Home » Ligo and Virgo Announce Discovery of an Intermediate Mass Black Hole
Intermediate Mass Black Hole

Ligo and Virgo Announce Discovery of an Intermediate Mass Black Hole

Above image: An artist’s rendition of the two black holes of GW190521 orbiting each other in a gas disk that surrounds a supermassive black hole. Image credit: R. Hurt/IPAC/Caltech.

On September 2, 2020 the LIGO Scientific collaboration and the Virgo collaboration announced the observation of an intermediate mass black hole, the missing link between stellar mass black holes and supermassive black holes in the core of most galaxies. It was formed in a collision of two black holes that radiated gravitational wave signal detected by the LIGO and Virgo detectors. The first confident detection of the signal was made by the data analysis program “coherent WaveBurst”—a search algorithm invented and developed at University of Florida.

The gravitational wave signal, named GW190521 [1], was detected on May 21, 2019 around 3am Greenwich time by the two LIGO detectors located in Hanford, Washington, and Livingston, Louisiana, and a third detector, Virgo, located near Pisa, Italy. Figure 1 shows the data recorded by the detectors and the GW190521 signal reconstructed by coherent WaveBurst. GW190521 was the collision of two black holes—one with about 85 times the mass of our Sun and the other 66 times the mass of our Sun—merging to produce a single, more massive spinning black hole that is about 150 times the mass of our Sun. The black holes of GW190521 are more than twice as massive as those of GW150914—the first gravitational wave discovered by LIGO in 2015 (another discovery made by coherent WaveBurst). An enormous amount of energy was released through gravitational waves in the collision, the same as if the mass of 8 stars like our Sun was turned into pure energy. GW190521 was by far the most distant event LIGO and Virgo have ever discovered. Moving with the speed of light, it took 7 billion years for the signal to reach our Solar System, which did not yet exist at the time of the collision.

GW190521
Figure 1. Gravitational wave signal recorded by the detectors (color) and reconstructed signal by coherent WaveBurst (black).

The detection of this collision is groundbreaking on multiple fronts [2]. It is the first case of a so-called intermediate-mass black hole, a previously elusive category with a mass range from a hundred to a hundred thousand times the mass of our Sun. We have many cases of lighter black holes that are formed by dying stars and of heavier, supermassive black holes that are found in the center of most galaxies. This intermediate-mass black hole category has so far not been seen and could represent a missing link between the two known black hole populations. “This discovery is a milestone in gravitational wave astronomy. Observations of an intermediate-mass black hole inform our understanding of the formation and evolution of the most massive stars and serve as probes of structure formation and growth of massive black holes”, says Sergey Klimenko, a University of Florida professor who pioneered development of searches for intermediate mass black holes in LIGO.

The mere existence of such a heavy black hole collision rewrites how we think of the formation and interaction of these mysterious cosmic objects. Because the heavier of the GW190521 component black holes couldn’t have been formed by dying stars, another explanation is needed. The emerging evidence suggests that it was the end product of previous collisions of smaller black holes. Consecutive collisions of black holes are expected in special cosmic locations where many black holes reside in a small space so that they can collide just by chance. The centers of galaxies are such locations, where tens of thousands of black holes can congregate within a light year from the center. “Despite the large number of discoveries by LIGO and Virgo, GW190521 is the first example in which we see a strong indication of where and how black hole collisions take place,” says Imre Bartos, an assistant professor at the University of Florida, who chairs the LIGO/Virgo’s working group that is searching for intermediate mass black holes.

The University of Florida (UF) has been a key part of the NSF-funded LIGO project since 1996 when the UF LIGO group was organized by UF professor Guenakh Mitselmakher. UF faculty David Tanner, Guido Mueller and David Reitze (who is the current Director of the LIGO Laboratory) took responsibility for LIGO’s Input Optics. At the same time, Sergey Klimenko, Guenakh Mitselmakher and Bernard Whiting started to work on LIGO data analysis. UF scientists led the design of key parts of the Advanced LIGO interferometer and continue to conduct research on materials and devices for future detectors. The Input Optics project is one of the most complex components of the LIGO detector. UF scientists Sergey Klimenko and Guenakh Mitselmakher led the development of the gravitational-wave detection algorithm coherent WaveBurst, which was the first LIGO algorithm to discover gravitational waves. The UF LIGO group has grown over the years. Stephen Eikenberry joined in 2011 and is working on optical follow-up of LIGO signals. Hai-Ping Cheng joined in 2012 and studies the reasons for excess noise in the optical coatings of the LIGO mirrors. John Conklin joined in 2015 and studies LIGO suspensions and controls. Imre Bartos joined 2017 and works on astrophysics of gravitational wave sources. Paul Fulda joined in 2018 and works on improving detector performance and on low-loss components for the next upgrade of LIGO. Over almost two decades, UF scientists, postdocs and students designed, built, installed and commissioned the input optics and developed novel detection algorithms for Advanced LIGO.

The discovery of the first intermediate black hole was carried out by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC) with key contributions from the University of Florida. LVC is a group of more than 1400 scientists worldwide who have joined together in the search for gravitational waves using the LIGO and Virgo detectors.

The LIGO/Virgo webinar presenting the GW190521 results is scheduled for 1500 CEST (0900 EDT, 0600 PDT, 220 JST) on Thursday September 3rd. The webinar will be broadcast on Zoom. Pre-registration is required at https://us02web.zoom.us/j/86896814123 to join the webinar.

GW190521 publications

[1] GW190521: A Binary Black Hole Merger with a Total Mass of 150 Msun, published in Physical Review Letters, download from journal website.
See also associated Viewpoint article in Physics

[2] Properties and astrophysical implications of the 150 Msun binary black hole merger GW190521, published in The Astrophysical Journal Letters, download from journal website.

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