Compact Objects Research Papers - Academia.edu (original) (raw)

Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate, and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, "The Physics of Stars", sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with "Stars and Statistical Physics" by Balian and Blaizot and "Neutron Stars for Undergraduates" by Silbar and Reddy, is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the Summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars.

The well-known Regge-Wheeler equation describes the axial perturbations of Schwarzschild metric in the linear approximation. From a mathematical point of view it presents a particular case of the confluent Heun equation and can be solved exactly, due to recent mathematical developments. We present the basic properties of its general solution. A novel analytical approach and numerical techniques for study the boundary problems which correspond to quasi-normal modes of black holes and other simple models of compact objects are developed.

- by Plamen Fiziev
- •
- Black Holes, Quantum Cosmology, Compact Objects, Cosmology

Black holes are extremely relativistic objects. Physical processes around them
occur in a regime where the gravitational field is extremely intense. Under such conditions,
our representations of space, time, gravity, and thermodynamics are pushed
to their limits. In such a situation philosophical issues naturally arise. In this chapter
I review some philosophical questions related to black holes. In particular, the relevance
of black holes for the metaphysical dispute between presentists and eternalists,
the origin of the second law of thermodynamics and its relation to black holes, the
problem of information, black holes and hypercomputing, the nature of determinisim,
and the breakdown of predictability in black hole space-times. I maintain that black
hole physics can be used to illuminate some important problems in the border between
science and philosophy, either epistemology and ontology.

- by Gustavo E Romero
- •
- Theoretical Physics, Philosophy, Philosophy of Science, Philosophy of Physics

There are many observational evidences for the existence of massive compact condensations in the range 106−1010Modot10^6 -10^{10} M_\odot106−1010Modot at the core of various galaxies. At present such condensations are commonly interpreted as Black Holes (BHs). However, we point out that while such Black Hole Candidates (BHCs) must be similar to BHs in many respects they, actually, can not be BHs because existence of Black Holes would violate the basic tenet of the General Theory of Relativity (GTR) that the worldline of a material particle must be timelike at any regular region of spacetime. On the other hand general relativistic collapse of very massive bodies should lead to Eternally Collapsing Configurations (ECOs) or Ultra Compact Objects (UCOs). While ECOs/UCOs may practically be as compact as corresponding BHs, they will have a physical surface. Also while BHs do not have any intrinsic magnetic field ECOs may have strong intrinsic magnetic field. We point out that despite many claims actually there i...

- by Abhas Mitra
- •
- Physics, General Relativity, Theoretical astrophysics, Active Galactic Nuclei

In this paper, have been found new class of solutions to the Einstein-Maxwell system for charged anisotropic matter which are relevant in the description of highly compact stellar objects. The equation of state is barotropic with a linear relation between the radial pressure and the energy density and we have considered a prescribed form for the gravitational potential Z. Variables as the energy density, radial pressure and the metric coefficients are written in terms of elementary and polynomial functions. The obtained models not admit singularities in the matter and the charge density.

- by Robert T Jantzen
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- Physics, Quantum Gravity, Special Relativity, Compact Objects
- by Scott Hughes
- •
- Quantum Physics, Monte Carlo Simulation, Numerical Relativity, Compact Objects
- by Manjari Bagchi
- •
- Mechanical Engineering, Aerospace Engineering, Space, Compact Objects

- by Plamen Fiziev
- •
- Physics, Black Holes, Quantum Cosmology, Compact Objects

The recently reported gravitational wave events GW150914 and GW151226 caused by the merg- ers of binary black holes [1–3] provide a formidable way to set constraints on alternative metric theories of gravity in the strong field regime. In this paper, we develop an approach where an arbitrary theory of gravity can be parametrised by an effective coupling Geff and an effective grav- itational potential Φ(r). The standard Newtonian limit of General Relativity is recovered as soon as Geff → GN and Φ(r) → ΦN. The upper bound on the graviton mass and the gravitational interaction length, reported by the LIGO-VIRGO collaboration, can be directly recast in terms of the parameters of the theory which allows an analysis where the gravitational wave frequency modulation sets constraints on the range of possible alternative models of gravity. Numerical results based on published parameters for the binary black hole mergers are also reported. Comparison of the observed phase of the GW150914 and GW151226 with the modulated phase in alternative theories of gravity does not give reasonable constraints due the large uncertainties in the estimated parameters for the coalescing black holes. In addition to these general considerations, we obtain limits for the frequency dependence of the α parameter in scalar tensor theories of gravity.

