scholarly journals Role of Structure Scalars on the evolution of Compact Objects in Palatini f(R) Gravity

2021 ◽  
Author(s):  
M.Z. Bhatti ◽  
Z. Yousaf ◽  
Z. Tariq
Keyword(s):  
Compact Objects ◽  
Structure Scalars

Role of structure scalars on viscous and heat conducting spherical systems in f(R,T) gravity

2017 ◽  
Vol 26 (14) ◽  
pp. 1750155 ◽  
Author(s):  
T. Hussain ◽  
M. Khurshudyan ◽  
S. Ahmed ◽  
As. Khurshudyan
Keyword(s):  
Evolution Equations ◽  
Energy Momentum Tensor ◽  
Gravitational Theory ◽  
Riemann Tensor ◽  
Momentum Tensor ◽  
Compact Objects ◽  
Heat Conducting ◽  
Structure Scalars ◽  
Dynamical Features

In this paper, we analyze some dynamical features of spherical celestial objects through structure scalars in [Formula: see text] gravitational theory, where [Formula: see text] and [Formula: see text] are the Ricci scalar and the trace of energy–momentum tensor, respectively. In this framework, we consider our relativistic geometry to be spherical in shape filled with radiating viscous and shearing fluid content. We formulate extended version of structure scalars by orthogonal decomposition of the Riemann tensor with and without constant [Formula: see text] and [Formula: see text] backgrounds. We discuss the effects of dark source corrections on the construction of expansion and shear evolution equations via scalar variables. It is inferred that like general relativity, one can investigate the evolutionary stages of stellar compact objects with the help of extended scalar parameters.

scholarly journals Black hole and neutron star mergers in galactic nuclei: the role of triples

2019 ◽  
Vol 488 (2) ◽  
pp. 2825-2835 ◽  
Author(s):  
Giacomo Fragione ◽  
Nathan W C Leigh ◽  
Rosalba Perna
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Neutron Star ◽  
Massive Stars ◽  
Star Clusters ◽  
Galactic Nuclei ◽  
Compact Objects ◽  
Large Numbers ◽  
Binary Case

ABSTRACT Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are thought to contain large numbers of black holes (BHs) and neutron stars (NSs), a fraction of which form binaries and could merge by Kozai–Lidov oscillations (KL). Triple compact objects are likely to be present, given what is known about the multiplicity of massive stars, whose life ends either as an NS or a BH. In this paper, we present a new possible scenario for merging BHs and NSs in galactic nuclei. We study the evolution of a triple black hole (BH) or neutron star (NS) system orbiting an SMBH in a galactic nucleus by means of direct high-precision N-body simulations, including post-Newtonian terms. We find that the four-body dynamical interactions can increase the KL angle window for mergers compared to the binary case and make BH and NS binaries merge on shorter time-scales. We show that the merger fraction can be up to ∼5–8 times higher for triples than for binaries. Therefore, even if the triple fraction is only ∼10–$20\rm{\,per\,cent}$ of the binary fraction, they could contribute to the merger events observed by LIGO/VIRGO in comparable numbers.

scholarly journals Neutrino trapping in extremely compact Tolman VII spacetimes

2021 ◽  
Vol 81 (6) ◽  
Author(s):  
Zdeněk Stuchlík ◽  
Jan Hladík ◽  
Jaroslav Vrba ◽  
Camilo Posada
Keyword(s):  
Energy Density ◽  
Compact Objects ◽  
Schwarzschild Solution ◽  
Energy Density Distribution ◽  
Uniform Energy Density ◽  
Related Distribution ◽  
Density Radius ◽  
Neutrino Emissivity ◽  
Trapping Regions

AbstractExtremely compact objects trap gravitational waves or neutrinos, assumed to move along null geodesics in the trapping regions. The trapping of neutrinos was extensively studied for spherically symmetric extremely compact objects constructed under the simplest approximation of the uniform energy density distribution, with radius located under the photosphere of the external spacetime; in addition, uniform emissivity distribution of neutrinos was assumed in these studies. Here we extend the studies of the neutrino trapping for the case of the extremely compact Tolman VII objects representing the simplest generalization of the internal Schwarzschild solution with uniform distribution of the energy density, and the correspondingly related distribution of the neutrino emissivity that is thus again proportional to the energy density; radius of such extremely compact objects can overcome the photosphere of the external Schwarzschild spacetime. In dependence on the parameters of the Tolman VII spacetimes, we determine the “local” and “global” coefficients of efficiency of the trapping and demonstrate that the role of the trapping is significantly stronger than in the internal Schwarzschild spacetimes. Our results indicate possible influence of the neutrino trapping in cooling of neutron stars.

