[astro123]

I don't think that is right, though I'm not a GW expert so please tell me if I'm wrong.

I think they know the difference just by looking at the mass. i.e. we think it is hard to form black holes smaller than ~3.3ish solar masses and we don't think neutron stars can be more massive than 2.2 solar masses.

This is why we get articles like [1] where there is an issue when we think we've found something between those numbers.

And yes, the foolproof way of checking whether a NS was involved is to follow up with telescopes. But the constraints on position from GW aren't always good and so you can't always find it.

[1] https://www.sciencemag.org/news/2020/06/gravitational-waves-...

[jessriedel]

Ok, I'm actually only 90% wrong! It turns out that there is in fact some information about the BH-NS distinction in the gravitational signal itself, but it is very weak and hasn't been detected yet. It is expected to be detected in the future though.

https://twitter.com/di_goldene_pave/status/13223229779841433...

[astro123]

I didn't know about that, that's really cool! Thanks for letting me know.

[jessriedel]

Yikes, on reflection I think you're right. Sorry about that. Thanks for the correction!