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Scientific Report 2007-2009
Theoretical physics
T8. Physics of Gravitational Wave Sources
The first generation of interferometric detectors of
gravitational waves (GWs) is now operating at the design
sensitivity: the European detectors VIRGO and GEO,
and the American project LIGO, are taking data which
will be analyzed in coincidence. Upgrading of these detectors
have already started and the second generation,
the advanced (Virgo and LIGO) detectors, will have a
sensitivity enhanced by an order of magnitude. Furthermore,
a design study for an even more sensitive, 3 rd
generation of detectors is in progress. Theoretical and
phenomenological studies of GW sources are strongly
needed, since accurate templates of the expected signals
enhance chances of detection, and provide an instrument
to investigate the physics of the emitting source, establishing
the basis for a gravitational wave astronomy.
Our research consists in studying various aspects of
the physics of GW astrophysical sources. The main topics
under investigation are: (i) non-radial oscillations and
instabilities of young and old neutron stars, (ii) interaction
of stars and black holes in binary systems, (iii)
structure and deformations of strongly magnetized neutron
stars, (iv) stochastic background of GWs.
(i) Compact stars like neutron stars (NSs) are expected
to pulsate in damped oscillations (quasi-normal
modes), which are associated to the emission of GWs.
The detection of these signals will allow to measure the
oscillation frequencies and damping times, which carry
information on the structure of the star and on the equation
of state (EOS) of matter in its core. This would offer
a unique opportunity to study the behaviour of matter at
supranuclear density. Considering a number of EOSs of
nuclear matter recently proposed, and including the possibility
that the compact star is composed of deconfined
quark matter, we have carried out a systematic study
on the star pulsation frequencies. We have shown that
a GW-detection from a pulsating star will enable us to
establish whether the emitting source is a NS or a quark
star, and to constrain its EOS (see Fig. 1). In addition,
ν f ( kHz )
3.2
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
Neutron stars
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
M/M o
+
Strange stars
APR2
APRB200
APRB120
BBS1
G240
Figure 1: The frequency of the fundamental mode as a function
of the mass of the star, for neutron stars described by
different EOSs, and for quark stars (shadowed region).
we have studied the oscillations of rapidly rotating NSs.
We have developed a new method to study the oscillation
modes of rapidly rotating NSs [1], finding the effects of
rotation on the frequencies of the quasi-normal modes.
(ii) We have studied the tidal disruption of NSs by
black holes in coalescing binaries, evaluating the critical
orbital separation at which the star is disrupted by the
black hole tidal field, for several EOSs describing the
NS matter and for a large set of the binary parameters.
When the disruption occurs before the star reaches the
innermost stable circular orbit, the gravitational wave
signal emitted by the system exhibits a cutoff frequency,
which is a distinctive feature of the waveform. We have
evaluated this quantity and shown that, if found in a
detected gravitational wave, this frequency will allow to
determine the NS radius with an error of a few percent,
providing valuable information on the EOS.
(iii) After the discovery of the Soft Gamma Repeaters
and Anomalous X-ray Pulsars, it has been proposed that
these sources are neutron stars with extremely strong
magnetic fields; these magnetars would have a surface
field as large as 10 15 G, and internal fields up to 10 16 G.
A consistent fraction of the NSs should become magnetars
at some stage of their evolution, and since a strong
magnetic field induces a large deformation in the stellar
structure, these stars could be strong sources of gravitational
waves. We have constructed a general relativistic
model of magnetars [2], finding the structure of the
magnetic field, the stellar deformation it induces, and
evaluating the expected GW emission.
(iv) Ten years ago, in a series of papers we studied
the stochastic background of gravitational waves
generated by cosmological populations of astrophysical
sources. In the meantime, significant advances have
been done in astrophysical observation, and a more
accurate estimate of the star-formation rate history is
available; moreover more accurate GW waveforms and
estimates of formation rates for different sources have
been produced by numerical relativity studies. Thus, we
have started a research program to update our previous
work on the subject. At present, we have completed
the study of the GW stochastic background generated
by Population III and Population II stars, determining
the corresponding power spectral density and assessing
whether these backgrounds might act as foregrounds for
signals generated in the inflationary epoch [3].
References
1. V. Ferrari et al., Phys. Rev. D 76, 104033 (2007).
2. R. Ciolfi et al., MNRAS 397, 913 (2009).
3. S. Marassi et al., MNRAS 398, 293 (2009).
4. E. Berti et al., Phys. Rev. Lett. 103, 239001 (2009).
Authors
O. Benhar 1 , R. Ciolfi, A. Colaiuda, V. Ferrari, L. Gualtieri,
S. Marassi, F. Pannarale, M. Valli
http://www.roma1.infn.it/teongrav/
Sapienza Università di Roma 31 Dipartimento di Fisica