scholarly journals Fast Radio Bursts: An Extragalactic Enigma

2019 ◽  
Vol 57 (1) ◽  
pp. 417-465 ◽  
Author(s):  
James M. Cordes ◽  
Shami Chatterjee
Keyword(s):  
Radiation Energy ◽  
Compact Objects ◽  
Host Galaxy ◽  
Dispersion Measure ◽  
Radio Bursts ◽  
Host Galaxies ◽  
Alternative Source ◽  
Propagation Effects ◽  
Source Models

We summarize our understanding of millisecond radio bursts from an extragalactic population of sources. Fast radio bursts (FRBs) occur at an extraordinary rate, thousands per day over the entire sky with radiation energy densities at the source about ten billion times larger than those from Galactic pulsars. We survey FRB phenomenology, source models and host galaxies, coherent radiation models, and the role of plasma propagation effects in burst detection. The FRB field is guaranteed to be exciting: New telescopes will expand the sample from the current ∼80 unique burst sources (and only a few secure localizations and redshifts) to thousands, with burst localizations that enable host-galaxy redshifts emerging directly from interferometric surveys. ▪ FRBs are now established as an extragalactic phenomenon. ▪ Only a few sources are known to repeat. Despite the failure to redetect other FRBs, they are not inconsistent with all being repeaters. ▪ FRB sources may be new, exotic kinds of objects or known types in extreme circumstances. Many inventive models exist, ranging from alien spacecraft to cosmic strings, but those concerning compact objects and supermassive black holes have gained the most attention. A rapidly rotating magnetar is a promising explanation for FRB 121102 along with the persistent source associated with it, but alternative source models are not ruled out for it or other FRBs. ▪ FRBs are powerful tracers of circumsource environments, “missing baryons” in the intergalactic medium (IGM), and dark matter. ▪ The relative contributions of host galaxies and the IGM to propagation effects have yet to be disentangled, so dispersion measure distances have large uncertainties.

scholarly journals Very young neutron stars and millisecond pulsars: the role of the accretion

2012 ◽  
Vol 8 (S290) ◽  
pp. 231-232
Author(s):  
Alexander F. Kholtygin ◽  
Andrei P. Igoshev
Keyword(s):  
Magnetic Fields ◽  
Neutron Stars ◽  
Binary Systems ◽  
Compact Objects ◽  
Millisecond Pulsars ◽  
Know How ◽  
Central Compact Objects ◽  
Low Magnetic Field ◽  
Age Determinations

AbstractWe consider the evolution of the very young neutron stars (NS) with moderate and low magnetic field values around 1E8 G to know how large is the share of the these objects among the those attributed as the millisecond pulsars (MSP). To exclude the contamination of accreted NS and young NS with moderate magnetic fields we study the observational evidences of the accretion on NS in the binary systems and different methods of age determinations. It was concluded that only central compact objects are appropriate candidates for NSs with small initial magnetic fields.

MAGNETO-PLASMA PROCESSES IN RELATIVISTIC ASTROPHYSICS: MODERN DEVELOPMENTS

2013 ◽  
Vol 22 (07) ◽  
pp. 1330016
Author(s):  
OLEG YU. TSUPKO
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Gravitational Lensing ◽  
Crucial Role ◽  
Compact Objects ◽  
Relativistic Astrophysics ◽  
Core Collapse ◽  
Core Collapse Supernova ◽  
Supernova Explosions

This contribution is a review of some talks presented at the session "Magneto-Plasma Processes in Relativistic Astrophysics" of the Thirteenth Marcel Grossmann Meeting MG13. We discuss the modern developments of relativistic astrophysics, connected with presence of plasma and magnetic fields. The influence of magneto-plasma processes on the structure of the compact objects and accretion processes is considered. We also discuss a crucial role of magnetic field for the mechanism of core-collapse supernova explosions. Gravitational lensing in plasma is also considered.

scholarly journals Modeling the outcome of supernova explosions in binary population synthesis using the stellar compactness