In this series of papers, we shall present a simplistic approach to the study of particle dynamics, fluid dynamics and numerical simulations of accretion flows and outflows around rotating black holes. We show that with a suitably modified effective potential of the central gravitating rotating object, one can carry out these studies very accurately. In this approach, one need not use the full general relativistic equations to obtain the salient features of the general relativistic flows provided the Kerr parameter remains within −1 ≤a≤ 0.8. We present the equatorial and the non-equatorial particle trajectories from our potential and compare salient properties in Kerr and in pseudo-Kerr geometries. Our potential naturally produces accurate results for motions around the Schwarzschild geometry when the black hole angular momentum is set to zero.

- by Sandip Mondal
- •
- General Relativity, Black Holes, Topology, Fluid Dynamics

The self-field of a charged particle has a singular component that diverges at the particle. We
use both coordinate and covariant approaches to compute an expansion of this singular field for
particles in generic geodesic orbits about a Schwarzschild black hole for scalar, electromagnetic and
gravitational cases. We check that both approaches yield identical results and give, as an application,
the calculation of previously unknown mode-sum regularisation parameters.
In the so-called mode-sum regularization approach to self-force calculations, each mode of the
retarded field is finite, while their sum diverges. The sum may be rendered finite and convergent
by the subtraction of appropriate regularization parameters. Higher order parameters lead to faster
convergence in the mode-sum. To demonstrate the significant benefit that they yield, we use our
newly derived parameters to calculate a highly accurate value of Fr = 0.000013784482575667959(3)
for the self-force on a scalar particle in a circular orbit around a Schwarzschild black hole.
Finally, as a second example application of our high-order expansions, we compute high-order
expressions for use in the effective source approach to self-force calculations.

- by Anna Heffernan and +1
- •
- Theoretical Physics, General Relativity, Theoretical astrophysics, Black Holes
- by Robert T Jantzen
- •
- Physics, Special Relativity, Compact Objects, Models
- by Shu-ichiro Inutsuka
- •
- Organic Chemistry, Compact Objects
- by Frank Löffler
- •
- Black Holes, Performance, Numerical Relativity, Compact Objects

From recent reports on terrestrial heavy ion collision experiments it appears that one may not obtain information about the existence of asymptotic freedom (AF) and chiral symmetry restoration (CSR) for quarks of QCD at high density. This information may still be obtained from compact stars - if they are made up of strange quark matter. Very high gravitational redshift lines (GRL), seen from some compact stars, seem to suggest high ratios of mass and radius (M/R) for them. This is suggestive of strange stars (SS) and can in fact be fitted very well with SQM equation of state deduced with built in AF and CSR. In some other stars broad absorption bands appear at about ~ 0.3 keV and multiples thereof, that may fit in very well with resonance with harmonic compressional breathing mode frequencies of these SS. Emission at these frequencies are also observed in six stars. If these two features of large GRL and BAB were observed together in a single star, it would strengthen the possibility for the existence of SS in nature and would vindicate the current dogma of AF and CSR that we believe in QCD. Recently, in 4U 1700-24, both features appear to be detected, which may well be interpreted as observation of SS - although the group that analyzed the data did not observe this possibility. We predict that if the shifted lines, that has been observed, are from neon with GRL shift z = 0.4 - then the compact object emitting it is a SS of mass 1.2 M_sun and radius 7 km. In addition the fit to the spectrum leaves a residual with broad dips at 0.35 keV and multiples thereof, as in 1E1207-5209 which is again suggestive of SS.

- by Manjari Bagchi
- •
- Quantum Chromodynamics, Equations of State, Compact Objects, X Rays

In a previous paper, we computed expressions for the Detweiler-Whiting singular field of point
scalar, electromagnetic and gravitational charges following a geodesic of the Schwarzschild spacetime.
We now extend this to the case of equatorial orbits in Kerr spacetime, using coordinate and covariant
approaches to compute expansions of the singular field in scalar, electromagnetic and gravitational
cases. As an application, we give the calculation of previously unknown mode-sum regularization
parameters. We also propose a new application of high-order approximations to the singular field,
showing how they may be used to compute m-mode regularization parameters for use in the m-mode
effective source approach to self-force calculations

- by Anna Heffernan
- •
- Theoretical Physics, General Relativity, Theoretical astrophysics, Black Holes

The effects implied for the structure of compact objects by the modification
of General Relativity (GR) produced by the generalization of the Lagrangian density
to the form f (R) = R+αR2,where R is the Ricci curvature scalar, have been recently
explored. It seems likely that this squared-gravity may allow heavier Neutron Stars
(NSs) than GR. In addition, these objects can be useful to constrain free parameters
of modified-gravity theories. The differences between alternative gravity theories are
enhanced in the strong gravitational regime. In this regime, because of the complexity
of the field equations, perturbativemethods become a good choice to treat the problem.
Following previous works in the field, we performed a numerical integration of the
structure equations that describe NSs in f (R)-gravity, recovering their mass-radius
relations, but focusing on particular features that arise from this approach in the profiles
of the NS interior.We show that these profiles run in correlation with the second-order
derivative of the analytic approximation to the Equation of State (EoS), which leads to
regions where the enclosed mass decreases with the radius in a counter-intuitive way.
We reproduce all computations with a simple polytropic EoS to separate zeroth-order
modified gravity effects.

