Vulpecula, pulsars and neutron stars

[Astronuc]

I was listening to a Star Date podcast regarding the constellation Vulpecula and learned about the discovery of pulsars and neutron stars.
https://stardate.org/radio/program/2023-03-19

Vulpecula, the fox, doesn’t have a lot of impressive stars. But it sure has a lot of impressive dead ones. That’s where astronomers discovered the first neutron star — which also happened to be the first pulsar. And a few years ago, it’s where they discovered the first fast radio burst in our home galaxy — an object that’s also a neutron star.

The first neutron star was discovered in Vulpecula in 1967.
https://en.wikipedia.org/wiki/PSR_B1919+21

In 2020, another neutron star there suddenly produced short bursts of radio waves that didn’t sync up with how fast it spins — the first sighting of such an event in the Milky Way.

A recent study says it was a result of an outburst of charged particles. The star generates an extremely powerful magnetic field.

https://stardate.org/astro-guide/pulsars
A spinning neutron star with a magnetic field on the order of one trillion Gauss. This magnetic field accelerates electrically charged particles along the magnetic poles, forming a beam of energy that shoots into space from the poles. If the beam shines toward Earth, astronomers see a flickering beacon. The most famous pulsar, the Crab Nebula, pulses 30 times a second.

https://en.wikipedia.org/wiki/PSR_J1946+2052

https://www.nasa.gov/mission_pages/chandra/images/chandra-studies-extraordinary-magnetar.html
On March 12, 2020, astronomers detected a new magnetar with NASA’s Neil Gehrels Swift Telescope. This is only the 31st known magnetar, out of the approximately 3,000 known neutron stars.
https://en.wikipedia.org/wiki/Swift_J1818.0–1607
https://www.esa.int/ESA_Multimedia/Images/2020/06/XMM-Newton_observes_baby_magnetar

Magnetars - https://en.wikipedia.org/wiki/Magnetar

Alpha Vulpeculae (α Vulpeculae, abbreviated Alpha Vul, α Vul), officially named Anser, is the brightest star in the constellation of Vulpecula. It is approximately 291 light-years from Earth. It forms a wide optical binary with 8 Vulpeculae.

https://en.wikipedia.org/wiki/Alpha_Vulpeculae
It is a M1 III red giant, with an estimated radius of 43.14 R☉.
https://in-the-sky.org/data/object.php?id=TYC2129-2772-1

https://en.wikipedia.org/wiki/23_Vulpeculae (triple star system)
https://en.wikipedia.org/wiki/3_Vulpeculae (binary star system)
https://en.wikipedia.org/wiki/31_Vulpeculae (binary star system)

It seems there is a lot of interesting objects in that direction.

Vulpecula is a faint constellation in the northern sky. Its name is Latin for "little fox", although it is commonly known simply as the fox. It was identified in the seventeenth century, and is located in the middle of the Summer Triangle (an asterism consisting of the bright stars Deneb, Vega, and Altair).
https://en.wikipedia.org/wiki/Vulpecula

Vulpecula is bordered by the constellations of Cygnus, Lyra, Hercules, Sagitta, Delphinus and Pegasus
https://en.wikipedia.org/wiki/RR_Lyrae
https://en.wikipedia.org/wiki/RR_Lyrae_variable

https://en.wikipedia.org/wiki/Y_Sagittarii - Y Sagittarii is a variable star in the constellation of Sagittarius. It is a Cepheid variable with an apparent magnitude that ranges around +5.77. The measure of its parallax by Hubble Space Telescope puts Y Sagittarii to 1,293 light-years away from the Solar System.

https://en.wikipedia.org/wiki/Sagittarius_(constellation)
https://en.wikipedia.org/wiki/Epsilon_Sagittarii - binary star system
https://en.wikipedia.org/wiki/Sigma_Sagittarii - B2.5V
https://en.wikipedia.org/wiki/Zeta_Sagittarii - triple star system with the formal name Ascella

 

[sbrothy]

Magnetars and their insane magnetic fields probably makes Tesla spin [sic] in his grave. Having read about the guy I suspect his first thought would probably be how to make a deathray out of it. They're just one of many insane things "out there".

My favorites are the SMBH at the core of galaxies:

"The central supermassive black hole in the active nucleus [NGC 1365], which has a mass of about 2 million solar masses, rotates at close to the speed of light."

wiki on NGC 1365

That something so big and heavy rotates so fast boggles the mind. I mean the darn thing drags spacetime around with it. Whatever that actually means! Fits nicely on the list of things you probably shouldn't poke with a stick. :)
These are probably not all new (or even correct). I'm just a happy amateur. Fascinating reads though:

"The Spin of Supermassive Black Holes"
-- https://arxiv.org/abs/1307.3246

"Constraints on models of fallback discs around millisecond magnetars"
-- https://arxiv.org/abs/2303.04055

"Determination of supermassive black hole spins in local active galactic nuclei"
-- https://arxiv.org/abs/2205.10623

Have fun!

 

[Hornbein]

"The central supermassive black hole in the active nucleus [NGC 1365], which has a mass of about 2 million solar masses, rotates at close to the speed of light."