2021 ◽  
Author(s):  
Maciej Dabrowny ◽  
Nicola Giacobbo ◽  
Davide Gerosa
Keyword(s):  
Black Holes ◽  
Neutron Stars ◽  
Binary Stars ◽  
Compact Objects ◽  
Population Synthesis ◽  
Subsequent Formation ◽  
Compact Binaries ◽  
Mass Gap ◽  
Supernova Explosions

AbstractFollowing the collapse of their cores, some of the massive binary stars that populate our Universe are expected to form merging binaries composed of black holes and neutron stars. Gravitational-wave observations of the resulting compact binaries can reveal precious details on the inner workings of the supernova mechanism and the subsequent formation of compact objects. Within the framework of the population-synthesis code mobse, we present the implementation of a new supernova model that relies on the compactness of the collapsing star. The model has two free parameters, namely the compactness threshold that separates the formation of black holes and that of neutron stars, and the fraction of the envelope that falls back onto the newly formed black holes. We compare this model extensively against other prescriptions that are commonly used in binary population synthesis. We find that the cleanest signatures of the role of the pre-supernova stellar compactness are (1) the relative formation rates of the different kinds of compact binaries, which mainly depend on the compactness threshold parameter, and (2) the location of the upper edge of the mass gap between the lightest black holes and the heaviest neutron stars, which mainly depends on the fallback fraction.

Role of modified scalar variables in the modeling of spherical fluids

2018 ◽  
Vol 15 (08) ◽  
pp. 1850140 ◽  
Author(s):  
A. Akram ◽  
A. Rehman Jami ◽  
S. Ahmad ◽  
M. Sufyan ◽  
R. Munir
Keyword(s):  
Degrees Of Freedom ◽  
Dust Cloud ◽  
Mass Function ◽  
Riemann Tensor ◽  
Ricci Scalar ◽  
Structure Scalars ◽  
Specific Distribution ◽  
Scalar Matter ◽  
Symmetric Geometry

The aim of this work is to analyze the role of shear evolution equation in the modeling of relativistic spheres in [Formula: see text] gravity. We assume that non-static diagonally symmetric geometry is coupled with dissipative anisotropic viscous fluid distributions in the presence of [Formula: see text] dark source terms. A specific distribution of [Formula: see text] cosmic model has been assumed and the spherical mass function through generic formula introduced by Misner-Sharp has been formulated. Some very important relations regarding Weyl scalar, matter variables and mass functions are being computed. After decomposing orthogonally the Riemann tensor, some scalar variables in the presence of [Formula: see text] extra degrees of freedom are calculated. The effects of the polynomial modified structure scalars in the modeling of through Weyl, shear, expansion scalar differential equations are investigated. The energy density irregularity factor has been calculated for both anisotropic radiating viscous with varying Ricci scalar and for dust cloud with present Ricci scalar corrections.

scholarly journals Role of tilted congruence and f(R) gravity on regular compact objects

2017 ◽  
Vol 95 (2) ◽  
Author(s):  
Z. Yousaf ◽  
Kazuharu Bamba ◽  
M. Zaeem-ul-Haq Bhatti
Keyword(s):  
Compact Objects

scholarly journals LOCALLY COMPACT OBJECTS IN EXACT CATEGORIES

2011 ◽  
Vol 22 (12) ◽  
pp. 1787-1821 ◽  
Author(s):  
LUIGI PREVIDI
Keyword(s):  
Compact Objects ◽  
Locally Compact ◽  
Exact Category ◽  
Mutual Relations ◽  
Exact Categories

We identify two categories of locally compact objects on an exact category [Formula: see text]. They correspond to the well-known constructions of the Beilinson category [Formula: see text] and the Kato category [Formula: see text]. We study their mutual relations and compare the two constructions. We prove that [Formula: see text] is an exact category, which gives to this category a very convenient feature when dealing with K-theoretical invariants, and study the exact structure of the category [Formula: see text] of Tate spaces. It is natural therefore to consider the Beilinson category [Formula: see text] as the most convenient candidate to the role of the category of locally compact objects over an exact category. We also show that the categories [Formula: see text], [Formula: see text] of countably indexed ind/pro-objects over any category [Formula: see text] can be described as localizations of categories of diagrams over [Formula: see text].

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