1,721,004 research outputs found
Constraining
No full textWe discuss spherically symmetric dynamical systems in the framework of a general model of gravity, i.e. , where is a dimensional quantity in squared length units [L]. We initially assume that the internal structure of such systems is governed by the Krori–Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar SAX J1748.9-2021, derived from bursting X-ray binaries located within globular clusters, we determine that is approximately equal to km. In particular, the model is capable of producing stable configurations for SAX J1748.9-2021, encompassing both its geometric and physical characteristics. We show that, within the framework of gravity, the Krori–Barua ansatz establishes semi-analytical connections between the radial () and tangential () pressures, and the density (). These relations are described as and . In this context, and denote the sound speeds in the radial and tangential directions, respectively. Meanwhile, pertains to the surface density, and is derived from the model parameters. These connections are consistent with the equations of state derived from the best-fit solutions identified in the ongoing investigation. Notably, within the framework of gravity where is negative, the maximum compactness, denoted as C, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or gravity with positive, where the compactness has the potential to asymptotically reach the black hole threshold (). The model predictions suggest a central density that largely exceeds the saturation nuclear density, which is g/cm. Also the surface density surpasses . We obtain a mass-radius diagram, corresponding to the boundary density, which is consistent with other observational data
Charged spherically symmetric black holes in scalar-tensor Gauss–Bonnet gravity
No full textWe derive a novel class of four-dimensional black hole (BH) solutions in Gauss–Bonnet (GB) gravity coupled with a scalar field in presence of Maxwell electrodynamics. In order to derive such solutions, we assume the ansatz for metric potentials. Due to the choice of the ansatz of the metric, the Reissner Nordström gauge potential cannot be recovered because of the presence of higher-order terms which are not allowed to be vanishing. Moreover, the scalar field is not allowed to vanish. If it vanishes, a function of the solution results undefined. Furthermore, it is possible to show that the electric field is of higher-order in the monopole expansion: this fact explicitly comes from the contribution of the scalar field. Therefore, we can conclude that the GB scalar field acts as non-linear electrodynamics creating monopoles, quadrupoles, etc in the metric potentials. We compute the invariants associated with the BHs and show that, when compared to Schwarzschild or Reissner–Nordström space-times, they have a soft singularity. Also, it is possible to demonstrate that these BHs give rise to three horizons in AdS space-time and two horizons in dS space-time. Finally, thermodynamic quantities can be derived and we show that the solution can be stable or unstable depending on a critical value of the temperature
Maxwell-f(Q) Theory
No full textExploring the four-dimensional AdS black hole is crucial within the framework of the AdS/CFT correspondence. In this research, four-dimensional stationary and rotating AdS solutions in the framework of the (Formula presented.) gravitational theory are investigated, considering the charged scenario. Author\u27s emphasis is on the power-law ansatz, which consistent with observations and is deemed the most viable. Because this solution does not have an uncharged version or relate to general relativity, it falls into a new category, which derives its features from changes in non-metricity and incorporates the Maxwell domain. The singularities of such a solution are analyzed, computing all the quantities of different curvature and non-metricity invariants. Author\u27s results indicate the presence of a central singularity, albeit with a softer nature compared to standard non-metricity or Einstein general relativity, attributed to the influence of the effect of (Formula presented.). Several physical characteristics of black hole from thermodynamics perspective and demonstrate the existence of an outer event horizon in addition to the inner Cauchy horizons are examined. However, under the conditions of sufficiently large electric charge, a naked singularity emerges. Finally, a class of rotating black hole in four-dimensional (Formula presented.) gravity that are asymptotically anti-de Sitter charged is derived
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Nonlinear Charged Black Hole Solution in Rastall Gravity
No full textWe show that the spherically symmetric black hole (BH) solution of a charged (linear case) field equation of Rastall gravitational theory is not affected by the Rastall parameter and this is consistent with the results presented in the literature. However, when we apply the field equation of Rastall’s theory to a special form of nonlinear electrodynamics (NED) source, we derive a novel spherically symmetric BH solution that involves the Rastall parameter. The main source of the appearance of this parameter is the trace part of the NED source, which has a non-vanishing value, unlike the linear charged field equation. We show that the new BH solution is Anti−de-Sitter Reissner−Nordström spacetime in which the Rastall parameter is absorbed into the cosmological constant. This solution coincides with Reissner−Nordström solution in the GR limit, i.e., when Rastall’s parameter is vanishing. To gain more insight into this BH, we study the stability using the deviation of geodesic equations to derive the stability condition. Moreover, we explain the thermodynamic properties of this BH and show that it is stable, unlike the linear charged case that has a second-order phase transition. Finally, we prove the validity of the first law of thermodynamics
The Effect of f( R, T) Modified Gravity on the Mass and Radius of Pulsar HerX1
No full textMillisecond pulsars are the perfect testable to examine potential matter-geometry coupling and its physical consequences in the context of the recent Neutron Star Interior Composition Explorer discoveries. We apply the field equations of modified gravity, f(R, T) = R + α T, to a spherically symmetric spacetime, where R is the Ricci scalar, α is a dimensional parameter, and T is the matter of the geometry. Five unknown functions are present in the output system of differential equations, which consists of three equations. To close the system, we make explicit assumptions about the anisotropy and the radial metric potential, g rr . We then solve the output differential equations and derive the explicit forms of the components of the energy-momentum tensor, i.e., density, radial, and tangential pressures. We look into the possibility that all of the physical parameters in the star can be reexpressed in terms of α and the compactness parameters, C = 2 GM Rc-2. We show that, for a given mass, the size permitted by Einstein\u27s general relativity is less due to the matter-geometry coupling in f(R, T). The validity of the hypothesis was validated by observations from an extra 21 pulsars. To achieve a surface density that is compatible with a neutron core at nuclear saturation density, the mass-radius curve enables masses up to 3.35M o˙. We emphasize that although there is no assumption of an equation of state, the model fits well with a linear behavior. When comparing the surface densities of these 20 pulsars, we divided them into three groups. We show that these three groups are compatible with neutron cores
Charged solution with equal metric ansatz in Gauss–Bonnet theory coupled to scalar field
No full textIn this study, we apply the Gauss–Bonnet equation of motions coupled with the scalar field and potential and derive a new charged spherically symmetric black hole (BH) solution. In this study, we derive this BH using an equal metric potential, i.e., gtt=grr. Among many things, we succeed in determining the form of the scalar field, the function of the scalar, and the Gauss–Bonnet term. We investigate the physics of this BH solution and show that we have additional terms than the Riessner Nordström BH solution, i.e., [Formula presented] and [Formula presented]. The existence of these additional terms is due to the contribution of the scalar field ξ. These additional terms are responsible for reproducing the multi-horizon BH solution. Due to the existence of these additional terms, the modified first law of thermodynamics and the corresponding Smarr relation are derived
Slow Kerr-NUT black hole solution in dynamical Chern-Simons modified gravity
No full textThe slow rotation of Kerr-NUT spacetime is explored by taking into account the linear form of rotation and NUT parameters in the dynamical Chern-Simon gravity theory, which can be formulated from a scalar field describing the background. We show that in the absence of the potential scalar field, the metric potential does not respect the effect of the NUT parameter, although the scalar field is affected by the rotation and NUT parameters. As a result, unlike the slowly rotating black hole solution presented in [1], the cross term of the metric potential, which has both rotation and NUT parameters, has no contribution up to the leading order of the first order perturbation
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