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Indeed. The unusual thing seems to be how two such objects ended up in such an orbit... But that doesn't seem to be something people would upvote. > Difficult

Indeed. The unusual thing seems to be how two such objects ended up in such an orbit... But that doesn't seem to be something people would upvote. > Difficulty of explaining the current wide orbit The current separation between the luminous star and dark companion is ≈ 2.2 au. This separation is uncom- fortable because it is both too tight for the progenitor of the NS to have fit inside as a red supergiant or AGB star, and too wide to be a result of common envelope evolution. Maybe upvoters think, based on the title, it's a near Earth supernova candidate or something? But (among other things) the compact object is too far away from it's compamion star and not close enough to the point where a neutron star would collapse. And its pretty far away.

mr_toad 3y ago
It’s already gone supernova. In theory in-falling matter could push it over the limit for it to collapse into a black hole. That would probably release a burst of gravitational radiation and not much else. However, there is a wide gap between the largest observed neutron stars and the smallest black holes that suggests that neutron stars collapsing into black holes is uncommon, or some process prevents it.
- raattgift 3y ago
  Gravitational radiation needs a (change of the (mass)) quadrupole moment, and you won't get that from a singleton neutron star. You'd need to break the rough spherical symmetry pretty extremely and over the course of at least a large fraction of the NS rotation to shed noteworthy gravitational waves. Bumps of realistic matter raised on NSes will tend to flatten out completely on the order of a couple light crossing times of the NS radius, which is super fast (~ 100 microseconds). Getting realistic matter that remains lumpy enough to count as a bump to the top of the crust of an NS is also really hard. More on that in three paragraphs. A binary system has a quadrupole moment (it's like two weights on either end of an imaginary bar rotating about an axis about half way along the bar; GWs are shed with the strength mainly concentrated in the plane of the rotation) but GWs from the (probably binary) system including the main sequence star discussed in the preprint at the top will be low amplitude and low frequency because the orbit is a wide and lasts a couple of years. The merger of a pair of neutron stars will have a strong quadrupole, and consequently will be strong sources of gravitational waves (and typically lots of gamma rays as a signature of various nuclear reactions like the r-process of heavy element generation). Accreting neutron stars can deflagrate with nuclear explosions. They also tend to shed lots of X-rays. Radiation pressure works against the tendency to collapse further; you need quite a dense flow of relatively cold matter onto the NS to turn it into a black hole. The extreme magnetic field of a typical (i.e., not magnetar) neutron star is likely to be relevant too. The lower mass limit on black holes was observational rather than theoretical (Ozel, 2012; Farr, 2011). However, it's no longer really observational. "Masses in the Stellar Graveyard" (Nov. 2021) <https://media.ligo.northwestern.edu/gallery/mass-plot https://media.ligo.northwestern.edu/gallery/mass-plot> (all events through the end of the third LIGO-Virgo-Kagra observing run (O3) with p(astro) > 0.5) and Ethan Siegal's write-up <https://bigthink.com/starts-with-a-bang/mass-gap-dead/ https://bigthink.com/starts-with-a-bang/mass-gap-dead/> has a reasonably accessible summary. That is, there are plenty of BHs close to the ~3 M_sun limit for NSes, although that does not guarantee that they formed by collapse of an isolated NS rather than another massive object scattering off or colliding with an NS progenitor of the BH. It's hard to contrive circumstances in which a slow sparse accretion onto an isolated NS keeps the NS high-frequency-quiet. But it could be hidden in a bunch in otherwise very UV-noisy and gas-rich environments like the central parsecs of galaxies, deep in globular clusters, etc. such that it could be missed by shallow surveys of those using e.g. the Chandra X-Ray observatory.
  - pfdietz 3y ago
    > Accreting neutron stars can deflagrate with nuclear explosions. This doesn't eject matter from the star, though. The gravitational binding energy of matter on the surface of a neutron star is greater than the energy released in its fusion.
    - raattgift 3y ago
      I'd be grateful if you'd pretend the sentence following the one you quote started "These and other processes tend" instead of "They also tend". I regret my sloppiness made it possible for you to read the sentence you quoted as suggesting that a subsonic nuclear explosion would eject bulk matter right out of an NS, rather than generating a burst of X-rays and gammas. The paragraph in question is an oblique reference to the NS analogy to the classical Eddington limit without delving too far into the drivers of eruptive and persistent far-UV "warm" disc winds and other accretion structure outflows. mr_toad's comment didn't warrant dealing with that in detail. And anyway none of this is relevant to the system described in the preprint at the top, which is nothing at all like an NS-LMXB.
      - pfdietz 3y ago
        I'm glad I was able to help you by clarifying your statement.

