Detected events and Intermediate-Mass Black Holes

During the first three observing runs of the LIGO-Virgo-KAGRA gravitational wave detectors network, some 93 gravitational waves were detected, as shown in the image below.  Many more events have been observed during the 4th observing run and their analyses are ongoing now.

Interestingly, 20 of these gravitational waves are due to binary black hole mergers where the masses of the individual components are rather large, often well above 50 Solar masses. Some cases, particularly GW190521, not only show a final black hole mass of nearly 150 solar masses and is, therefore, an intermediate-mass black hole, but also one of the two merging black holes had a mass of nearly 85 solar masses. This poses the question of how such a heavy black hole came to be in the first place. Was it the result of previous mergers? Or was it produced in some unknown stellar evolution process? The observation of GW190521, shown on the side panel was the first direct evidence of the existence of an intermediate-mass black hole. Intermediate-mass black holes, or IMBHs in short, are black holes whose mass, in the range between 100 and 100000 solar masses, falls between the range of stellar black holes and supermassive black holes. 

A very interesting and fascinating hypothesis is that IMBHs might indeed represent the link between stellar and supermassive black holes, being an intermediate state that, via hierarchical mergers, from stellar evolves into supermassive black. 

I research the gravitational wave signatures and properties associated with the formation of IMBHs and I am developing algorithms that might help in their detection. In fact, although these events can be very loud, in current detectors they appear as short bursts, making the detection of a signal buried in the noise very challenging. In the future, however, with the LISA (Laser Interferometer Space Antenna) interferometer in space, and with the terrestrial observatory multiband observations will be possible as shown in figure below.

The gravitational waves produced in the coalescence of massive black holes will first be detected by the LISA (Laser Interferometer Space Antenna) interferometer, a space-based detector designed to observe low-frequency gravitational waves that terrestrial detectors like LIGO and Virgo cannot access. LISA, scheduled for launch in the 2030s, will be capable of detecting these waves well before they enter the sensitive range of ground-based observatories, providing valuable early information about the source, its mass, and its location in the sky. This early detection, which could occur days to years before the waves reach terrestrial detectors, allows ground-based observatories to fine-tune their analysis pipelines, optimizing them for the incoming signal. This collaborative approach enhances the overall precision of gravitational wave observations, offering a more complete understanding of the cosmic events driving these signals.