- 4349
- NEUTRON STAR or BLACKHOLE? Astronomers have found a new and unknown
object in the Milky Way that is heavier than the heaviest neutron stars known
and yet simultaneously lighter than the lightest black holes known.
-------------------------- 4349 - NEUTRON STAR or BLACKHOLE?
- Using the “MeerKAT Radio Telescope”,
astronomers found an object in orbit around a rapidly spinning millisecond
pulsar located around 40,000 light years away in a dense group of stars known
as a globular cluster.
-
- Using the clock-like ticks from the
millisecond pulsar they showed that the massive object lies in the black hole
mass gap. It could be the first
discovery of the much-coveted radio pulsar, black hole binary; a stellar
pairing that could allow new tests of Einstein's general relativity and open
doors to the study of black holes.
-
- When a neutron star, the ultra-dense
remains of a dead star, acquires too much mass, usually by consuming or
colliding with another star, it will collapse. What they become after they
collapse is the cause of much speculation, but it is believed that they could
become black holes, objects so gravitationally attractive that even light
cannot escape them.
-
- Astronomers believe that the total mass
required for a neutron star to collapse is 2.2 times the mass of the sun.
Theory tells us that the lightest black holes created by these stars are much
larger, at about 5.0 times more massive than the sun, giving rise to what is
known as the "black hole mass gap."
-
- The ability of the extremely sensitive
“MeerKAT telescope” to reveal and study these objects is enabling a great step
forward and provides us with a glimpse of what will be possible with the
“Square Kilometer Array”.
-
- The discovery of the object was made while
observing a large cluster of stars known as “NGC 1851” located in the southern
constellation of “Columba”, using the MeerKAT telescope.
-
- The globular cluster NGC 1851 is a dense
collection of old stars that are much more tightly packed than the stars in the
rest of the galaxy. Here, it is so crowded that the stars can interact with
each other, disrupting orbits and in the most extreme cases colliding.
-
- It is the collision between two neutron
stars that is proposed to have created the massive object that now orbits the
“radio pulsar”. Astronomers could detect
faint pulses from one of the stars, identifying it as a radio pulsar which is a
type of neutron star that spins rapidly and shines beams of radio light into
the universe like a cosmic lighthouse.
-
- The pulsar spins more than 170 times a
second, with every rotation producing a rhythmic pulse, like the ticking of a
clock. The ticking of these pulses is incredibly regular and by observing how
the times of the ticks change, using a technique called pulsar timing, they
were able to make extremely precise measurements of its orbital motion.
-
- It like being able to drop an almost perfect
stopwatch into orbit around a star almost 40,000 light years away and then
being able to time those orbits with microsecond precision.
-
- The regular timing also allowed a very
precise measurement of the system's location, showing that the object in orbit
with the pulsar was no regular star but an extremely dense remnant of a
collapsed star. Observations also showed that the companion had a mass that was
simultaneously bigger than that of any known neutron star and yet smaller than
that of any known black hole, placing it squarely in the “black-hole mass gap”.
-
- Uncovering the true nature of the companion
will be a turning point in our understanding of neutron stars, black holes, and
whatever else might be lurking in the black hole mass gap.
-
- The supernova observed, “SN 2022jli”,
occurred when a massive star died in a fiery explosion, leaving behind a
compact object, a neutron star or a black hole. This dying star, however, had a
companion which was able to survive this violent event. The periodic
interactions between the compact object and its companion left periodic signals
in the data, which revealed that the supernova explosion had indeed resulted in
a compact object.
-
- The explosive deaths of supermassive stars,
supernovae, lead to the creation of
black holes or neutron stars, right? At least, that’s the evolutionary path
that astronomers suggest happens. And, these compact objects exist throughout
the Universe. But, no one’s ever seen the actual birth process of a neutron
star or black hole in action before.
-
- That changed when supernova “SN 2022jli”
occurred in the nearby galaxy NGC 157. This catastrophic stardeath event was
discovered in May 2022. Measurements and radiation detected showed something
unusual, not like a “normal” supernova.
Their light curve becomes a useful window on the creation of either a
neutron star or a black hole.
-
- This is the first time that repeated
periodic oscillations, over many cycles, have been detected in a supernova
light curve. Supernovae occur pretty
frequently in the Universe. Astronomers study them and chart how their
brightness changes over time.
-
- After the initial explosion, the light it
generates fades out over some time. Usually, it’s a pretty smooth change in the
light curve. But, SN 2022jli didn’t fit the “normal” curve. Instead of fading out smoothly, the
brightness of light from the explosion oscillated in a 12-day-long period.
-
- What story does SN 2022jli’s strange light
curve tell us about the creation of black holes or neutron stars? Starting with
the explosion itself. It was a fine example of what astronomers call “Type II
supernovae”. Basically, at the end of its life, a supermassive star collapses
and then explodes outward. The remaining core collapses further to create one
of two types of massive objects. A neutron star is one.
-
It’s what’s left over after
the rapidly collapsing core of the star crushes the remaining protons and
neutrons of matter into neutrons. It’s essentially a ball of neutrons. Most
neutron stars have about the mass of the Sun crushed inside themselves. But,
they are small compared to their progenitor stars. Most are
20 kilometers across.
-
- Stellar-mass black holes also come from the
deaths of supermassive stars that were at least 20 times the mass of the Sun or
more. The core collapses during the event, the same as with a neutron star.
But, the mass is so great that the event creates a black hole, crushing all the
leftover core material into a pinpoint of dense matter.
-
- Like many massive stars, the progenitor of
SN 2022jli appears to have had at least one companion star. It probably
survived the supernova explosion. The outburst threw out huge amounts of
material, and the companion star interacted with it. That caused its atmosphere
to “puff up”.
-
- The newly created compact object passes
through the orbit of the star and sucks hydrogen gas away from the star. That
material funnels into an accretion disk around the compact object. Those
periodic episodes of matter theft from the star release lots of energy, which
gets picked up as regular changes of brightness in the light curve measurements
as well as the gamma-ray signals.
-
- Of course, we can’t see light coming from
the compact object itself, whether it’s a neutron star or a black hole. But, we
do see radiation from the heated material drawn into the accretion disk around
the compact object. And, since astronomers were able to track the changes in
the light curve due to activity by the massive object, it amounted to watching
its formation.
-
- The next step is to figure out exactly what
astronomers saw being formed. Was it a neutron star with tremendously strong
magnetic fields and gravity, or a black hole with gravity so strong nothing
(not even light) could escape it?
-
- Determining that requires additional
observations and the capabilities of telescopes not yet online, such as the
“Extremely Large Telescope” due to begin operations in a few years.
-
-
February 10, 2023 NEUTRON
STAR or BLACKHOLE? 4349
------------------------------------------------------------------------------------------
-------- Comments appreciated and Pass it on to
whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for
copies or Index of all reviews
--- to:
------
jamesdetrick@comcast.net
------ “Jim Detrick” -----------
--------------------- --- Saturday, February 10,
2024
---------------------------------
No comments:
Post a Comment