Giacomo Bruno, Jishnu Suresh

The stochastic gravitational wave background (SGWB) originates from the superposition of GWs emitted by a large number of
unresolved and uncorrelated sources. Its detection is considered to be one of the ”holy grails” of
GW astronomy, because of its possible cosmological origin and consequently its impact on our
comprehension of the Universe.

The Louvain GW group has been contributing a major effort to the search
for an anisotropic SGWB and the publication of its results. The group is esponsible for one
of the three data analysis algorithms of the LIGO/Virgo/KAGRA (LVK) Collaboration, called broadband radiometer analysis.
Dr J. Suresh has been acting as the anisotropic sub-group chair for the LVK SGWB
group. Not having found any evidence for an SGWB signal, upper limits have been set as a
function of the sky direction.

Millisecond pulsars are one of the potential candidates contributing to the anisotropic stochastic
gravitational-wave background observable in the ground-based gravitational-wave detectors.
We have been contributing to a project aiming to estimate and detect the
stochastic gravitational-wave background produced by millisecond pulsars in the Milky Way.

We have contributed significantly to the published results of a search that looks for
persistent stochastic gravitational-wave sources in all directions of the sky at all frequencies at
which the detectors are sensitive.

Our group has also published a search that is capable of setting constraints on the
ensemble properties of neutron stars, like their average ellipticity, from cross-correlation-based
stochastic gravitational-wave background measurements.

Giacomo Bruno, Antoine Depasse, Andrew Miller

Asymmetrically rotating neutron stars (NS) are the canonical sources of continuous gravitational
waves, which is the name given to long-duration, almost monochromatic GW signals. There has
been a growing number of other sources of similar signals, which are in general very weak, but
because they are almost monochromatic and of very long duration, they can be integrated over
observation periods lasting up to years and become observable. Sophisticated corrections need to be devised
in order to capture these long and weak signals. These corrections
take into account the deviation from perfect mono-chromaticity of the signal frequency spectrum, which are caused either
by the source dynamics of by the relative movement of the source and the detector.

Our group has been setting up a search for new ultra-light bosons, which could be dark matter (DM) candidates and could accumulate around spinning black holes (BH) via superradiance. In particular, we have been
focusing on the search for vector boson accumulating in known X-ray binaries in our galaxy.
In addition to the movement of the earth, the signal will be modulated by the Doppler effect due to the motion of the source black hole (BH)
around its barycenter.

Our group is also active on studies aiming to detect planetary-mass (10^-7 to 10^-2 M⊙) primordial BHs (PBH) with continuous-wave
methods. The method applies to binary
systems that are still far from the merger and has allowed to constrain the rates and abundance of PBHs in the universe. Limits on the
fraction of DM made of such PBHs (in the galactic halo, in the galactic centre, and in the solar
system vicinity) have also been calculated, for LIGO/Virgo as well as ET.

Ultra-light (10^-13 - 10^-11 eV) bosons could interact with the baryons and leptons in the LIGO/Virgo
mirrors, causing a constant, narrowband signal in the instruments, very similar to a quasi-monochromatic GW.
This project displays synergy between particle physics and GW physics and shows that we can
now directly look for DM candidates with GW instruments. Both short-author list and Collaboration-wide publications have resulted from this project.