Earlier this week, the large international team associated with three gravitational wave projects unveiled the results of their latest observing campaign. The collaboration had already published two key detections from this race, the first-ever crashes of black holes with neutron stars. But this long-awaited third catalog adds significantly to the full tally of researchers, bringing the total number of gravitational wave events to 90.
The catalog includes compact objects collided during the second half of the collaboration’s third observing run (called O3b), which ran from November 2019 to March 2020. This run included both the detectors European Virgo and LIGO based in the United States; Japan’s KAGRA project joined the party for the last two weeks of the campaign.
The four detectors use lasers bouncing off mirrors to measure infinitesimal changes in distance as spacetime constricts and stretches as a gravitational ripple passes through. Observatories reveal thousands of potential events, which scientists eliminate using complex computer algorithms.
From O3, the LVK collaboration issues public alerts for gravitational wave events, to enable rapid response from astronomers looking for fleeting glows from neutron stars or black holes that have merged. (Black hole pairs shouldn’t blink unless they’re surrounded by gas.) O3b issued 39 alerts, none of which came with light.
The new analysis combs through the data more carefully than those initial passes. It removes about half of the 39 and adds an additional 17 events that escaped previous detection, for a total of 35. Combined with the recently revised list of previous detections (which increased the count from 50 to 55), this gives us gives a total of 90.
As with previous runs, the new catalog contains mostly black hole mergers – 32 of the total 35 pairs were binary black holes. But there are also the two neutron star-black hole collisions, and an event of indeterminate type: it could have been a black hole squeaking a neutron star, but there is a good chance that the smaller object was not just a tiny black hole. (And by tiny, I mean about 2.8 times the mass of the Sun.)
Not all of these events are guaranteed to be real; the team estimates that around 10% are false alarms, given their generous inclusion of all events with more than a 50% chance of being legitimate. Even so, scientists now have nearly 100 examples of crashing objects creating ripples in the fabric of spacetime – this is spectacular given that six years ago we detected zero. .
Revelations about neutron stars and black holes
The new catalog, called GWTC-3, contains several notable events. A crash involved one of the lightest neutron stars ever detected by any method, at 1.2 solar masses. Another involved snappy black holes (about 87 and 61 Suns) that added to astronomers’ discomfort (more on that later).
The list also includes black holes that spun like tops in the same direction as their circuit around each other, an alignment more likely to occur if the stars that died to form the black holes were paired from birth. But there was also at least one binary in which black holes rotated backwards relative to their orbit.
Yet as regular S&T readers may know, what interests me most is the big picture. In addition to the detailed catalog, the LVK collaboration has published three other articles, one of which is a 60-page population-level analysis. This type of study focuses on statistics and not on individual events. The researchers used the 76 most reliable events in the catalog and looked at what they tell us as a cohort.
Studying the population reveals several interesting findings, three of which caught my attention:
First, there is a sharp drop in the number of objects just above 2 solar masses. Astronomers had predicted that they would not see objects between about 3 and 5 solar masses, due to physical limitations of the size of a neutron star as well as observations of binary systems in our galaxy. But the gravitational wave data doesn’t show a hard upper edge to this putative gap, nor does it appear entirely empty. Neutron stars can’t be above about 2.3 suns, according to the data, so maybe black holes can be smaller than we thought. The previous results support this idea.
Second, not all black holes are created equal. If you look at the larger of the two black holes involved in each merger, these primaries cluster around three different masses: 10, 17, and 35 Suns. Those with 10 solar masses have a good explanation: black holes with such low masses are unlikely to pair up after they formed in a star cluster, so the binaries could all have been remnants of stars who were born and died as fraternal twins. But it’s unclear why some black holes would more often have masses of 17 or 35 suns.
Third, there is no evidence yet for a higher mass difference. This result is definitely something to watch out for. Astronomers have long predicted that there should be a shortage of black holes between about 50 and 120 solar masses, because stars large enough to create black holes that size will tear apart when they die, leaving no remnants. The oversized black holes of the previous observation campaign had already made astronomers squirm. But while the latest gravitational wave data shows a drop to masses above about 45 suns, it’s not precipitous. We haven’t seen any black holes above 120 Suns either, so there’s no upper limit to the predicted outage. Calculations suggest that if there is is a gap, it starts above 75 solar masses – much higher than expected.
Perhaps unexpectedly large black holes do not come from normal stars. Instead, they could be second-generation black holes from mergers, or they could have grown stronger from the gas they extracted. These different scenarios would cause black holes to spin in a certain way, but so far the spin measurements obviously don’t support any theory.
LVK will return for the fourth observation campaign in late 2022, when further upgrades could triple the number of detections. We might see alerts five times a week!
The LIGO Scientific Collaboration, the Virgo Collaboration and the KAGRA Scientific Collaboration. “GWTC-3: Compact binary coalescences observed by LIGO and Virgo during the second part of the third observation period.”
The LIGO Scientific Collaboration, the Virgo Collaboration and the KAGRA Scientific Collaboration. “Population of merged compact binaries inferred using gravitational waves via GWTC-3.”
You can also find easy-to-read summaries of these research papers on LIGO’s outreach page, in multiple languages.