LF chapter finished

main/LF.tex

@@ -30,7 +30,9 @@ Currently, the ground-based observatories are tuned to detect binary systems sou
 The first detection of gravitational waves happened on the 14th September 2015 and confirmed the Theory General Relativity, opening a new window on the Universe: the signal from a merger of two black holes have been observed thanks to the emission of gravitational waves, confirming the existence of these objects, still mostly unknown \cite{first}. The detector responsible of the new discovery is based in the USA and it is one of the terrestrial interferometers currently in use for gravitational waves detection \footnote{The working principles of the interferometers and details about the US instrument are exposed in Chapter \ref{LIGO}}.
 
 \section{Opening the low frequency window}
-As we will see in the next chapter, the grund-based detectors involved in the search of gravitational waves are cover a wide range of frequencies, but they are affected by some noises which make them unable to detect waves from sources emitting below 30 Hz. We will see later the nature of these noises. The reason why it is important to open the lower frequency window is that it can give access to the detection of gravitational waves emitted by sources which physical structure and astrophysical features are still unknown.\\
+As we will see in the next chapter, the ground-based detectors involved in the search of gravitational waves cover a wide range of frequencies, but they are affected by some noises which make them unable to detect waves from sources emitting below 30 Hz. We will see later the nature of these noises. The reason why it is important to open the lower frequency window is that it can give access to the detection of gravitational waves emitted by sources which physical structure and astrophysical features are still unknown.\\
+
+\noindent
 This is the effort towards which a huge part of the scientific collaboration is involved.
 
 \subsection{Frequencies of emission}
@@ -42,11 +44,11 @@ We saw in the previous chapter that the emitted amplitude depends on the masses
 \end{equation}
 
 \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 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}.}.\\
+This equation is particularly useful if we want to know information about the radiation emitted by 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 \cite{branchesi}.\\
+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 are 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. 
 
-\subsection{Redshift}
+\subsection{Redshifted frequencies}
 When dealing with cosmological objects, we need to take into account the contribution of the redshift: in the case of gravitational waves, the redshift acts on the observed frequency. In a cosmological context, the time-scale is redshifted, and so it is the frequency observed $f_{obs}$ with respect to the emitted one $f_{em}$ by:
 
 \begin{equation}
@@ -58,10 +60,10 @@ f_{obs} = f_{em}/(1+z).
 The implication of this effect lies in a factor (1+z) multiplied to the masses involved.\\
 An important consequence is that if the instrument could be able to detect in a broader range of lower frequencies, it could be 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}
-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 to localize and study the source under several other points of view.\\
+\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 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 if the gravitational-wave detector is able to provide a location quickly and accurately.\\
+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 if the gravitational-wave detector is able to provide a location 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 see 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 received the coordinates. Moreover, the more the two inspirilling objects are far from coalescence, the more are they far from each other, increasing the volume of observation in the sky.
 
 
@@ -77,14 +79,14 @@ N_c = \int f_{gw}(t) dt.
 This quantity defines for how many cycles (and hence how much time) the detector can follow the evolution of a signal in a given frequency band. The ground-based detectors are sensitive to operate for thousands of cycles \footnote{Interesting examples on typical duty cycles for ground- and space-based detectors can be found in \cite{mag}.}. Lowering the frequency band and increasing the sensitivity would increase the number of cycles, allowing to follow a signal for more time \cite{mag}. The consequent advantage is more precise waveform predictions based on these observations, besides to the detection of objects still unknown.
 
 \section{The goals of the gravitational-wave collaboration}
-The efforts of the scientific collaboration towards the opening of the low frequency window are devoted to the development of new technologies for active control of the noise sources responsible of the lack of sensitivity below 30 Hz \cite{lf1} \cite{lf2} \cite{lf3}. This has been the target focussed on during the workshops dedicated to the low frequency band, which the author attended between 2018 and 2021.\\
+The efforts of the scientific collaboration, towards the opening of the low frequency window, are devoted to the development of new technologies for active control of the noise sources, responsible of the lack of sensitivity below 30 Hz \cite{lf1} \cite{lf2} \cite{lf3}. This has been the target focussed on during the workshops dedicated to the low frequency band, which the author attended between 2018 and 2021.\\
 The goal of these meetings is to update the state-of-the-art of the topic and work together on new ideas and possible new solutions.\\
-The strategy investigated is based on noise subtraction: in particular, modelling, controls and reduction of the noise of seismic platforms are currently under exam for increasing the sensitivity below 30 Hz. Besides this, the study for  the development of lower-noise sensors is also an up-to-date topic of discussion.\\
+We will see in the next chapters that one of the most important noise sources, affecting the detectors in the low frequency range, is the seismic noise. The strategy investigated is based on the subtraction of this noise source: in particular, modelling, controls and reduction of the noise of seismic platforms are currently under exam for increasing the sensitivity below 30 Hz. Besides this, the study for  the development of lower-noise sensors is also an up-to-date topic of discussion.\\
 
