21 Feb 2021 16:47:32 UTC

Topic 224886

(moderation:

New observations of the first black hole ever detected have led astronomers to question what they know about the Universe's most mysterious objects. Published today in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.

https://phys.org/news/2021-02-black-hole-massive-thought.html

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With a mass originally estimated at 15 times that of the sun, Cygnus X-1 is one of the most massive and most luminous of the X-ray binary systems known in the Milky Way. New measurements have now raised that figure to 21 solar masses. The makeover does not change the overall perception of the cosmos; Cygnus X-1 is still a black hole, an almost science-fictional manifestation of Einsteinian weirdness in celestial reality. But the details of how Cygnus X-1 became a black hole are now in doubt.

“A significant change in the mass of such a classic and historical astronomical source is a big deal (at least to astronomers),” Daniel Holz, a theoretical astrophysicist at the University of Chicago who was not part of the study, wrote in an email. Also by email, James Miller-Jones of the International Center for Radio Astronomy Research at Curtin University in Australia wrote: “We realized that a 21-solar-mass black hole was too massive to form in the Milky Way with the best existing estimates of the amount of mass lost by massive stars in stellar winds.”

https://www.nytimes.com/2021/02/18/science/cygnus-black-hole-astronomy.html

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## OK, they found it to be at a

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OK, they found it to be at a different distance than originally thought, this changes the estimate of mass of the central black hole in this system. Most interestingly it is

in rotation. That raises a pertinent question : what exactly is it that is rotating ? A black hole is externally defined by it's event horizon, the surface beyond which nothing can be detected. Time for a thought experiment ....extremalIn the first instance let's imagine a black hole that we presume not to be rotating, and to keep the example simple have nothing else around it. You could test for rotation by dropping something into the hole, call it a

. Watch the path of such a test mass on the way down ( have it radiate some signal to localise it ). If you did it carefully, one could make sure that the initial velocity of the test mass was directed towards the centre of the hole. With a black hole how do you know what direction that centre is, given that it doesn't radiate ? I suppose there is only one answer : aim toward the centre of the black disc that the hole appears to be. So if you do not see anytest mass( say, with respect to a distant starfield ) of the test mass on the way down then you'd say it isn't rotating. The test mass would always appear to be centred on the hole. Such a test mass would have no angular momentum all the way down to the event horizon ( it has no component of it's velocity tangential to the vector to the centre of the hole ).deviation in directionHence there's a clue for

holes. Send a test mass toward the apparent centre of the hole as above, but if it starts to deviate then that is evidence for the hole's rotation. It's path will skew towards the direction of rotation. It will appear to gain angular momentum and start to circulate as it descends to the event horizon ! But it didn't start with any angular momentum by the way we started the test mass on it's journey. It gained the angular momentum from the hole by what is called 'frame dragging'. I can't think of any meaningful analogy to this effect but it is real : it was measured for the Earth by the Gravity Probe B mission. It's a purely GR phenomenon.rotatingWhat is the

part of the situation? It means that the speed of rotation of our test mass near the event horizon is near the speed of light. It is awkward to visualise what one would see. The angular momentum of the test mass around the hole would increase greatly. The test mass wouldn't get to the speed of light, but it's inertial mass could increase without bound in a like manner as per particles in accelerators.extremalThe article doesn't say precisely how they found that the black hole was extremal. But there is stuff falling inwards from an accretion disk around the black hole, so perhaps it would radiate in some tell tale fashion.

{ This is in addition to other noticeable effects on a test mass approaching an event horizon, which I've left out of the description. }

Cheers, Mike.

( edit ) Here is an article describing the extremal measurement. Their result depends on measured spectra and a model : "our obtained spin parameter is based purely on the theoretical model."

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## Mike Hewson wrote:( edit )

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Mike Hewson wrote:BlackHoles@Home was brought to our attention by RBPEAKE some time ago.

https://einsteinathome.org/content/blackholeshome

They still are not up and running yet, but I checked with the project head last summer, and he is still working on it. I think one of the parameters they are looking for is spin.

http://astro.phys.wvu.edu/bhathome/

Maybe you can jog him more authoritatively.

## Interesting. The home page

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Interesting. The home page was last updated on 08 February 2020 and of course alot has happened in West Virginia since then. A press conference presentation indicates funding through to this year. Here is the author's home page at West Virginia University and he has a very impressive CV. To sign up for a newsletter is easy with a Google account and I see the last entry as :

