The LIGO and Virgo Collaboration, which includes scientists from the Institute for Gravitational Wave Astronomy at the University of Birmingham, have reported the discovery of a signal from what may be the most massive black hole merger yet observed in gravitational waves.
The signal, labelled GW190521, was detected on May 21, 2019, with the LIGO and Virgo detectors. Lasting less than one-tenth of a second in the sensitive band of the detectors, the signal was generated by a source that is about 17 billion light years from Earth, making it one of the most distant gravitational-wave sources detected so far. The discovery is reported in papers published in Physical Review Letters and The Astrophysical Journal.
Using a powerful suite of state-of-the-art computational and modeling tools, scientists concluded that GW190521 was most likely generated by an exceptional binary black hole merger, involving two black holes, one about 85 times and the second one about 65 times the mass of the sun. This spectacular smash-up created an even more massive black hole of about 142 solar masses, and released an enormous amount of energy equivalent to around 8 solar masses in gravitational waves.
“As GW190521 is such a heavy system, it provides us with a fantastic opportunity to study the complex physics of merging binary black holes as predicted by General Relativity,” says Dr Geraint Pratten, a researcher at the Institute for Gravitational Wave Astronomy and LIGO team member.
In addition, the LIGO-Virgo team also found that, if GW190521 was indeed produced by the merger of two black holes, the black holes were likely spinning in such a way that the spin axis was not parallel to the orbital angular momentum. Such misaligned spins would cause the orbit to precess or “wobble”.
“In this unusual merger, we see the first hint that black holes may be tumbling in space. Future observations will certainly shed more light on this phenomenon”, says Dr Patricia Schmidt, lecturer at the Institute for Gravitational Wave Astronomy.
The uniquely large masses of the two inspiraling black holes, as well as the final black hole, raise a slew of questions regarding their formation. All of the black holes observed before this new observation fit within two categories: stellar-mass black holes, which are thought to form when massive stars die; or supermassive black holes, such as the one at the center of the Milky Way galaxy.
However, in the case of GW190521, the final 142-solar-mass black hole lies within an intermediate mass range between stellar-mass and supermassive black holes — the first of its kind ever detected.
The two progenitor black holes that produced the final black hole also seem to be unique in their size. They are so massive that scientists suspect they may not have each formed from a collapsing star, as most stellar-mass black holes do.
In particular, there is a phenomenon known as “pair instability”, which prevents the formation of black holes with masses between approximately 60 and 130 solar masses from the collapse of massive stars at the end of their lives. And yet, the two black holes that produced the GW190521 signal, in particular the 85 solar mass one, are the first that scientists have detected with masses within this “pair instability mass gap”. Professor Alberto Vecchio, director of the Institute for Gravitational Wave Astronomy, says: “When stars are too massive they are believed to blow up completely when they collapse, leaving nothing behind. A black hole of 85 solar masses should not exist. This is a beautiful discovery and a fascinating puzzle. Now we need to figure out how Nature could have possibly assembled such an object.”
One possible explanation could be a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes.
Riccardo Buscicchio, PhD student at the University of Birmingham involved in the discovery says: “We expect such a chain of mergers to happen in a soup of black holes. The properties of GW190521 indicate that it may have formed in such an environment, although we will need to observe more such high-mass events to better understand their origin.”
There are many outstanding questions regarding GW190521, and scientists are still considering the possibility that the signal isn’t from a binary black hole merger at all.
In the case of GW190521, searches designed to look for gravitational waves that are not necessarily produced by the merger of two black holes, picked up a slightly clearer signal. This opens up the very small possibility that GW190521 arose from something other than a binary merger, such as a collapsing star in our galaxy or from a cosmic string produced as the universe inflated in its earliest moments.
Lucy Thomas, a PhD student at the University of Birmingham and LIGO member, says: “Since the first detection of gravitational waves five years ago, our understanding of black holes and neutron stars has grown substantially. GW190521 offers a small possibility of being produced by an unexpected source. This could add to or even challenge our understanding, which is extremely exciting.”
LIGO research activities at the University of Birmingham are supported by the U.K. Science and Technology Facilities Council (STFC).
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