- by Gustavo E Romero
- •
- Stellar Astrophysics, General Relativity, Neutron Stars, Astrophysics
- by D. Koo
- •
- Organic Chemistry, Astrophysical Plasma, Dark Matter, Galaxy Formation and Evolution
- by Daniela Pérez
- •
- Theoretical Physics, Gravitation, General Relativity, Black Holes
- by Gustavo E Romero
- •
- X-ray Binaries, Dust (Astronomy & Astrophysics), Magnetic field, Compact Objects
- by Stephen Rafter
- •
- Organic Chemistry, Compact Objects
- by Luca Zampieri
- •
- Black Holes, Compact Objects, Accretion Disks, Cosmology
- by D. Bini
- •
- Quantum Gravity, Special Relativity, Compact Objects, Models

In this paper we consider spherically symmetric interior spacetimes filled by anisotropic fluids in the context of Hořava gravity and Einstein-aether theory. We assume a specific non-static configuration of the aether vector field and show that the field equations admit a family of exact analytical solutions which can be obtained if one of the two metric coefficients is assigned. We study as an illustrative example the case in which the metric of the interior spacetime reproduces the Newtonian potential of a fluid sphere with constant density.

We developed a code that is able to calculate the analytic ADM mass, angular momen-
tum, compaction, radius, central density, and other physical parameters for polytropes
with index n = 2-3 by solving the energy equation using linear interpolation and extrap-
olations for necessary constants. From these parameters the dynamical time, and Alfven
time if magnetic eld is present, are also estimated. In addition, the code is capable of cal-
culating models with spins spanning from static to mass-shedding limits. These analytic
results crudely estimate the physical properties of massive stars(n > 2:5) such as super-
massive stars (SMS, n . 3) and compact stars with sti er equation of states(n < 2:5).
It provides an useful approach of searching marginally stable initial models for numerical
evolution.

- by Lunan Sun
- •
- Equations of State, Astrophysics, Numerical Relativity, Compact Objects
- by Bülent KIZILTAN
- •
- Organic Chemistry, Compact Objects

The accretion disk around a compact object is a nonlinear general relativistic system involving magnetohydrodynamics. Naturally the question arises whether such a system is chaotic (deterministic) or stochastic (random) which might be related to the associated transport properties whose origin is still not confirmed. Earlier, the black hole system GRS 1915+105 was shown to be low dimensional chaos in certain temporal classes. However, so far such nonlinear phenomena have not been studied fairly well for neutron stars which are unique for their magnetosphere and kHz quasi-periodic oscillation (QPO). On the other hand, it was argued that the QPO is a result of nonlinear magnetohydrodynamic effects in accretion disks. If a neutron star exhibits chaotic signature, then what is the chaotic/correlation dimension? We analyze RXTE/PCA data of neutron stars Sco X-1 and Cyg X-2, along with the black hole Cyg X-1 and the unknown source Cyg X-3, and show that while Sco X-1 and Cyg X-2 are low dimensional chaotic systems, Cyg X-1 and Cyg X-3 are stochastic sources. Based on our analysis, we argue that Cyg X-3 may be a black hole.

- by Jayanta Dutta
- •
- Organic Chemistry, Magnetohydrodynamics, Astrophysical Plasma, Black Holes
- by Gustavo Romero
- •
- Mathematical Physics, Quantum Physics, Stellar Astrophysics, General Relativity
- by Subharthi Ray
- •
- Mechanical Engineering, Aerospace Engineering, Space, Compact Objects

In this paper, we study the formation and dynamical evolution of black hole–black hole (BH–BH) binaries in young star clusters (YSCs), by means of N-body simulations. The simulations
include metallicity-dependent recipes for stellar evolution and stellar winds, and have been run for three different metallicities (Z = 0.01, 0.1 and 1 Z ). Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the
stellar remnants depends on the metallicity of the progenitor stars. We find that BH–BH binaries form efficiently because of dynamical exchanges: in our simulations, we find about
10 times more BH–BH binaries than double neutron star binaries. The simulated BH–BH binaries form earlier in metal-poor YSCs, which host more massive black holes (BHs) than in
metal-rich YSCs. The simulated BH–BH binaries have very large chirp masses (up to 80 M ), because the BH mass is assumed to depend on metallicity, and because BHs can grow in mass
due to the merger with stars. The simulated BH–BH binaries span a wide range of orbital periods (10−3 –107 yr), and only a small fraction of them (0.3 per cent) is expected to merge
within a Hubble time. We discuss the estimated merger rate from our simulations and the implications for Advanced VIRGO and LIGO.

