@@ -42,7 +42,7 @@ We saw in the previous chapter that the emitted amplitude depends on the masses
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\noindent
This equation is particularly useful if we want to know information about the radiation emitted my a certain mass, at a certain frequency at a certain time before the merger. Predictions about this time and the frequency where it is possible to detect the radiation is essential for several reasons, from efficiency of the detector in terms of variety of sources to Multimessenger astronomy, in which timing is important to assure a correct localization of the sourceCITA MULTI.\\
This equation is particularly useful if we want to know information about the radiation emitted my a certain mass, at a certain frequency at a certain time before the merger. Predictions about this time and the frequency where it is possible to detect the radiation is essential for several reasons, from efficiency of the detector in terms of variety of sources to Multimessenger astronomy, in which timing is important to assure a correct localization of the source.\\%CITA MULTI.\\
In our case of interest, if we apply the lowest range of frequency available by ground-based detectors ($\sim$ 10 Hz in order of magnitude and consider M$_c$ = 1.21 M$_{\odot}$, it is possible to observe the radiation emitted at $\tau$ = 17 minutes to coalescence. This equation says that the larger is the time to coalescence, the smaller is the masses involved \footnote{A useful exercise to prove this is by applying the Kepler's law for different emitting frequencies and masses. Some interesting examples are given in \cite{mag}.}.\\
Recalling Fig. \ref{spec}, the range of the frequencies of emission below 10 Hz lies almost all in the space-based detectors dominion. Opening this frequency window would allow the ground-based detectors to access to a frequency bandwidth which is still not investigated and would allow the detection from sources whose physics is still unknown.
@@ -245,7 +245,7 @@ This test shows that HoQI2 is in general noisier than HoQI1, especially above 1
\section{Laser stabilization: tests and results}
The tests have been performed measuring the stability of the beat-note peak around the 60 Hz setpoint: the frequency counter used for this measurements is a Keysight 53230A 350 MHz - 20 ps. The output of the fast photoreceiver is DC-coupled and can be directly connected to the counter. The measurements has been recorded on a USB drive: the data provided by the counter are in frequency (Hz).\\
Several tests have been taken in different conditions for noise hunting along the frequency range of interest. QUI CI VANNO I TEST COL COUNTER DI CUI PARLAVA CONOR: LO SCOPO E' DIMOSTRARE CHE IL SETUP E' STATO ADEGUATAMENTE TESTATO
Several tests have been taken in different conditions for noise hunting along the frequency range of interest. %QUI CI VANNO I TEST COL COUNTER DI CUI PARLAVA CONOR: LO SCOPO E' DIMOSTRARE CHE IL SETUP E' STATO ADEGUATAMENTE TESTATO
The discovery of gravitational waves opened a new way to look at the Universe and offered new opportunities to shed light on the still unknown aspects of physical sciences. The work presented in this thesis wants to give a contribution to the development of this new type of research: the author chose to focus on the improvement of the instruments able to detect the gravitational waves. This field is important to make the detectors more sensitive, in order to see more gravitational-wave sources and help to complete the mosaic of the astrophysical science. In particular, the detectors currently in use are interferometers, which are especially blind in a range of frequency below 30 Hz: this affects the chance to detect sources emitting in this frequency band.\\
This lack of sensitivity is mainly due to seismic motion, and the work exposed in this thesis focussed on new techniques to lower this noise source and allow the instruments to be sensitive below 30 Hz.\\
During the studies, the development and test of devices able to to potentially reduce the seismic motion have been performed, such as optical levers for tilt motion reduction and laser stabilization for low frequency readout; a new concept of the seismic system on one of the interferometers (LIGO) has also been proposed.\\
The results are promising to provide suppression of the seismic motion in the bandwidth of interest and show that it is possible for a ground-based instrument to be seismically more stable and able to detect gravitational waves where it is now forbidden.
The optical levers can in principle reduce tilt motion below 1 Hz; the use of capacitive position sensors in a new software configuration for LIGO can help to suppress ground motion by a factor of 3 in order of magnitude below 0.1 Hz. A competitive frequency stabilization to 8 $\times$ 10$^3$ Hz/$\sqrt{Hz}$ at 1 Hz for readout at low frequency is possible with a compact and easy to handle setup. These results are promising to provide suppression of the seismic motion in the bandwidth of interest and show that it is possible for a ground-based instrument to be seismically more stable and able to detect gravitational waves where it is now forbidden.
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@@ -196,6 +196,12 @@ There are three appendices useful to make the work more complete: appendix A ill
\bibitem{lf3} Low Frequency Workshop, University of Birmingham, August 2018
\bibitem{yu} H. Yu et al, \textit{Prospects for Detecting Gravitational Waves at 5 Hz with Ground-Based Detectors}, Phys. Rev. Lett. 120, 141102, 2018
\bibitem{goulding} A. D. Goulding et al, \textit{Discovery of a Close-separation Binary Quasar at the Heart of a z$\sim$0.2 Merging Galaxy and Its Implications for Low-frequency Gravitational Waves}, The Astrophysical Journal Letters, 879:L21 (7pp), 2019
\bibitem{whyte} J. S. B. Whyithe et al, \textit{Low-frequency gravitational waves from massive black holes binaries: predictions for LISA and PULSAR timing arrays}, The Astrophysical Journal, 590:691–706, 2003
