That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)?
One question for me is how do they know which collision is BH+BH, BH+star, Or star+star? That sounds incredibly interesting detail
I suspect it is a piecewise combination of two linear functions. There was probably a ~fixed sensitivity across Runs 1-2, with some upgrades before Run 3[0], which resulted in an improved sensitivity for that run. So in effect the slope of detections/time is steeper with the improved sensitivity.
[0] https://indico.cern.ch/event/577856/contributions/3422625/
For the second question: they can infer the masses of the objects in question from the gravitational waves. Generally if the mass is < 2.5 solar masses it has to be a neutron star, and if it’s > 5 it must be a black hole. Keep in mind this isn’t rigorous! The theoretical maximum for neutron stars is a little less than 3 solar masses, and the estimated minimum for a stellar black hole is much more empirical - I think the smallest known is about 3.5. There’s a lot of interesting physics between 2.5 and 5 solar masses - perhaps this is how we’ll discover “quark stars.”
I believe our detectors are not sensitive enough to detect the acceleration of normal stars in any physically plausible scenario - I think they would have to be accelerating way too fast to generate waves that are indistinguishable from noise.
a) the orbit would be so large that the actual acceleration is fairly small, or
b) the centripetal forces of a small, relativistic orbit would rip the star into pieces. In particular this would dramatically reduce the effective density of the accelerating mass.
Having enough gravitational force to retain coherent structure while accelerating quickly enough to generate gravitational waves seems like a tough circle to square.
[0]https://astronomy.com/news/2019/12/new-technology-improves-g...
Virgo is the 3rd actually. LIGO consists of 2 separate facilities. But yes, having more detectors decreases the degrees of freedom when calculating the origin of the signal.
We’re just getting better at detecting them.
IANAA, but I imagine that they can detect the source of the waves and check electromagnetic observations from the same area. BH+BH = no electromagnetic radiation, star+star = electromagnetic radiation from 2 stars.
Of course, it's not easy to detect such fine measurements and correlate between different instruments.
Of course, this can be corroborated with electromagnetic observations. But, to my knowledge, not all NS-NS mergers are witnessed in telescopes for various technical limitations.
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-...
https://twitter.com/di_goldene_pave/status/13223229779841433...