On August 26, 2025, while the fourth observing period (O4) of the LIGO, Virgo and KAGRA detectors was still in progress, version 4.0 of the Gravitational-Wave Transient Catalog (GWTC-4.0) was published. It contains the events detected during the first part of O4, between May 2023 and January 2024.

This catalog extends our knowledge of binary black hole systems and mixed systems with a black hole and a neutron star. For example, the attached graphic illustrates the large population of compact binary object systems observed by the LIGO-Virgo-KAGRA Collaboration, and how these observations expand our knowledge of these objects, which are rarely detectable by electromagnetic observations.

The publication of the catalog on the website of the Gravitational Wave Open Science Center (GWOSC, www.gwosc.org) is accompanied by a collection of scientific articles that includes not only the results themselves (arXiv:2508.18082) but also an introduction to gravitational wave detectors and general concepts about transient gravitational wave signals produced by the merger of compact astrophysical objects (arXiv:2508.18080) and a detailed description of the methods used in searching for these signals (arXiv:2508.18081).

Also published along with the catalog are all the data analyzed to produce it so that scientists outside the LIGO-Virgo-KAGRA collaboration can study it independently in the spirit of “open science.” The characteristics of the public data along with details of the procedure by which they are calibrated and on the quality of the data are discussed in a separate article (arXiv:2508.18079) published along with the other three already mentioned.

Researchers from the University of Trieste and of the Trieste section of the INFN were actively involved in both article writing and preparation of data for publication, in particular Agata Trovato (UniTS-INFN) and Panagiotis Iosif (UniTS-INFN) are part of the working group that wrote the data article (arXiv:2508.18079), Agata Trovato is responsible for preparing the public data, and Agata Trovato and Edoardo Milotti (UniTS-INFN) participated in the internal review of the results article (arXiv:2508.18082).

For information: Agata Trovato (agata.trovato@units.it), Edoardo Milotti (milotti@ts.infn.it)