Through two observations that match theoretical expectations for a Black Hole neutron star merger humanity can now place limits on how often these occur.  About once per month within a one billion light year radius of the Earth Black Holes and Neutron Stars merge.  We do not seem to see any from any from inside our own galaxy.  If we did the signals would be stronger.  This is not the first time we may have observed such an event but there was much more question of just what it was.  In any case, as I showed for public consumption with a simple calculation the result is more than likely a Black Hole. The reason we do not see these from within our own galaxy is likely because these high mass systems evolve from fast short-lived Blue and White giant stars to Black holes and neutron star binaries, to a single black hole relatively quickly.  So, most all such binaries in our galaxy are already isolated Black holes of low to intermediate mass.

Two Events

GW200105, GW200115 so named for having been observed via gravitational waves were observed as reported by LIGO – VIRGO in (1).  GW200105 was observed with only one detector. GW200115 was a standard multi detector event.  To say that these meet theoretical expectations of a black hole neutron star mergers means that General Relativity a model we understand very well and have tested very well would have to be radically wrong for this to be anything else.

When gravitational waves are used to localize the areas in the sky where an event may have taken place, we are only able to localize to broad areas of the sky. This is because it is like we have an eye with just one light sensitive cell in it. With two of these eyes, we can localize much better. Therefore, the two-detector event, seen at bottom is much more localized. The more gravitational wave detectors we have the more precise. However, this broad area of sky that must be searched is part of why we may not see an electromagnetic companion signal. Image courtesy of (1) Published open access fair use claimed.

When events are observed GCN circulars, which notify astronomers so that they may observe the areas of the sky the came from for any optical signals.  None were seen for this.  However, it is possible that an analysis of past data could be done which could identify a candidate for a neutron star orbiting a Black hole, which when we check is no longer there.  The major difficulty in finding and confirming this would be the distances involved.  Neutron stars are however a bit easier to observe because of their strong radio signals it is hardly impossible that some VLBI (Very Long Baseline Interferometry, the same technique used to observe the Black Hole by the Event Horizon Telescope) survey cataloged objects that may have had an unseen partner.

Not In Our Galaxy.

All of that said, this is just another great day for science.  As a theoretical astrophysicist I now have something to engaged in some informed speculation about.  We know there are both neutron stars and black holes in our own galaxy so why wouldn’t we detect a merger in our own galaxy?   Here are some possibilities.

Binary star systems that would contain a neutron star and a Black hole would involve two high mass stars.  The higher the mass of a star the faster it will burn its nuclear fusion fuel, hydrogen.  Once a star burns its core hydrogen it starts to die.  Stars like our Sun swell up and become red giants then slowly shed their outer layers.  Stars of 8 solar masses or more will die more violently and much more quickly. 

I like to use the analogy to stars in Hollywood.  High mass stars are very bright, very intense, burn extremely fast and die young they are the John Belushis of the sky.  Stars like our Sun are also very bright but do not make as much of a splash they are the Jim Belushis of the universe.  They also last much longer.

Using this analogy, the big bright luminous stars that would die and make black holes and neutron stars ran through their fuel and died.  Then over time as they orbited gravitational waves radiated away energy that caused them to spiral in leaving only a solitary black hole.  Because these signals only travel at the speed of light, in our galaxy we only see the Black holes that are the result.    Much as now we all know about Animal house and can still laugh at a clip from John Belushi on SNL while his brother can host an unremarkable weekend of SNL.

All the above is in simple terms and based on fully accepted physics.  The observation reported on here did not observe any deviation from general relativity to within the precision of their observations but then many modified gravity models fit well within the error bars (1). If one adds the very theoretical but well-grounded models of modified gravity that would see some energy radiated away through gravitational coupling to one or more additional fields or higher order terms in the mathematical model, then it is even more likely that we would not observe such a merger within our galaxy.  Not impossible but not likely that we will observe such an event from within our middle-aged galaxy.  By now all or almost all such pairs of objects will have merged.

References

1. Abbott, R, Abbott, T. D. and Et, Al.Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences 915. The Astrophysical Journal Letters. 2021, Vol. 915, 1.

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