
A black hole in a low mass X-ray binary star - dnetesn
https://phys.org/news/2017-04-black-hole-mass-x-ray-binary.html
======
antognini
To provide a little bit of context, it is known that globular clusters should
produce a lot of black holes, but it has long been believed that they should
retain very few, if any of these black holes. One reason for this is that
black holes should receive a kick upon their formation. The gravitational
potential of a globular cluster is weak enough that this would generally be
enough to kick the black hole out of the cluster.

For those that remain in the cluster, the black holes would be much more
massive than the rest of the stars in the cluster. Globular clusters undergo a
phenomenon called mass segregation, where the most massive stars settle to the
center of the cluster and lighter stars occupy the outer regions. So over time
the black holes would sink to the center of the cluster where they would
interact and form binaries with each other. These binary-binary interactions
would, over time, kick single black holes out of the cluster. Since these
interactions happen on timescales that are much shorter than the lifetime of
the cluster, the received wisdom about twenty years ago was that globular
clusters should have only a couple of black holes left, if any at all.

In 2007, Thomas Maccarone (who is an author on this paper) discovered a black
hole in a globular cluster in another galaxy. It wasn't until 2012 that any
were discovered in a globular cluster around the Milky Way. Now that computers
have gotten fast enough that it has started to become possible to simulate the
dynamics of globular clusters, theorists have found that globular clusters
retain a much higher fraction of black holes than they originally thought. The
reason for this seems to be that the distribution in black hole masses leads
the black hole population to remain better mixed with ordinary stars than was
originally thought.

~~~
Florin_Andrei
> _Globular clusters undergo a phenomenon called mass segregation, where the
> most massive stars settle to the center of the cluster and lighter stars
> occupy the outer regions._

Intuitively, this seems simple, since "heavier objects" should sink further
down, no?

But thinking about this for a while, it doesn't seem so simple anymore. No
matter what's the mass of the star, if it's already on a given orbit around
the cluster, it will tend to stay on that orbit, unless there are close
interactions with other stars of comparable mass.

So what would be an intuitive explanation for this phenomenon?

~~~
antognini
This is one of those rare cases where one's immediate intuition isn't so far
off. Globular clusters are very dense stellar environments. For context, the
radius of a globular cluster is roughly the distance to the nearest star to
our Sun (a few light-years). In this space there are packed roughly a million
stars. So dynamical interactions are frequent.

A consequence of this is that stars exchange energy with each other
frequently, and when you have frequent energy exchanges, you end up with a
system that is in energy equipartition --- i.e., every star has about the same
amount of energy as every other. Since more massive stars have the same energy
as lighter stars, they must move slower, and consequently sink to the center
of the cluster. Ultimately energy equipartition is the same reason that there
is gravitational settling in a gas or a liquid on Earth.

~~~
effie
> _when you have frequent energy exchanges, you end up with a system that is
> in energy equipartition_

Could you support this claim with some explanation? This can be proven for the
Maxwell-Boltzmann distribution of velocities, which is valid for gas in
thermodynamic equilibrium, but probably not so valid for stars as these
interact via long range forces, not through collisions like molecules do.

EDIT Also, the stars that move too fast will escape the system so the
equipartition cannot be sustained for stars whose mass is lower than some
value corresponding to average quadratic speed being equal to escape speed.

~~~
antognini
I'm not sure if it has been proven mathematically. The statement that the
long-term evolution of the system will tend towards equipartition is
equivalent to the system being ergodic. Ergodicity means that statistically
any particular state is as likely as any other. These systems are observed
(both in reality and in simulation) to be consistent with being ergodic. But
I'm not sure it's been proven. I'm also not sure that it has been proven that
gasses are ergodic either, even though experiments are consistent with them
being ergodic.

~~~
effie
> _These systems are observed (both in reality and in simulation) to be
> consistent with being ergodic._

How could one possibly conclude that system is ergodic from observation? I
thought ergodicity is a property of infinite time averages. Globular clusters
do not exist indefinitely in the same macrostate, they lose stars as they
evolve.