mr_toad 3y ago

It’s already gone supernova. In theory in-falling matter could push it over the limit for it to collapse into a black hole. That would probably release a burst of gravitational radiation and not much else. However, there is a wide gap between the largest observed neutron stars and the smallest black holes that suggests that neutron stars collapsing into black holes is uncommon, or some process prevents it.

raattgift 3y ago
Gravitational radiation needs a (change of the (mass)) quadrupole moment, and you won't get that from a singleton neutron star. You'd need to break the rough spherical symmetry pretty extremely and over the course of at least a large fraction of the NS rotation to shed noteworthy gravitational waves. Bumps of realistic matter raised on NSes will tend to flatten out completely on the order of a couple light crossing times of the NS radius, which is super fast (~ 100 microseconds). Getting realistic matter that remains lumpy enough to count as a bump to the top of the crust of an NS is also really hard. More on that in three paragraphs. A binary system has a quadrupole moment (it's like two weights on either end of an imaginary bar rotating about an axis about half way along the bar; GWs are shed with the strength mainly concentrated in the plane of the rotation) but GWs from the (probably binary) system including the main sequence star discussed in the preprint at the top will be low amplitude and low frequency because the orbit is a wide and lasts a couple of years. The merger of a pair of neutron stars will have a strong quadrupole, and consequently will be strong sources of gravitational waves (and typically lots of gamma rays as a signature of various nuclear reactions like the r-process of heavy element generation). Accreting neutron stars can deflagrate with nuclear explosions. They also tend to shed lots of X-rays. Radiation pressure works against the tendency to collapse further; you need quite a dense flow of relatively cold matter onto the NS to turn it into a black hole. The extreme magnetic field of a typical (i.e., not magnetar) neutron star is likely to be relevant too. The lower mass limit on black holes was observational rather than theoretical (Ozel, 2012; Farr, 2011). However, it's no longer really observational. "Masses in the Stellar Graveyard" (Nov. 2021) <https://media.ligo.northwestern.edu/gallery/mass-plot https://media.ligo.northwestern.edu/gallery/mass-plot> (all events through the end of the third LIGO-Virgo-Kagra observing run (O3) with p(astro) > 0.5) and Ethan Siegal's write-up <https://bigthink.com/starts-with-a-bang/mass-gap-dead/ https://bigthink.com/starts-with-a-bang/mass-gap-dead/> has a reasonably accessible summary. That is, there are plenty of BHs close to the ~3 M_sun limit for NSes, although that does not guarantee that they formed by collapse of an isolated NS rather than another massive object scattering off or colliding with an NS progenitor of the BH. It's hard to contrive circumstances in which a slow sparse accretion onto an isolated NS keeps the NS high-frequency-quiet. But it could be hidden in a bunch in otherwise very UV-noisy and gas-rich environments like the central parsecs of galaxies, deep in globular clusters, etc. such that it could be missed by shallow surveys of those using e.g. the Chandra X-Ray observatory.
- pfdietz 3y ago
  > Accreting neutron stars can deflagrate with nuclear explosions. This doesn't eject matter from the star, though. The gravitational binding energy of matter on the surface of a neutron star is greater than the energy released in its fusion.
  - raattgift 3y ago
    I'd be grateful if you'd pretend the sentence following the one you quote started "These and other processes tend" instead of "They also tend". I regret my sloppiness made it possible for you to read the sentence you quoted as suggesting that a subsonic nuclear explosion would eject bulk matter right out of an NS, rather than generating a burst of X-rays and gammas. The paragraph in question is an oblique reference to the NS analogy to the classical Eddington limit without delving too far into the drivers of eruptive and persistent far-UV "warm" disc winds and other accretion structure outflows. mr_toad's comment didn't warrant dealing with that in detail. And anyway none of this is relevant to the system described in the preprint at the top, which is nothing at all like an NS-LMXB.
    - pfdietz 3y ago
      I'm glad I was able to help you by clarifying your statement.