@@ -18,7 +18,7 @@ The challenging goal of detecting gravitational waves opened a research field de
This research is important, because detecting gravitational waves provides information on the sources which produced them. There is still a gap in the knowledge of many astrophysical objects, such as Black Holes (BH), Neutron Stars (NS) and Supernova events: this new-born branch of astrophysics will help to fill the gap and increase our knowledge of the Universe.\\
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The detectors currently in use are sensitive to events from sources emitting at frequencies above $\sim$10 Hz, but there is still a broad range of frequencies to which the detectors are blind. Looking at different frequencies of emission means looking at different objects emitting gravitational waves. This would broaden the catalogue of observed objects and the chances to better understand their nature.\\
The detectors currently in use are sensitive to events from sources emitting at frequencies above $\sim$30 Hz, but there is still a broad range of frequencies to which the detectors are blind. Looking at different frequencies of emission means looking at different objects emitting gravitational waves. This would broaden the catalogue of observed objects and the chances to better understand their nature.\\
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The work carried on during my PhD studies and presented in this thesis has been dedicated to the improvement of the sensitivity of the detectors at frequencies below 10 Hz, by the development of new ideas and technologies to reduce noise sources affecting the low-frequency bandwidth, in particular the seismic motion.
@@ -64,7 +64,7 @@ The implication of this effect lies in a factor (1+z) multiplied to the masses i
An important consequence is that if the instrument could detect in a broader range of lower frequencies, it is possible to identify objects located at higher redshifts, i.e. more ancient, or apparent high masses increased by the cosmological distance \cite{yu}. Examples of these objects are Intermediate Mass Black Holes (IMBH) or stellar-mass BHs, whose nature and physics are still unknown.
\subsection{Multi-messenger astronomy and low frequencies}
Multi-messenger astronomy is a branch of astronomy born with the discovery of the first gravitational wave. It has been seen that the signal of a gravitational wave can be followed up by observatories operating in other frequency bands (say, the electromagnetic bandwidth), to localize and study the source under several other points of view \footnote{A general overview about multi-messenger astronomy can be found in \cite{branchesi}. An interesting paper about a multi-messenger GW-source detection and its implications is \cite{multi}.}.\\
Multi-messenger astronomy is a branch of astronomy born with the first gravitational wave detections. It has been seen that the signal of a gravitational wave can be followed up by observatories operating in other frequency bands (say, the electromagnetic bandwidth), to localize and study the source under several other points of view \footnote{A general overview about multi-messenger astronomy can be found in \cite{branchesi}. An interesting paper about a multi-messenger GW-source detection and its implications is \cite{multi}.}.\\
It is then important that the communication between these observatories is the best of the efficiency: the joint-collaboration is determinant to provide a precise localization of the source in the sky and a complete set of data to study the object in all its details \cite{bird}.\\
The main challenge when an electromagnetic observatory tries to follow up a signal from a gravitational-wave detector is the time spent in the communication of the signal, and in the adjustments of the instrument towards the right position in the sky. This can be achieved faster and precisely if the gravitational-wave detector is able to provide coordinates quickly and accurately.\\
A significant contribution to this goal could be added by the opening of the lower frequency window of ground-based gravitational-wave detectors. As seen in the previous section, the time to coalescence scales with frequency as $f^{-8/3}$. Lowering the frequency of observation would increase the time of observation before the coalescence. This would give more time for the electromagnetic detectors to adjust the position once they have received the coordinates. Moreover, the further the two inspiraling objects are from coalescence, the further they are from each other, thus increasing the volume of observation in the sky.