I'm not sure but I believe Wikipedia is wrong about that. Other sources say "close to what general relativity allows" and implies no number can be stipulated.

 

[Ibix]

"The central supermassive black hole in the active nucleus [NGC 1365], which has a mass of about 2 million solar masses, rotates at close to the speed of light."

I'm not sure but I believe Wikipedia is wrong about that. Other sources say "close to what general relativity allows" and implies no number can be stipulated.

You are correct to be skeptical. There is a maximum angular momentum a black hole of a given mass can have, and attempts to increase it beyond that will inevitably increase the mass of the hole at or above the rate at which it increases the angular momentum. But you can't assign a single speed to a rotating object anyway, and even less so a black hole that doesn't have a material surface that you could mark in some way in order to track its spin.

 

[snorkack]

You are correct to be skeptical. There is a maximum angular momentum a black hole of a given mass can have, and attempts to increase it beyond that will inevitably increase the mass of the hole at or above the rate at which it increases the angular momentum. But you can't assign a single speed to a rotating object anyway, and even less so a black hole that doesn't have a material surface that you could mark in some way in order to track its spin.

Actually, unlike a general case of a rotating object, it should be easier to assign a single linear speed to a rotating black hole.
While the mass of an extended object is usually at various distances from its axis, a rotating black hole has only one dimension. Its whole mass is in the ring singularity, and all parts of the ring singularity are at equal distance from the axis. Thus you could just divide the angular moment of the black hole with the mass of black hole and radius of the ring singularity to get the linear speed of the ring singularity.

Of course, the significance of that result´will depend on the physical interpretation of the radius of ring singularity. Still, what do you get?

 

[Ibix]

The circumference of the ring singularity isn't a well-defined notion. Also, I don't think you can naively assume Euclidean geometry, which you are doing when you just divide by the radius.

I also wouldn't define "black hole" as meaning just the singularity - the event horizon and its interior is the usual definition.

 

[snorkack]

While the mass of an extended object is usually at various distances from its axis, a rotating black hole has only one dimension. Its whole mass is in the ring singularity, and all parts of the ring singularity are at equal distance from the axis. Thus you could just divide the angular moment of the black hole with the mass of black hole and radius of the ring singularity to get the linear speed of the ring singularity.

Of course, the significance of that result´will depend on the physical interpretation of the radius of ring singularity. Still, what do you get?

Checked the derivation.
Kerr black hole parametres include parametre
a=J/Mc
which is commonly interpreted as the radius of the ring singularity.
Under that interpretation, the ring singularity would always be rotating at speed of light, right? Whereas the outer event horizon may be further away.

 

[sbrothy]

Actually, unlike a general case of a rotating object, it should be easier to assign a single linear speed to a rotating black hole.
While the mass of an extended object is usually at various distances from its axis, a rotating black hole has only one dimension. Its whole mass is in the ring singularity, and all parts of the ring singularity are at equal distance from the axis. Thus you could just divide the angular moment of the black hole with the mass of black hole and radius of the ring singularity to get the linear speed of the ring singularity.

Of course, the significance of that result´will depend on the physical interpretation of the radius of ring singularity. Still, what do you get?

Y
The precsecim2on

You are correct to be skeptical. There is a maximum angular momentum a black hole of a given mass can have, and attempts to increase it beyond that will inevitably increase the mass of the hole at or above the rate at which it increases the angular momentum. But you can't assign a single speed to a rotating object anyway, and even less so a black hole that doesn't have a material surface that you could mark in some way in order to track its spin.

The correctness doesnt really matter in the sense that Im still awestruck by these giant objects spinning ridiculously fast. Often several times a second. It still boggles my mind! :)

 

[Hornbein]

I believe the limit is the amount of spin that would expose the singularity, whatever that means.

 

[snorkack]

I believe the limit is the amount of spin that would expose the singularity, whatever that means.

Right.
I have seen diverse symbols for the parameters of the Kerr black hole, and diverse expressions.
The outer radius of the ergosphere at equator is equal to the Schwarzschild radius
R_s=2GM/c²
The parametre expressing the flattening of the black hole... choosing χ, it is
χ=Jc/GM²
Now the radius of the ring singularity is χ*GM/c²=(GM/c²)*(Jc/GM²)=J/Mc
The radius of the event horizon (which is spherical) is (GM/c²)*(1+√1-χ²)
On the limit of small but nonzero χ, the ring singularity is small compared to event horizon - but deep inside the event horizon it still rotates at exactly the speed of light. On the limit of χ=1, the radius of event horizon becomes equal to the radius of ring singularity, meaning that the ring singularity rotates, at the speed of light, right at the equator of the horizon.

 

[Ken G]

It's certainly a nice picture to help understand how Kerr black holes work. What I don't understand is that in principle, stars could do the same thing-- they could contract until their angular momentum says they have to be a ring, rather than a sphere. They can't quite do that, a ring diffuses out into a disk, but the accretion efficiency in that scenario would be poor unless the angular momentum per gram was very low. But even with relatively high angular momentum per gram, stars find a way to shed angular momentum and create that central sphere we all know so well. So why don't supermassive black holes do the same thing, and end up spinning way less than GR allows, just like stars do?