35 new events detected by LIGO and Virgo in their latest
observation period bring to 90 the gravitational waves detected to date by
the global three-interferometer network.
Most of the new signals originate from the whirling spiral of two
merging black holes: cosmic quakes that shake the fabric of spacetime,
generating a powerful burst of gravitational waves. Two other events,
one already reported last June, were
instead identified as mergers between a neutron star and a black hole, a
source observed for the first time in this last LIGO-Virgo run. A further
event, detected in February, 2020, could come from either a pair of black
holes or from a mixed pair of a black hole with a neutron star.
The dataset, published today in the so-called third Catalog Paper,
outlines the features of new populations of black holes, the masses of which,
together with those of the observed neutron stars, may provide clues about
how stars live and die, further broadening the horizons of gravitational
The Catalog is accompanied by three other publications, focusing
on the cosmological and astrophysical consequences of the results and on the
multi-messenger search for gravitational-wave signals in coincidence with 86
very energetic bursts (Gammy Ray Bursts) detected in space by the Fermi/GBM
and Swift/BAT instruments, during the second part of the third observation
period. No confident GW counterparts have been observed; in parallel, no
signals of a different kind (e.g. light and neutrinos) have thus far been
reported by telescopes and observatories on Earth or in space following-up
the GW signals, i.e. searching for signals emitted by the same source as that
which emitted the gravitational waves.
At the same time, the LIGO, Virgo and KAGRA scientific
collaborations have today also released the full set of calibrated data
recorded by the LIGO and Virgo detectors from November, 2019, to March, 2020.
This allows the whole research community to perform independent analyses and
checks, maximising the wealth of scientific results.
The progress achieved in a few years by gravitational-wave
scientists has been amazing, passing from the first detection to the
observation of a number of events per month. This has been possible thanks to
the programme of continuous technological upgrades, which have transformed
the first pioneering instruments into increasingly sensitive detectors. The
progress in detector sensitivity due to the technological upgrades and
commissioning is evident, considering that, of the 90 gravitational-wave
events published today, as many as 79 refer exclusively to the most recent
observation period, which ran from April, 2019, to March, 2020.
The LIGO and Virgo observatories are currently undergoing a
further upgrade and will start the upcoming fourth observing period, in the
second half of 2022, with an even greater sensitivity, corresponding to a
volume of the Universe almost 5 times larger than before and, therefore, a
much greater probability of picking up gravitational signals.
"Among the other upgrades at Virgo, we have realised an additional
optical cavity (the so-called signal-recycling cavity), which allows to
improve the sensitivity band of the detector at high frequencies", said
Sebastian Steinlechner, assistant professor at Maastricht University and
Nikhef. "This corresponds to an increased capacity of the detector to
'listen' to the final stages of the coalescing pairs, when two black holes or
stars merge into one."
The KAGRA detector in Japan is being commissioned and KAGRA plans
to join the next observing period. The expansion of the network of detectors
able to jointly take data will further increase the accuracy of source
localisation, a key feature for future developments in multi-messenger