~~~
antognini
This is not my specialty so I may be getting some details wrong here. I don't
mean so much that globular clusters are ergodic as few-body interactions are
ergodic. It has at least been proven that three-body interactions are chaotic,
which implies that all microstates are accessible to the system. This is a
necessary requirement for ergodicity, but I don't believe that it implies that
the interactions are necessarily ergodic. All I really know is that in
practice globular clusters are observed to obey equipartition of energy, as
are the average outcomes of large numbers of few-body interactions.

------
xenophonf
Look! The actual scientific data and analysis!

[https://academic.oup.com/mnras/article/3052451/The](https://academic.oup.com/mnras/article/3052451/The)

------
dave_sullivan
Yes, it's unlikely and pretty tin-foil-hat, but wouldn't it be nuts if it
turned out to be a manufactured black hole?
[http://accelerating.org/articles/transcensionhypothesis.html](http://accelerating.org/articles/transcensionhypothesis.html)

 _Intelligences could be expected to slowly accrete the mass of their parent
star, rather than lifting or collapsing that mass and then inefficiently
recreating fusion within the black hole (if the laws of physics even allow the
latter, which they may not). In other words, you might absorb your star in a
passive and gravitationally-driven process that would look a lot like a low
mass X-ray binary (LMXRB) system to external observers, but in which the black
hole companion begins as a planet mass black hole on the order of 1000 times
less massive than the star companion, which should be a main-sequence star,
likely with a spectral class G, like our Sun._

 _Approximately 100 LMXRBs have been discovered in our galaxy to date, and
about 13 of these have been found in the globular clusters, areas at the rim
of our galaxy that may not harbor intelligence. They have also been found in
many distant galaxies, again often in globular clusters. Few have involved G
class stars, and none have yet been discovered with the very high, 1,000:1
mass ratio the hypothesis appears to predict. This may simply be a problem of
detection. XRBs emit X-rays only when they are "eating" their companion sun, a
transient phenomenon. Chandra, our best X-ray observatory, may also not have
the sensitivity or persistence needed to detect very high mass ratio LMXRBs,
or those that absorb their star's matter very infrequently or in very small
doses. More research and theory is needed in this area._

~~~
Florin_Andrei
Macroscopic black holes seem like a good source of energy mid-term. But if you
consider the very long term (tens of billions of years and beyond), this seems
like a waste of matter. You're sending lots of precious baryonic stuff
literally down the cosmic drain.

Microscopic black holes seem more efficient. They are one of the very few ways
to achieve total mass conversion (complete conversion of mass into energy).

~~~
Tloewald
This was, iirc, the energy source used in Arthur C. Clarke's Earthlight et al.

------
pavel_lishin
> There are many neutron stars in clusters, but black holes that form in these
> dense stellar environments are expected either to sink down to the center of
> the cluster or else to be gravitationally ejected from the cluster after
> they are formed.

Why would this be the case? Unless you're extremely close to it, a black hole
is just a mass, like any other stellar object.

------
relieferator
These globular clusters have always seemed peculiar to me. One would surmise
that such a structure would result in a massive black hole without the
rotational effects of say a galaxy.

~~~
xioxox
Spiral galaxies tend to have regular rotation, but elliptical galaxies are
much more like a globular cluster, although globular clusters aren't thought
to contain dark matter. It's just a lot of stars going round in random orbits.
They would have to lose angular momentum in order to collapse into the centre.
They can get stripped, however, by tidal gravitational effects.

------
rusanu
> such as would be produced by a black hole of about one solar mass

How do such low mass black holes form?

------
userbinator
The title would be so much clearer with the addition of one more word: "star".

On the other hand, it did make me click to find out what "binary" was about...

~~~
dotancohen
At least the star offers binaries. I hate it when I get to a system and I have
to compile everything myself.

~~~
m_mueller
Programmer dad?

