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5 mins read 18 May 2020

Using Gravitational Waves to find the 'Missing Link'

Despite an absence of data on intermediate-mass black holes, industrious astronomers are using gravitational wave detectors to learn more about them.

Intermediate mass black-hole candidate weighing 50,000 times the mass of the Sun 3XMM J215022.4−055108 imaged by the Hubble Space Telescope. Credit: NASA/ESA/D. Lin

One of the biggest ever moments in the history of the study of black holes occurred just about one year ago. An image, arguably as iconic as the Apollo 8 crew’s Earthrise, or the Pillars of Creation taken by the Hubble Space Telescope, had gone viral on social media, and was being talked about in homes and workplaces across the world. This was not a moment celebrated just by scientists, but by everyone; it was truly something that captured the public’s attention, bringing astronomy into the mainstream and making overnight successes of a few humble researchers. This was a photo of the supermassive black at the heart of the galaxy M87, and it was incredible.

As you can imagine, black holes are not the easiest objects to photograph. No light escapes from the gigantic gravitational wells they inhabit, and whether they are large or small their presence must be inferred by observing infalling material or from the orbits of nearby stars. They also tend to be a long way away. But even finding a garden variety stellar-mass black hole in our own backyard is difficult. Earlier this month astronomers announced they had spotted the closest one yet, just 1,000 light years away in the southern constellation Telescopium. Its very existence at that distance suggests the presence of many, many more black holes, but to date scientists only have a handful of probable candidates.

Even more perplexing is that there are no particularly good candidates for a type of black hole known as an intermediate-mass black hole (IMBH). These black holes are a missing link between the stellar-mass black holes that are a bit more massive than our Sun, and the supermassive black holes inhabiting the centres of most galaxies. Despite years of searching, astronomers only have a few indirect pieces of evidence for their existence. They are smaller and less active than supermassive black holes, and much more sparsely located than their stellar mass counterparts. Perhaps the best evidence of their existence to date is a poorly resolved image from the Hubble Space Telescope of an object caught expelling copious amounts of X-ray, but the evidence is far from conclusive.

Chart illustrating the relative masses of super-dense cosmic objects. Researchers suspect that a class of intermediate-mass black holes exist, with masses up to more than 100,000 times that of our sun, but the mystery remains unsolved. Credit: NASA/JPL-Caltech

But in the search for intermediate-mass black holes, there is still plenty of science to be done. Gravitational wave detectors have been detecting the mergers of stellar-mass black holes by their distinctive chirping since 2015, and intermediate-mass black hole systems are larger in mass and so emit more energetic waves. Despite this, their signals, known as bursts, can be detected only within a very short time window and none have yet been observed. So astronomers are using the lack of detections to work out limits on just how many mergers must actually be occurring.

In a new paper from the ARC Centre of Excellence for Gravitational Wave discovery (OzGrav) researchers used supercomputer simulations to figure out what was going on. They took data from the LIGO and Virgo gravitational wave detectors, injected simulated intermediate-mass black hole binary signals, and then tried to recover those signals from the data. Unlike previous studies that assumed the black holes were spinning in an orientation that was aligned to their orbits, they accounted for the case where a black hole’s orbital plane precessed. Doing so allowed them to place better limits on the merger rates.

OzGrav alumnus and former research fellow at Monash University in Melbourne, and a co-author of the paper, Dr. Juan Calderon Bustillo, explained how this study expanded on a previous paper by scientists at LIGO and Virgo. “We extend this study to the case of precessing binaries, in which the orbital plane is tilted up and down successively. This is similar to what happens when you spin a coin on a table and you wait long enough; at the end different sides of the coin touch the table alternatively.”

An artist's impression of an intermediate-mass black hole orbiting with a stellar-mass black hole. Credit: Josh Valenzuela/UNM

This produced mergers with somewhat different signatures. “It seems these more complicated collisions are louder, and hence easier to observe. In other words, we can observe them from further distances. The fact that we haven't found any allows us to obtain better constraints of their merger rate.” Based on data from the first observing run of the LIGO and Virgo detectors, their numbers indicate that no more than 0.36 mergers/Gpc3/yr are occurring, although that number will decrease when data from the second observing run is taken into account because of upgrades done on the detectors.

After a brief hiatus of a couple of years during which the LIGO and Virgo detectors underwent upgrades, they are now at the end of the third observing run. With the more sensitive equipment, astronomers are hoping for some detections of intermediate-mass black holes, but if not, they will at least be able to further refine the merger rates. And they have the even more sensitive next generation gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer to look forward to. For now though the missing link between stellar-mass and supermassive black holes remains every bit as mysterious as you’d expect a black hole to be.

The paper appears on the arXiv preprint server