Purpose of the OG51 collaboration is modeling gravitational wave (GW) sources
through theoretical studies and numerical simulations. As will be clear from
what follows, several topics are common to different nodes and are the subject
of intense collaborations. In what follows the scientific activity of each node
will be briefly summarized.

ROMA I
The activity in Rome essentially regards: 1) the PHYSICS OF NEUTRON STARS
(NS's), exploring all processes that are relevant to GW emission, and 2)
the STUDY OF SIGNALS EMITTED BY BINARY SYSTEMS.
About 1) we have computed frequencies and damping times at which a star made of
quark matter oscillates and emits GWs. We show that the combined knowledge of
these frequencies and of the star mass (or the star radiation radius) allows one
to discriminate between strange stars and NS and set stringent bounds on the bag
constant. We have started the study of neutrino emission rates in dense nuclear
matter, whose knowledge is relevant to GW emission and to the cooling of newly
born NSs, using a non relativistic many body theory approach. The same
formalism and dynamical model is currently being applied to the calculation of
the shear viscosity of nuclear matter, which is crucial to evaluate the amount
of energy emitted in GWs. Next year we also plan to construct reliable models of
NSs with a strong magnetic field, including toroidal fields, meridional
currents, stellar rotation, a magnetosphere, and a superconducting core,
contributions that are either partially neglected in the literature, or treated
in a Newtonian framework unappropriate to describe a NS.
About 2), for the first time we have computed the GW-signal emitted as a
consequence of the tidal deformation of a white dwarf (m=1 Msun) which moves
close to a black hole (BH) (M=10 Msun) on various orbits using the affine model
approach introduced by Carter and Luminet in 1983. In this work, the tidal
tensor and the orbits have been computed using Newtonian gravity. We will now
extend this study to a general relativistic framework. This will allow to treat
much closer encounters bewteen stars and black holes than those considered in
the Newtonian approach and compute the much stronger emitted GW-signal.
Moreover, we have developed a new method to compute the GW-signal emitted by a
black hole perturbed by extended sources, which couples the nonlinear
hydrodynamics equations to the relativistic wave equations which describe black
hole perturbations in the frequency domain. The method proves to be very
accurate and has been applied to compute the GW-signal emitted by an oscillating
high-density torus, formed in the aftermath of a gravitational collapse,
orbiting around a black hole. Besides the points mentioned before, next year we
plan to compute, using appropriate models, the frequencies of the L=1 unstable
g-modes which, as shown by recent simulations of Supernova explosions, can
strongly be excited in the deep core of a newly born proto-neutron stars. This
dipole instability, which we plan to study, may be source of a strong
GW-emission, if the coupling of the dipolar unstable mode with other
non-axisymmetric modes is significant.

TRIESTE
Trieste's work is focused on studying the physics of black holes and neutron
stars, as possible sources of GWs. In the past years the group has played a
leading role in developing the "Whisky" code for making 3D general relativistic
hydrodynamical simulations. Its first major application has been for studying
gravitational collapse to form black holes, following the matter dynamics,
appearance of trapped surfaces and GW-emission. The codes has also been used to
1) investigate the nonlinear dynamics of high-density thick tori around black
holes with 2D numerical simulations, showing that these systems may either
become unstable to the runaway instability or exhibit oscillatory behaviour
leading to strong GW-emission. 2) Study the merger of binary systems consisting
of a stellar-mass BH and a NS. Preliminary calculations for mergers of these
systems have been made and work is in progress to calculate the GW-emission. 3)
Study extreme mass ratio inspirals which would allow to map the black hole
spacetime using LISA data. Whisky has now been extended to include general
relativistic magneto-hydrodynamics and will be used to compute the dynamical
growth of bar-mode instabilities during collapse, which could provide a powerful
source of GWs, and the behaviour of high-density discs around black holes. This
is of interest in connection with gamma-ray bursters and possible associated
GW-emission. We have also been studying the oscillation modes of NSs
investigating the effects on them of strong internal magnetic fields or very
rapid rotation. Next year we will be focusing on NS inertial-mode oscillations,
taking into account the possible presence of strong magnetic fields, solid
crusts and superfluid interiors. We aim to obtain improved model calculations so
as to understand better the implications of future observations for
asteroseismology. We will also study possible GW production resulting from
phase transitions.

PARMA
The activity in Parma has focused on the numerical study of the bar-mode
dynamical instability of rapidly rotating compact stars in full General
Relativity (GR). This instability is characterized by the formation of
spiral-arms and we find that the resulting bar-like deformation is not
persistent. Moreover, we show that imposing an initial perturbation near the
threshold for the instability onset the dynamics slightly changes and that the
non-persistence of the bar-like deformation is mainly due to non linear mode
coupling and to the growth of odd-modes, mainly the m=1 one. We were able to
precisely determine, for the first time, the critical value of the parameter
beta=(rotational kinetic energy)/(gravitational binding energy) for the onset of
the instability in full GR. In addition we have studied various ways to extract
the GW-signal emitted by the quasi-periodic oscillations of high density thick
accretion disks orbiting a Schwarzschild black hole. Next year the group will
proceed on the study of non-axisymmetric instabilities of compact objects and
their role as a possible source of GW-emission during stellar core collapse. In
particular we will further study the bar-mode instability, since it provides a
very effective way of generating strong quadrupole deformations in axisymmetric
stellar configurations and therefore it gives a very efficient mechanism for
generating GWs. In particular we will study the dependence of the threshold for
the instability onset on the stellar compactness, on the velocity profile and on
the equation of state of nuclear matter. Parma's group also plans to study the
non-linear non-axisymmetric pulsation of rotating (and differentially rotating)
NSs in full GR in the time domain using 3D simulations.

TORINO
In Torino, Einstein's equations are solved using Cactus/Whisky numerical
codes. These codes are being developed in collaboration with LSU-Baton Rouge and
the A. Einstein Institute in Potsdam (AEI). Torino's group has contributed to
developing and testing of parts of these codes. Last year stellar oscillations
have been studied solving Einstein's equations perturbed to second order: we
studied the effects of radial and non-radial mode coupling for odd-parity
perturbations, and we found that when the frequency of the radial oscillations
is close to the w-mode frequency, the second order part of the signal is
amplified due to the non-linear coupling. Moreover, using the Effective One Body
(EOB)approach developed by Buonanno and Damour, assuming extreme mass ratio and
zero spin, we have computed analytically the GW-signal emitted by a binary black
hole merger up to when the two bodies are so close that their quasi normal modes
are excited. In collaboration with Valencia and AEI, we have completed the
study of GWs generated by fluid matter accretion onto a Schwarzschild black hole
using test fluid approximation and perturbation theory; in collaboration with
M. Tiglio, LSU, we have started a study of perturbations of Kerr black holes and
we plan to investigate the plunge of a massive particle with a trajectory driven
by radiation reaction. Furthermore, an analysis has been performed aimed at
classifying general relativistic effects which can be revealed in the signals
from pulsars in binary systems, with the aim of finding a signature of black
holes as being the compact companions of the pulsar. Next year using
Cactus/Whisky we will compare, from the same set of initial data, non-linear
evolution of oscillation modes of stars and black hole with perturbative
evolution described by a linear code written in the past by
A. Nagar. Furthermore, we will extend our previous work on second order, odd,
stellar perturbations to even-parity perturbations. We will extend the EOB
approach to the equal mass case and further study, using Kerr-Schild
(horizon-penetrating) coordinates, the hypercritical accretion flows onto a
black hole and the induced excitation of the oscillation modes.

FERRARA
We have studied the motion of a binary system, composed by two massive objects,
in the field of a supermassive black hole (SMBH). The relevant equations have
been numerically integrated in the case when the SMBH is a non rotating,
Schwarzschild black hole, but we are starting to investigate how to integrate
the corresponding equations for a rotating (Kerr) SMBH. Next year we shall
evaluate the gravitational signal emitted by these systems (compact binaries +
SMBH) and extend our calculations to the case of rotating SMBHs, since
astronomical observations indicate that the SMBH sitting at the center of our
galaxy is indeed a Kerr black hole with angular momentum per unit mass a ~ 0.22
in geometrical units.
Subsequently, we shall extend these results to a system of 10^4 compact binaries
orbiting the central black hole, which has been indicated to contribute to the
X-ray emission of the central region of our galaxy. This system of binaries is
expected to contribute to the stocastic background of gravitational waves of
galactic origin.