- by Brunetto Marco Ziosi
- •
- Black Holes, Gravitational Waves, Compact Objects, N-body simulations

We analyze the thermodynamical properties of the regular static and spherically
symmetric black hole interior model presented by Mbonye and Kazanas. Equations for the
thermodynamical quantities valid for an arbitrary density profile are deduced, and from them
we show that the model is thermodynamically unstable. Evidence is also presented pointing
to its dynamical instability. The gravitational entropy of this solution based on the Weyl curvature
conjecture is calculated, following the recipe given by Rudjord, Grøn and Sigbjørn,
and it is shown to have the expected behavior.

- by Gustavo E Romero
- •
- Gravitation, General Relativity, Black Holes, Compact Objects
- by Gustavo E Romero
- •
- Mathematical Physics, Physics, Quantum Physics, Stellar Astrophysics
- by Manuel Linares
- •
- Physics, Organic Chemistry, Astrophysical Plasma, Compact Objects

- by Bidya Binay Karak
- •
- Organic Chemistry, Magnetohydrodynamics, Astrophysical Plasma, Black Holes
- by Roland Haas
- •
- Black Holes, Performance, Numerical Relativity, Compact Objects
- by Roland Haas
- •
- Quantum Gravity, General Relativity, Black Holes, Gravitational Waves
- by Gustavo E Romero
- •
- Gamma Ray Astronomy, Dust (Astronomy & Astrophysics), Compact Objects, X Rays
- by Juan Jose Moya Madrid
- •
- Organic Chemistry, Astrophysical Plasma, Galaxy Clusters, Compact Objects
- by Leonid Verozub
- •
- Compact Objects, Event horizon

In 2006, Chandra observations of Cygnus X-3 reveled our best view to date of an unique feature associated with this well know microquasar. Extended emission located 16" from Cygnus X-3 was found to be varying in (anti)phase with Cygnus X-3. Also from previous Chandra observations it is shown that the total flux from the feature varies with Cygnus X-3's total flux. If both this feature and Cygnus X-3 lie at the same distance the separation between the two would be 2.4 light years (assume a distance of 9 kpc).From a study of the spectra, flux and time variations of this feature we believe that this feature is a dust cloud that is located along our line of sight to Cygnus X-3. From these observations we are able to deduce the location, size, and properties of this cloud. We will present this analysis and the insights that this give us into the nature of Cygnus X-3 and its environment. We will also present models which will explain why this phenomenon has only been observed, b...

- by Michael McCollough
- •
- Compact Objects, Spatial resolution, Line of sight
- by C. Flynn
- •
- Dark Matter, Gravitational Lensing, Compact Objects, Optical Thickness
- by Chris Flynn
- •
- Dark Matter, Gravitational Lensing, Compact Objects, Optical Thickness
- by Enrique Pazos
- •
- Quantum Gravity, Gravitational Waves, Compact Objects, Mathematical Sciences
- by Paolo Pani
- •
- Black Holes, Compact Objects, Cosmology, Instability

Massive X-ray binaries are usually classified depending on the properties of the donor star in classical, supergiant and Be X-ray binaries. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in previous works but never studied in detail. We discuss possible scenarios and conclude that long-term quasi-periodic variations in the mass-loss rate of the primary are probably driving such variability in the measured X-ray flux. We obtain an improved orbital period of 9.5591 d with maximum X-ray flux at MJD 51856.6. Our study of the orbital X-ray variability in the context of wind accretion suggests a moderate eccentricity around 0.15. Moreover, the low value of v_inf solves the long-standing problem of the relatively high X-ray luminosity for the unevolved nature of the donor, BD +53 2790, which is probably an O9.5 V star. We note that changes in v_inf and/or the mass-loss rate of the primary alone cannot explain the diferent patterns displayed by the orbital X-ray variability. We finally emphasize that 4U 2206+54, together with LS 5039, could be part of a new population of wind-fed HMXBs with main sequence donors, the natural progenitors of supergiant X-ray binaries. (Abridged)

- by Pere Blay
- •
- Mass Transfer, X-ray Binaries, Dust (Astronomy & Astrophysics), Compact Objects