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Spectacular discovery of the first Intermediate mass black hole through gravitational waves

On May 21, 2019, at 03:02:29 UTC the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaboration observed a short duration of the gravitational-wave signal, named GW190521. The detected signal is consistent with the merger of two black holes with masses of 85 and 66 solar masses. The mass of the remnant black hole is estimated to be around 142 solar masses, which makes the historic event GW190521 the first observational confirmation for the existence of an intermediate-mass black hole (IMBH). 

Artist's impression of binary black holes about to collide. Source: Caltech


Black holes can be grouped into 4 categories depending on their masses. Firstly, there are the supermassive black holes that reside exclusively at the heart of galaxies having masses in the range from a few million to billions of solar masses. Then there are the intermediate-mass black holes having masses between a hundred to a few thousands of solar masses. Next are the stellar-mass black holes which form from the supernova explosions having masses in the range from 5 to several tens of solar masses and lastly, there are the quantum mechanical black holes which are currently hypothetical objects having masses above 22 micrograms to the mass of the Moon. Both supermassive and stellar black holes have been observed through various observations while there's no concrete evidence for the existence of the intermediate-mass black holes. Although several intermediate-mass black hole candidates have been reported in the literature none are definite. Since black holes do not emit electromagnetic radiation (except Hawking radiation) but can distort the fabric of space-time, gravitational waves are therefore an excellent way to study their properties.  
The LIGO was constructed to test a major prediction of Einstein's theory of general relativity, namely the existence of gravitational waves (GWs). Through successful detection of GWs from 10 binary black holes (BH) mergers, and one binary neutron star merger, the LIGO/Virgo have demonstrated a new way to observe the Universe. Indeed we can now not only see the Universe but hear it as well.        
 LIGO currently has two gravitational wave observatories that operate simultaneously: the LIGO Livingston Observatory in Livingston, Louisiana, and the LIGO Hanford Observatory located near Richland, Washington. The observatories are separated by 3,002 kilometers straight line distance through the earth, but 3,030 kilometers over the surface. Since GWs travel at the speed of light, this distance corresponds to a difference in GW arrival times of up to ten milliseconds. These two observatories work in conjunction with the Virgo observatory located in Europe to pinpoint the actual source of the GW by using a method called True range multilateration. 

This chart compares the event GW190521 witnessed by LIGO and Virgo and indicates that the remnant of the GW190521 merger falls into a category known as an intermediate-mass black hole. Source: Caltech

The event GW190521 is an extremely short GW signal which is consistent with the merger of two massive black holes of masses 85 and 66 solar masses leaving behind a single black hole with a mass of roughly 142 solar mass. This makes GW190521 the first direct detection of the formation of an intermediate-mass black hole. The team notes that the properties of the primary black hole (having a mass of 85 solar masses) may not result from a supernova explosion, therefore challenges our current understanding of stellar astrophysics. The team notes that the waveform of the event is compatible with the predictions of general relativity, albeit the short duration of the event is suspicious and has room for alternate explanations. Therefore, more advanced detectors such as the Laser Interferometer Space Antenna (LISA) coupled with ground-based detectors would be necessary to understand such systems vividly. The study has been published in the Physical Review Letters. 

Article Information: R. Abbott et al (LIGO Scientific Collaboration and Virgo Collaboration), "GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙", Physical Review Letters 125, 101102 (2020)

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