Here's something I've always wondered. Could you get close enough to a neutron star to see it with the naked eye? Or would you be dead? If you could see it, what would it look like?
It is 100% possible. There are optical pulsars (i.e. pulsars that emit visible light) and there should exist optical non-pulsing neutron stars as well.
One of the most famous neutron stars, the crab pulsar actually emits visible light and the Hubble has pictures of it taken in the visible light spectrum.
If you look at the above picture closely, in the center of the nebula you can see some ripples. The star is just barely visible in the center of that. If you look at the JWST's much higher resolution (but admittedly infrared) image of the crab nebula, it's far easier to make out.
Now seeing them without a telescope or anything else, we just need to be a bit closer (but still safely far enough away) to be able to see them like we can the other stars in the sky. But if you want to be able to see it like you would the sun or the moon, that is probably a different story.
Yes. They usually are quiet, but if something drop on surface, emit Roentgen, etc.
But anywhere, gravitational forces will be very strong, and their magnetic field also very strong, so you will feel them.
Experience might be something like MRT. - With all precautions, probably nothing special, but if do sudden movements, will be strange feelings, also gut may not feel comfortable.
The system offers up a bunch of "firsts" in terms of observations.
Why the dark object is only a neutron-star candidate is interesting.
The Discussion section (section 8, starting on the bottom page 12 of the PDF version) and particularly the list of possibilities in
§8.2 (starting on page 14) offers ways to distinguish from other possible multiple object systems.
Given more than a dozen pages describing years of observational data, effectively concluding "yet more data is needed to understand what this weird object is" is somewhat surprising, especially since (as you say) many neutron stars (and white dwarf binaries) at comparable distances have been found. On the other hand, "Unlike BHs and WDs, neutron star (NS) companions have not yet been unambiguously identified from the Gaia data".
Penultimately, if it turns out to be a main sequence star orbiting a Gaia-dark neutron star, the former's orbit reopens questions about how such binaries might form in the first place.
Finally, "dark" is in the spectrophotometry sense: the Gaia BP/RP instrument is sensitive in blue (and a little higher frequency) and red (and a little lower frequency), and the radial velocity spectrometry looks for the calcium Fraunhofer line. Neither is great for measuring a neutron star with nothing much falling onto it from the main sequence companion, and the lack of infall is relatable to the luminous star's wide and highly circular orbit. The "dark" object is likely to be loud in radio.
This is a binary system with a total mass of 3.887⊙ ± 0.004 ... which puts the companion in the system at 2.09⊙ to 2.71⊙ ... which may be in the "heavier than any theoretical neutron star, but lighter than any known black hole.
A recent Dr. Becky video on this: FOUND in the MASS GAP: The heaviest neutron star OR the lightest black hole? https://youtu.be/8PHt7NcwllA
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.
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.
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> (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/> 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.
> 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.
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.
Blog posts from a guy who made a whoopie cushion app used to make it to the front page, so I’m not sure what standards of importance you feel are being violated.
The question I suggest to you is: what can't be explained by an advanced alien civilization?
Pyramids? Advanced alien civilization. Rise of the hominids? Advanced alien civilization. An unusual Type IIA supernova? Advanced alien civilization. A solar system where the planets line up in a nice resonance? Advanced alien civilization. Bad run of luck last week? Advanced alien civilization.
