than using brightness fluctuations,
they 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.
-
- 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.
-
- This result is
just the first step. The team continues to observe III Zw 002 and hopes to be
able to study how the accretion disk precesses around the black hole over time,
which will tell us about the dynamics between the two.
-
-
October 1, 2023 PULSARS - used
to discover blackholes? 4175
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