* Astronomy

Members Login
Post Info TOPIC: Neutron stars


L

Posts: 131433
Date:
RE: Neutron stars
Permalink  
 


Title: Why I-Love-Q
Author: Kent Yagi, Leo C. Stein, George Pappas, Nicolas Yunes, Theocharis A. Apostolatos

Black holes are said to have no hair because all of their multipole moments can be expressed in terms of just their mass, charge and spin angular momentum. The recent discovery of approximately equation-of-state-independent relations among certain multipole moments in neutron stars suggests that they are also approximately bald. We here explore the yet unknown origin for this universality. First, we investigate which region of the neutron star's interior and of the equation of state is most responsible for the universality. We find that the universal relation between the moment of inertia and the quadrupole moment is dominated by the star's outer-core, a shell of width (50-95)% of the total radius, which corresponds to the density range (10^14-10^15)g/cm³. Second, we study the impact on the universality of approximating stellar isodensity contours as self-similar ellipsoids. An analytical calculation in the Newtonian limit reveals that the shape of the ellipsoids does not affect the universal relations, but relaxing the self-similarity assumption can completely destroy it. Third, we investigate the eccentricity profiles of rotating relativistic stars and find that the ellipticity is roughly constant, with variations of roughly (20-30)% in the region that matters to the universal relations. Fourth, we repeat the above analysis for non-compact, regular stars and find that the ellipticity is not constant, with variations that easily exceed 100% and universality is lost. These findings suggest that universality arises as an emergent approximate symmetry: as one flows in the stellar-structure phase space from non-compact star region to the relativistic star region, the eccentricity variation inside stars decreases, leading to approximate self-similarity in their isodensity contours, which then leads to the universal behavior observed in their exterior multipole moments.

Read more (1058kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

NASA'S Swift Reveals New Phenomenon in a Neutron Star

Astronomers using NASA's Swift X-ray Telescope have observed a spinning neutron star suddenly slowing down, yielding clues they can use to understand these extremely dense objects.
Read more



__________________


L

Posts: 131433
Date:
Permalink  
 

Neutron Stars as Laboratories

From 6-10 May, astronomers from all over the world join in Amsterdam for a symposium about neutron stars. In debate centre Felix Meritis, they will discuss the most recent results of the research about neutron stars, collapsed cores of giant stars that have exploded as supernovas. Because of their extreme compact matter, neutron stars are excellent laboratories to explore extreme phenomena in space.
Read more



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: I-Love-Q forever
Authors: Andrea Maselli, Vitor Cardoso, Valeria Ferrari, Leonardo Gualtieri, Paolo Pani

Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia I, the Love number lambda and the spin-induced quadrupole moment Q of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal I-lambda-Q relations pave the way for a better understanding of neutron stars, most notably via gravitational-wave emission. Gravitational-wave observations will probe highly-dynamical binaries and it is important to understand whether the universality of the I-lambda-Q relations survive strong-field and finite-size effects. We apply a Post-Newtonian-Affine approach to model tidal deformations in compact binaries and show that the I-lambda relation depends on the inspiral frequency, but is insensitive to the equation of state. We provide a fit for the universal relation, which is valid up to a gravitational wave frequency of ~900 Hz and accurate to within a few percent. Our results strengthen the universality of I-lambda-Q relations, and are relevant for gravitational-wave observations with advanced ground-based interferometers. We also discuss the possibility of using the Love-compactness relation to measure the neutron-star radius with an uncertainty of about 10% or smaller from gravitational-wave observations.

Read more (243kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: I-Love-Q
Authors: Kent Yagi, Nicolas Yunes

Neutron stars are not only described by their mass and radius, but also by how fast they spin (moment of inertia) and how much they can be deformed (Love number and quadrupole moment). These depend sensitively on the star's internal structure. We find universal relations between the moment of inertia, the Love number and the quadrupole moment that are independent of the equation of state. These relations can be used to learn observationally about neutron star deformations, break degeneracies in gravitational wave observations, and test General Relativity independently of nuclear-structure.

