[dwaltrip]

I believe that black hole mergers are dark in the entire electromagnetic (EM) spectrum. This is why the recent binary neutron star merger was so groundbreaking -- it emitted both gravitational waves and a broad spectrum of EM waves.

Indeed, though, it would be awesome to witness the collision and resulting gwaves personally, if such a thing was possible :)

[raverbashing]

Nah I think there are some effects in the visible range. You probably have some kind of accretion disk that gets shaken up by the merger, there might also be some effects due to Hawking radiation

But they're probably only visible from close range (like the 1AU mentioned above) and are too faint to be seen from 1Bi light years like the event that they captured

[russdill]

That's assuming they are "naked" and not surrounded by rotating disks of gas and dust.

[dwaltrip]

AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward (and emitting significant amounts of light in the process). Even then, they only actively feed in that way for a short period of time -- I think something like 10k years.

All of the LIGO observations have been of more basic stellar mass black holes merging together.

[raattgift]

> AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward

Stellar binaries are extremely common, and there is a reasonably large supply of binarys where one star has become a compact object. Their companion stars often drop lots of matter onto them, resulting in a reasonable supply of black holes. Diskoseismologists and others working on Swift have catalogued hundreds of stellar mass black hole accretion disks.

Examples from Swift:

http://adsabs.harvard.edu/abs/2013ApJ...769...16R https://arxiv.org/abs/1112.2249 (preprint version)

Swift also spotted ASASSN-14li which was a star being shredded by an SMBH and forming an early accretion structure. The event has been followed up by other observatories (notably Chandra and the European very long baseline interferometry network). ASASSN-14li is an easy google search term (the trick is knowing the term in the first place :-) ), hopefully you will enjoy some of the hits. :-)

[dwaltrip]

Ah, interesting. That makes a lot of sense. Would it be correct to say that if both objects in a binary pair are SMBHs, they would very likely not have an accretion disk, as the companion would be unable to send over any material?

[raattgift]

> if both objects in a binary pair are SMBHs, they would very likely not have an accretion disk, as the companion would be unable to send over any material

BH's don't let what's in the horizon out unless outside is verrrrrrrrrrrry cold (the universe will have to keep expanding for a long tine before it's cold enough for even isolated stellar-mass BHs to lose net mass to evaporation) or the BH is very small.

On the other hand, SMBHs will typically be found in galactic centres, where there is a lot of dust and gas.

So each of the mutually orbiting SMBHs may well have a substantial accretion disk. They may interact, or they might not (the disks might not be in the same plane, for instance).

Given the number of intensely active galactic nuclei we see in the sky, I don't think it's terribly unlikely for a central black hole to have an enormous accretion disk.

However, the Milky Way doesn't have an active galactic nucleus the central parsec is relatively quiet. The dense object in the central parsec is also pretty low-mass compared to that in many galaxies. https://www-xray.ast.cam.ac.uk/xray_introduction/AGN_intro.h...

[dwaltrip]

Hmmm. I wouldn't have guessed that! I suppose it does make sense that SMBHs in galactic centers could have relatively significant accretion disks. Thanks for the informative response.

[krylon]

> I believe that black hole mergers are dark in the entire electromagnetic (EM) spectrum.

Now that I think about it - yes, of course. It's why they are called _black_ holes. ;-) facepalm

[vecter]

That doesn't follow from the fact that black holes are "black". For example, black holes can emit a ton of radiation from accretion disk as matter accelerates and falls inwards, radiating huge amounts of energy [0]. Two naked black holes merging probably wouldn't emit much (if any) EM radiation, but if they had very active accretion disks, then it's certainly possible there would be a ton of EM activity.

[0] https://en.wikipedia.org/wiki/Accretion_disk

[krylon]

Yes. I had not thought of that. But if either of the black holes (or both) had an accretion disk at the time of the collision, they would have been "visible" before.

I am not sure, though, if regular (stellar-mass) black holes with an accretion disk emit enough EM radiation to be visible at such distances, or if it would become lost among the radiation emitted by the rest of the galaxy.

(A merger between two supermassive black holes with active accretion disks must be a spectacular sight even from a safe distance.)