 \noindent
-The importance of the improvement of seismic motion has been widely outlined and highlighted \cite{lantztalk}: the final goal is to reduce the noise coupling into the gravitational-wave signal and an important contribution could be provided by the efforts of the people working on the seismic noise suppression. \\
+The importance of the improvement of seismic motion has been widely outlined and highlighted \cite{lantztalk}: the final goal is to reduce the noise coupling into the gravitational-wave signal, and an important contribution could be provided by the efforts of the people working on the seismic noise suppression. \\
 
 \noindent
-It is in this frame that the work exposed in this thesis finds place. The experiments carried on cover both the studies for noise suppression of seismic platforms on gravitational-wave detectors and the development of new devices for sensing seismic motion.
+It is in this frame that the work exposed in this thesis finds place. The experiments carried on cover both the studies for noise suppression of seismic platforms on gravitational-wave detectors, and the development of new devices for sensing and reduce seismic motion.
 
 

main/main.tex

@@ -195,6 +195,11 @@ There are three appendices useful to make the work more complete: appendix A ill
 
 \bibitem{bird} S. Bird et al., \textit{Did LIGO detect dark matter?}, Phys. Rev. Lett. 116, 201301, 2016
 
+\bibitem{branchesi} M. Branchesi, \textit{Multi-messenger astronomy: gravitational waves, neutrinos, photons, and cosmic rays}, 2016 J. Phys.: Conf. Ser. 718 022004
+
+\bibitem{multi} LIGO Scientific Collaboration and Virgo Collaboration, Fermi GBM, INTEGRAL, IceCube Collaboration, AstroSat Cadmium Zinc Telluride Imager Team, IPN Collaboration, The Insight-Hxmt Collaboration, ANTARES Collaboration, The Swift Collaboration, AGILE Team, The 1M2H Team, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, GRAWITA: GRAvitational Wave Inaf TeAm, The Fermi Large Area Telescope Collaboration, ATCA: Australia Telescope Compact Array, ASKAP: Australian SKA Pathfinder, Las Cumbres Observatory Group, OzGrav, DWF (Deeper, Wider, Faster Program), AST3, and CAASTRO Collaborations, The VINROUGE Collaboration, MASTER Collaboration, J-GEM, GROWTH, JAGWAR, CaltechNRAO, TTU-NRAO, and NuSTAR Collaborations, Pan-STARRS, The MAXI Team, TZAC Consortium, KU Collaboration, Nordic Optical Telescope, ePESSTO, GROND, Texas Tech University, SALT Group, TOROS: Transient Robotic Observatory of the South
+Collaboration, The BOOTES Collaboration, MWA: Murchison Widefield Array, The CALET Collaboration, IKI-GW Follow-up Collaboration, H.E.S.S. Collaboration, LOFAR Collaboration, LWA: Long Wavelength Array, HAWC Collaboration, The Pierre Auger Collaboration, ALMA Collaboration, Euro VLBI Team, Pi of the Sky Collaboration, The Chandra Team at McGill University, DFN: Desert Fireball Network, ATLAS, High Time Resolution Universe Survey, RIMAS and RATIR, and SKA South Africa/MeerKAT \textit{Multi-messenger Observations of a Binary Neutron Star Merger} ApJL, 848:L12, 2017
+
 \bibitem{lf1} P. Fritschel, B. Lantz, \textit{Report on the workshop on Low Frequency Noise in LIGO, April 6 -7, 2021}, Low Frequency Workshop, April 2021 DCC L2100055
 
 \bibitem{lf2} ERC Workshop, Frebruary 2021