Read more (56kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The Neutron Star Zoo
Authors: Alice K. Harding

Neutron stars are a very diverse population, both in their observational and their physical properties. They prefer to radiate most of their energy at X-ray and gamma-ray wavelengths. But whether their emission is powered by rotation, accretion, heat, magnetic fields or nuclear reactions, they are all different species of the same animal whose magnetic field evolution and interior composition remain a mystery. This article will broadly review the properties of inhabitants of the neutron star zoo, with emphasis on their high-energy emission.

Read more (2052kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: How "free" are free neutrons in neutron-star crusts and what does it imply for pulsar glitches ?
Authors: N. Chamel

The neutron superfluid permeating the inner crust of mature neutron stars is expected to play a key role in various astrophysical phenomena like pulsar glitches. Despite the absence of viscous drag, the neutron superfluid can still be coupled to the solid crust due to non-dissipative entrainment effects. Entrainment challenges the interpretation of pulsar glitches and suggests that a revision of the interpretation of other observed neutron-star phenomena might be necessary.

Read more (23kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Lev Landau and the conception of neutron stars
Authors: Dmitry G. Yakovlev, Pawel Haensel, Gordon Baym, Christopher J. Pethick

We review the history of neutron star physics in the 1930s that is related to L. Landau. According to recollections of Rosenfeld (1974, Proc. 16th Solvay Conference on Physics, p. 174), Landau improvised the concept of neutron stars in a discussion with Bohr and Rosenfeld just after the news of the discovery of the neutron reached Copenhagen in February 1932. We present arguments that the discussion took place in March 1931, before the discovery of the neutron, and that they in fact discussed the paper written by Landau in Zurich in February 1931 but not published until February 1932 (Phys. Z. Sowjetunion, 1, 285). In his paper Landau mentioned the possible existence of dense stars which look like one giant nucleus; this can be regarded as an early theoretical prediction or anticipation of neutron stars, prior to the discovery of the neutron. The coincidence of the dates of the neutron's discovery and the paper's publication has led to an erroneous association of the paper with the discovery of the neutron. In passing, we outline the contribution of Landau to the theory of white dwarfs and to the hypothesis of stars with neutron cores.

Read more (2212kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Modelling pulsar time noise with long term power law decay modulated by short term oscillations of the magnetic fields of neutron stars
Authors: Shuang-Nan Zhang, Yi Xie

We model the evolution of the magnetic fields of neutron stars as consisting of a long term power-law decay modulated by short term small amplitude oscillations. Our model predictions on the timing noise \ddot\nu of neutron stars agree well with the observed statistical properties and correlations of normal radio pulsars. Fitting the model predictions to the observed data, we found that their initial parameter implies their initial surface magnetic dipole magnetic field strength ~ 5E14 G at ~0.4 year old and that the oscillations have amplitude between E-8 to E-5 and period on the order of years. For individual pulsars our model can effectively reduce their timing residuals, thus offering the potential of more sensitive detections of gravitational waves with pulsar timing arrays. Finally our model can also re-produce their observed correlation and oscillations of the second derivative of spin frequency, as well as the "slow glitch" phenomenon.

Read more (576kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Stellar astrophysics helps to explain the behaviour of fast rotating neutron stars in binary systems

Pulsars are among the most exotic celestial bodies known. They have diameters of about 20 kilometres, but at the same time roughly the mass of our sun. A sugar-cube sized piece of its ultra-compact matter on the Earth would weigh hundreds of millions of tons. A sub-class of them, known as millisecond pulsars, spin up to several hundred times per second around their own axes. Previous studies reached the paradoxical conclusion that some millisecond pulsars are older than the universe itself. The astrophysicist Thomas Tauris from the Max Planck Institute for Radio Astronomy and the Argelander Institute for Astronomy in Bonn could resolve this paradox by computer simulations. Through numerical calculations on the base of stellar evolution and accretion torques, he demonstrated that millisecond pulsars loose about half of their rotational energy during the final stages of the mass-transfer process before the pulsar turns on its radio beam. This result is in agreement with current observations and the findings also explain why radio millisecond pulsars appear to be much older than the white dwarf remnants of their companion stars - and perhaps why no sub-millisecond radio pulsars exist at all. The results are reported in the February 03 issue of the journal "Science".
Read more

__________________
1 2 3  >  Last»  | Page of 3  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard