• gravitational waves
• multimessenger astrophysics
• early warning
• localization
• neutron star

The joint detection of the gravitational wave event GW170817 and its electromagnetic coun-
terpart opened the era of multimessenger astronomy with binary neutron stars and allowed to
open a new window on the Universe. The chances of observing more of these events critically
depend on the capability to localize the source with sufficient confidence as quickly as possible.
Moreover, if the gravitational wave signal spends enough time in the detector sensitivity band
it is possible to detect and localize the event before the merger happens and alert in advance
telescopes for EM follow-up. The so-called early warning requires good low-frequency sensi-
tivity of gravitational wave detectors and enhances the probability of having a multimessenger
event. The LIGO-Virgo-KAGRA collaboration has programmed a list of observing runs. The last
of these, called O5, is expected to last until 2030. Moreover third-generation gravitational wave
detectors, such as Einstein Telescope and Cosmic Explorer, are expected to be ready at the end of
next decade. In the meantime, in the “post-O5” era between O5 and third-generation detectors,
upgrade programs such as Virgo nEXT are planned to boost the sensitivity of existing detectors
such as Advanced Virgo. In particular, newly proposed seismic attenuation techniques are par-
ticularly promising to enable the increasing of sensitivity in the low-frequency range. This thesis
aims to understand how the different expected noise budgets at low frequencies for Virgo nEXT
will impact the localization and early warning capabilities of the scheduled network of detectors
in the post-O5 timeframe. The forecasting is performed by simulating a population of binary
neutron stars at different fixed distances and analyzing them using the Fisher Matrix formalism
with a new library called GWFiNe. This thesis will report the expected localization and early
warning capabilities of the network of detectors in post-O5 for different Virgo configurations.
The obtained results will be useful to draw a path in the developing and upgrading plans.