10 New Merger Events in Gravitational-Wave Data

An independent team has found additional black hole mergers in LIGO data.

Still image from a simulation of the merger of two black holes of very different masses wobbling around each other in their inspiraling orbit. Redder colors represent stronger waves. This simulation matches the characteristics of GW190412, which both the LVK and IAS pipelines detected.

N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes (SXS) Collaboration expanding pattern of waves created by black hole merger still image from a simulation of the merger of two black holes of very different masses wobbling around each other in their inspiraling orbit. Redder colors represent stronger waves. This simulation matches the characteristics of GW190412, which both the LVK and IAS pipelines detected.

N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes (SXS) Collaboration

A TASTE FOR THE EXOTIC

Before gravitational-wave discoveries, astronomers expected black holes to have masses between 5 and 50 Suns. The lower bound is a mysterious one, based on the fact that we haven’t found a bunch of these smaller black holes; it has no theoretical underpinnings. The upper limit is due to stellar physics: Stars massive enough to make a black hole between 50 and 120 solar masses won’t do so — instead, they’ll blow themselves to smithereens. (Well, probably.)

LVK detections have included a smattering of objects outside these bounds, and the IAS candidates add several more. These include GW190711_030756, the merger of a roughly 80-solar-mass black hole with an 18-solar mass one, and GW190704_104834, in which a 7-solar-mass black hole merged with a 3.2-solar-mass object (likely a black hole). 

The few low-mass objects don’t solve the mystery at low masses — it’s easier to make less massive stars, so there should be more small black holes than big ones, and there aren’t. Perhaps stars explode in a way that naturally makes remnants of at least 5 solar masses. Conversely, the beefy black holes are potential byproducts of a previous generation of mergers, or they might indicate that fusion reactions inside stars aren’t as efficient as we think.

The IAS pipeline also spotted a candidate event that involved a big black hole spinning nearly on its head around its companion: GW190910_012619, the merger of a 34-solar-mass black hole with a 2.9-solar-mass object. “It would be so unlikely for that to come from two black holes that were together for their whole lifespan,” Olsen says. Although we’ve seen signs of misaligned spins before, this one is dramatic. It leaves no doubt that the two black holes paired up late in life. “Everybody will say, ‘Yeah, that’s probably a capture or some crazy multiple systems that have caused this.’” Notably, however, this event has one of the lowest likelihoods of being a real astrophysical signal.

Exotic events like these are key for disentangling how binary black holes form and how big or small a black hole can be.

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