"May 20, 2019, 5:16:29 AM

Black hole & gravitational wave facts & newsFacts: Gravitational waves from colliding black holes (technically "binary black hole mergers") are the most common type observed by our most sensitive gravitational wave detectors. We have detectedmore than a dozengravitational waves from black hole collisions so far! One of the main goals of BlackHoles@Home is to produce the theoretical predictions necessary to extract important science from these gravitational waves.News: Our best gravitational wave detectors are currently online and observing with unprecedented sensitivity. Even more exciting: the rate at which we are detecting gravitational waves right now is near the upper end of our original projections!technical].technical].Current BlackHoles@Home progressInstructions: Go to the BlackHoles@Home homepage http://blackholesathome.net , click Download, and follow theQuick Startguide.Technical details: We confirmed the new diagnostic works well by analyzing the gravitational waves from a black hole collision. In the process of collision, the two black holes become one black hole that oscillates and emits gravitational waves. The amplitude of oscillation drops exponentially and the amplitude and frequency of the resulting gravitational waves are well understood using separate, "black hole perturbation theory" calculations. When we compare the gravitational waves from our new diagnostic with the separate "perturbation theory" calculations, we obtain agreement over more than 8 orders of magnitude. Most other software that attempts this obtains about 3-4 orders of magnitude agreement.]Technical details: The horizon diagnostic will enable us to map out the black holes' horizons and compute important information about the black holes during each calculation. The software repository implementing the basic infrastructure is here: https://bitbucket.org/zach_etienne/et_minimal_2019_03/ . Our plan is to include this diagnostic in the BlackHoles@Home software, both for scientific and aesthetic reasons (imagine a neat and accurate visualization of the black hole horizons during the calculation, with a display of important information about the black holes!)]How you can helpby necessityan inclusive project, andeveryone can help! Here's how:Tell your friendsabout the project, encourage them to sign up for the newsletter (https://groups.google.com/forum/#!forum/blackholesathome). When the project starts, we'll need everyone's help!Documentationneeded: Some nice, step-by-step picture tutorials for downloading NRPy+ (the software behind BlackHoles@Home; go to http://blackholesathome.net/ and clickDownload) and getting the Jupyter notebooks up and running on various operating systems would be greatly appreciated. These tutorials would enable interested folks tomake use of their own computersandperform early black hole simulationsto help get BlackHoles@Home ready to go! Setting up each tutorial in a Google Doc might be easiest. Contact me at zachetie **at** gmail.com if you are able to help.translation, we'd like to translate the basic content on the BlackHoles@Home homepage into other languages. Contact me at zachetie **at** gmail.com if you are interested.Prof. Zachariah Etienne

Hmmm. It seems his professional email address per his home page is zbetienne-[at]-mail.wvu.edu and I have just penned an inquiry ! Thanks for bringing this to my attention.

Spin is always going to be a parameter for BH systems study for at least these reasons :

- unless there has been an accident of history all BH systems will have it ie. incredibly unlikely that they were created without any angular momentum of the ingoing components.

- there's only three observables for BHs : mass, spin and electric charge. I doubt if any ( excess ) charge would ever be significant enough for us to detect an effect at distance. That's not to say there wouldn't necessarily be some electromagnetic signal from whatever surrounds the BHs during a merger.

- yet another opportunity to test Einstein's general relativity.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## Frame dragging in direction

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Frame dragging in direction of rotation causes a repulsiv garvitational effect (gravitomagnetism, if I recall right), wich means, that the event horizon is shrinking.(See Bernard Schutz - Gravity from the ground up)

If Cygnus-1 is rotating so extremly fast, how much smaller is the event horizon, compared to a non rotating Schwarzschild black hole?

## Rechenkuenstler099

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Rechenkuenstler099 wrote:That's one way to look at it. The photons down deep in the gravitational well of a rotating black hole get an effective repulsion component from gravitomagnetism ( they have a mass equivalent ) acting upon them. Thus they can probe the region closer to the hole without getting captured ( compared to the non rotating case ).

Short Answer : It approaches half the Schwarzschild radius.

Long Answer : If one

an event horizon as where a coordinate singularity occurs in thedefinesof a chosen metric, then Kerr black holes have two event horizons. There's a quadratic involved and thus two solutions where the relevant expression becomes infinite. For the outer one, mathematically it looks like :radial component=r+ SQRT[MM^{2 }- (/L)M^{2}] ( where G = c = 1 ){ The inner horizon is at

=r- SQRT[MM^{2 }- (/L)M^{2}] and I really don't know what that physically means. }is the hole's angular momentum andLis the mass of the hole. The second square-root term goes to zero whenM->LM^{2}, thus->r.M{ If

-> 0 thenL-> 2rwhich is the Schwarzchild radius. It is proportional toM. The units work out to a radius of ~3km per solar mass. }M{ The radii I speak of are measured in the equatorial plane of the hole ie. defined as the plane perpendicular to the angular momentum vector. }

is what happens whenBut the niggly bit>LM^{2}as that makes for a negative expression under the square root and no real solutions for. Does that mean there are no event horizons and the central singularity is 'naked' ? Opinions differ, where for instance Roger Penrose has coined his cosmic censorship principle that prevents a singularity from being seen. But I understand that hasn't been proven yet. ;-)rAnother interesting point about frame dragging means that if you start descending into the hole with an orbit in the opposite sense to the hole's rotation, once you get sufficiently close in you just have to 'go with the flow' as it were and rotate with the hole.

Cheers, Mike.

( edit ) I get my information from a ripper of a book called

by Tevian Dray. It bypasses all the tensor kerfuddling and gets right into the meaning of the various metrics. You just have to accept the solutions to the GR equations as discovered and get on with the job of interpretation. Since I'm not in the business of finding new solutions then one can avoid the gorrible tensor mess. Differential geometry has been called, tongue in cheek,Differential Forms and the Geometry of General Relativitythe study of objects that are invariant with respect to a change in notation. :-)( edit )

means the method by which displacements in spacetime are measured in the limit of small 'distances'. Think of it as aMetricto determine how to take a tiny step. If you want to go a long way then you add up ( mathematicallyrule) lots of tiny steps to determine a distance travelled along some path. Geodesics are the paths of freely falling bodies which have the property that they minimise total 'timelike distance' travelled : the 'shortest' lines are taken by light. So the idea of gravitational forces converts to the demonstration of geodesic paths in non-Euclidean or 'curved' geometries. The above analysis looks at where the metric expression 'blows up' and that's a challenge to interpret.integrateI have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## Mike Hewson

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Mike Hewson wrote:March 2021http://astro.phys.wvu.edu/bhathome/

OK, I am staying tuned.

## OK! I guess we'll see

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OK! I guess we'll see soon.

I have a couple of computing devices that are so far behind the tech curve that they're not suitable for E@H. I could use them for this project maybe.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal