Quasi-normal modes of superfluid neutron stars

We study non-radial oscillations of neutron stars with superfluid baryons, in a general relativistic framework, including finite temperature effects. Using a perturbative approach, we derive the equations describing stellar oscillations, which we solve by numerical integration, employing different models of nucleon superfluidity, and determining frequencies and gravitational damping times of the quasi-normal modes. As expected by previous results, we find two classes of modes, associated to superfluid and non-superfluid degrees of freedom, respectively. We study the temperature dependence of the modes, finding that at specific values of the temperature, the frequencies of the two classes of quasi-normal modes show avoided crossings, and their damping times become comparable. We also show that, when the temperature is not close to the avoided crossings, the frequencies of the modes can be accurately computed by neglecting the coupling between normal and superfluid degrees of freedom. Our results have potential implications on the gravitational wave emission from neutron stars.Comment: 16 pages, 7 figures, 2 table

Dissipation in relativistic superfluid neutron stars

We analyze damping of oscillations of general relativistic superfluid neutron stars. To this aim we extend the method of decoupling of superfluid and normal oscillation modes first suggested in [Gusakov & Kantor PRD 83, 081304(R) (2011)]. All calculations are made self-consistently within the finite temperature superfluid hydrodynamics. The general analytic formulas are derived for damping times due to the shear and bulk viscosities. These formulas describe both normal and superfluid neutron stars and are valid for oscillation modes of arbitrary multipolarity. We show that: (i) use of the ordinary one-fluid hydrodynamics is a good approximation, for most of the stellar temperatures, if one is interested in calculation of the damping times of normal f-modes; (ii) for radial and p-modes such an approximation is poor; (iii) the temperature dependence of damping times undergoes a set of rapid changes associated with resonance coupling of neighboring oscillation modes. The latter effect can substantially accelerate viscous damping of normal modes in certain stages of neutron-star thermal evolution.Comment: 25 pages, 9 figures, 1 table, accepted for publication in MNRA

Non-radial oscillation modes as a probe of density discontinuities in neutron stars

A phase transition occurring in the inner core of a neutron star could be associated to a density discontinuity that would affect the frequency spectrum of the non-radial oscillation modes in two ways. Firstly, it would produce a softening of the equation of state, leading to more compact equilibrium configurations and changing the frequency of the fundamental and pressure modes of the neutron star. Secondly, a new non-zero frequency g-- mode would appear, associated to each discontinuity. These discontinuity g--modes have typical frequencies larger than those of g--modes previously studied in the literature (thermal, core g-- modes, or g--modes due to chemical inhomogeneities in the outer layers), and smaller than that of the fundamental mode; therefore they should be distinguishable from the other modes of non radial oscillation. In this paper we investigate how high density discontinuities change the frequency spectrum of the non-radial oscillations, in the framework of the general relativistic theory of stellar perturbations. Our purpose is to understand whether a gravitational signal, emitted at the frequencies of the quasi normal modes, may give some clear information on the equation of state of the neutron star and, in particular, on the parameters that characterize the density discontinuity. We discuss some astrophysical processes that may be associated to the excitation of these modes, and estimate how much gravitational energy should the modes convey to produce a signal detectable by high frequency gravitational detectors.Comment: submitted to MNRA

How to cluster in parallel with neural networks

Partitioning a set of N patterns in a d-dimensional metric space into K clusters - in a way that those in a given cluster are more similar to each other than the rest - is a problem of interest in astrophysics, image analysis and other fields. As there are approximately K(N)/K (factorial) possible ways of partitioning the patterns among K clusters, finding the best solution is beyond exhaustive search when N is large. Researchers show that this problem can be formulated as an optimization problem for which very good, but not necessarily optimal solutions can be found by using a neural network. To do this the network must start from many randomly selected initial states. The network is simulated on the MPP (a 128 x 128 SIMD array machine), where researchers use the massive parallelism not only in solving the differential equations that govern the evolution of the network, but also by starting the network from many initial states at once, thus obtaining many solutions in one run. Researchers obtain speedups of two to three orders of magnitude over serial implementations and the promise through Analog VLSI implementations of speedups comensurate with human perceptual abilities

Gravitational signals emitted by a point mass orbiting a neutron star: effects of stellar structure

The effects that the structure of a neutron star would have on the gravitational emission of a binary system are studied in a perturbative regime, and in the frequency domain. Assuming that a neutron star is perturbed by a point mass moving on a close, circular orbit, we solve the equations of stellar perturbations in general relativity to evaluate the energy lost by the system in gravitational waves. We compare the energy output obtained for different stellar models with that found by assuming that the perturbed object is a black hole with the same mass, and we discuss the role played by the excitation of the stellar modes. Ouresults indicate that the stellar structure begins to affect the emitted power when the orbital velocity is v >0.2c (about 185 Hz for a binary system composed of two canonical neutron stars). We show that the differences between different stellar models and a black hole are due mainly to the excitation of the quasinormal modes of the star. Finally, we discuss to what extent and up to which distance the perturbative approach can be used to describe the interaction of a star and a pointlike massive body.Comment: 22 pages, 6 figures, to appear in Phys. Rev. D. Revised version, added one table and extended discussio

Gravitational waves from extreme mass-ratio inspirals in Dynamical Chern-Simons gravity

Dynamical Chern-Simons gravity is an interesting extension of General Relativity, which finds its way in many different contexts, including string theory, cosmological settings and loop quantum gravity. In this theory, the gravitational field is coupled to a scalar field by a parity-violating term, which gives rise to characteristic signatures. Here we investigate how Chern-Simons gravity would affect the quasi-circular inspiralling of a small, stellar-mass object into a large non-rotating supermassive black hole, and the accompanying emission of gravitational and scalar waves. We find the relevant equations describing the perturbation induced by the small object, and we solve them through the use of Green's function techniques. Our results show that for a wide range of coupling parameters, the Chern-Simons coupling gives rise to an increase in total energy flux, which translates into a fewer number of gravitational-wave cycles over a certain bandwidth. For space-based gravitational-wave detectors such as LISA, this effect can be used to constrain the coupling parameter effectively.Comment: RevTex4, 18 pages, 7 figures, 1 tabl

Black hole particle emission in higher-dimensional spacetimes

In models with extra dimensions, a black hole evaporates both in the bulk and on the visible brane, where standard model fields live. The exact emissivities of each particle species are needed to determine how the black hole decay proceeds. We compute and discuss the absorption cross-sections, the relative emissivities and the total power output of all known fields in the evaporation phase. Graviton emissivity is highly enhanced as the spacetime dimensionality increases. Therefore, a black hole loses a significant fraction of its mass in the bulk. This result has important consequences for the phenomenology of black holes in models with extra dimensions and black hole detection in particle colliders.Comment: 4 pages, RevTeX 4. v3: Misprints in Tables correcte

The effect of zr addition on melting temperature, microstructure, recrystallization and mechanical properties of a cantor high entropy alloy

The effect of Zr addition on the melting temperature of the CoCrFeMnNi High Entropy Alloy (HEA), known as the “Cantor’s Alloy”, is investigated, together with its micro-structure, mechanical properties and thermomechanical recrystallization process. The base and Zr-modified alloys are obtained by vacuum induction melting of mechanically pre-alloyed powders. Raw materials are then cold rolled and annealed. recrystallization occurred during the heat treatment of the cold-rolled HEA. The alloys are characterized by X-ray diffraction, electron microscopy, thermal analyses, mechanical spectroscopy and indentation measures. The main advantages of Zr addition are: (1) a fast vacuum induction melting process; (2) the lower melting temperature, due to Zr eutectics formation with all the Cantor’s alloy elements; (3) the good chemical alloy homogeneity; and (4) the mechanical properties improvement of re-crystallized grains with a coherent structure. The crystallographic lattice of both alloys results in FCC. The Zr-modified HEA presents a higher recrystallization temperature and smaller grain size after recrystallization with respect to the Cantor’s alloy, with precipitation of a coherent second phase, which enhances the alloy hardness and strength

Multi-mode TES bolometer optimization for the LSPE-SWIPE instrument

In this paper we explore the possibility of using transition edge sensor (TES) detectors in multi-mode configuration in the focal plane of the Short Wavelength Instrument for the Polarization Explorer (SWIPE) of the balloon-borne polarimeter Large Scale Polarization Explorer (LSPE) for the Cosmic Microwave Background (CMB) polarization. This study is motivated by the fact that maximizing the sensitivity of TES bolometers, under the augmented background due to the multi-mode design, requires a non trivial choice of detector parameters. We evaluate the best parameter combination taking into account scanning strategy, noise constraints, saturation power and operating temperature of the cryostat during the flight.Comment: in Journal of Low Temperature Physics, 05 January 201

Influence of Different Filler Metals on the Mechanical and Microstructural Characteristics of Arc-Welded Joints Made of Dissimilar Titanium Alloys

In the motorsport industry, the choice of material for manufacturing the heat resistant components often falls on titanium alloys. In most cases, the production flow for this kind of part involves CNC machining and subsequent assembly by welding process, to other parts obtained by cold plastic forming and possibly made using different titanium alloys. Hence, the alloying element-content in the joint area can be extremely heterogeneous and variable point-by-point. To investigate this topic further, dissimilar welding of the alpha/beta alloy Ti6Al4V and of the oxidation-resistant alpha alloy KS-Ti 1.2 ASN-EX was made by GTAW technology and using different filler metals. Chemical and mechanical properties of the welds were investigated by XRD, SEM-EDS, microhardness maps, and tensile and bending tests. Results show that, despite the different alloying elements present in the two filler wires investigated, static properties of the welds are similar. Results also show that the local V/Al content ratio affects the microhardness as it is responsible for the creation of supersaturated alpha phases during the cooling of the weld beads