We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. ... more We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. The models were obtained in the test fluid approximation by assuming that the tori is a linear media, making it is possible to characterize the magnetic polarization of the fluid through the magnetic susceptibility χ m. The magnetohydrodynamic (MHD) structure of the models was solved by following the Komissarov approach, but with the aim of including the magnetic polarization of the fluid, the integrability condition for the magnetic counterpart was modified. We build two kinds of magnetized tori depending on whether the magnetic susceptibility is constant in space or not. In the models with constant χ m , we find that the paramagnetic tori (χ m > 0) are more dense and less magnetized than the diamagnetic ones (χ m < 0) in the region between the inner edge, r in , and the center of the disk, r c ; however, we find the opposite behavior for r > r c. Now, in the models with non-constant χ m , the tori become more magnetized than the Komissarov solution in the region where ∂χ m /∂r < 0, and less magnetized when ∂χ m /∂r > 0. Nevertheless, it is worth mentioning that in all solutions presented in this paper the magnetic pressure is greater than the hydrodynamic pressure. These new equilibrium tori can be useful for studying the accretion of a magnetic media onto a rotating black hole.
Two new families of exact solutions to the Einstein equations for a conformastatic spacetime with... more Two new families of exact solutions to the Einstein equations for a conformastatic spacetime with axial symmetry are presented which describe thin disks of dust immersed in a spheroidal halo. The solutions are obtained by expressing the metric function in terms of an auxiliary function which satisfies the Laplace equation, a characteristic property of the conformastatic spacetimes. The first family of solutions is obtained from the displacement, cut and reflexion method, which introduces a discontinuity in the first z-derivate of the metric tensor across the plane of the disk. The second family of solutions is obtained by using the oblate spheroidal coordinates because they adapt to the shape of the source and introduce naturally a cutting radius for the disk. The energy densities of the disk and the halo are everywhere positive and well behaved and their energy-momentum tensor agrees with all the energy conditions. Some particular solutions for the energy density of the disk and the halo are presented and the rotational curves are obtained by solving the geodesic equation for a particle that moves in circular orbits in the plane of the disk.
In astronomical surveys, such as the Zwicky Transient Facility, supernovae (SNe) are relatively u... more In astronomical surveys, such as the Zwicky Transient Facility, supernovae (SNe) are relatively uncommon objects compared to other classes of variable events. Along with this scarcity, the processing of multi-band light-curves is a challenging task due to the highly irregular cadence, long time gaps, missing-values, few observations, etc. These issues are particularly detrimental to the analysis of transient events: SN-like light-curves. We offer three main contributions: 1) Based on temporal modulation and attention mechanisms, we propose a Deep attention model (TimeModAttn) to classify multi-band light-curves of different SN types, avoiding photometric or hand-crafted feature computations, missing-value assumptions, and explicit imputation/interpolation methods. 2) We propose a model for the synthetic generation of SN multi-band light-curves based on the Supernova Parametric Model, allowing us to increase the number of samples and the diversity of cadence. Thus, the TimeModAttn model is first pre-trained using synthetic light-curves. Then, a fine-tuning process is performed. The TimeModAttn model outperformed other Deep Learning models, based on Recurrent Neural Networks, in two scenarios: late-classification and early-classification. Also, the TimeModAttn model outperformed a Balanced Random Forest (BRF) classifier (trained with real data), increasing the balanced-F 1 score from ≈ .525 to ≈ .596. When training the BRF with synthetic data, this model achieved similar performance to the TimeModAttn model proposed while still maintaining extra advantages. 3) We conducted interpretability experiments. High attention scores were obtained for observations earlier than and close to the SN brightness peaks. This also correlated with an early highly variability of the learned temporal modulation.
Considering the growing interest of the astrophysicist community in the study of dissipative flui... more Considering the growing interest of the astrophysicist community in the study of dissipative fluids with the aim of getting a more realistic description of the universe, we present in this paper a physical analysis of the energy-momentum tensor of a viscous fluid with heat flux. We introduce the general form of this tensor and, using the approximation of small velocity gradients, we relate the stresses of the fluid with the viscosity coefficients, the shear tensor and the expansion factor. Exploiting these relations, we can write the stresses in terms of the extrinsic curvature of the normal surface to the 4-velocity vector of the fluid, and we can also establish a connection between the perfect fluid and the symmetries of the spacetime. On the other hand, we calculate the energy conditions for a dissipative fluid through contractions of the energy-momentum tensor with the 4-velocity vector of an arbitrary observer. This method is interesting because it allows us to compute the conditions in a reasonably easy way and without considering any approximation or restriction on the energy-momentum tensor.
The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angul... more The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angular momentum and dissipation of energy within hot, ionized accretion discs. This instability is produced through the interactions of a differentially rotating plasma with an embedded magnetic field. Like all substances in nature, the plasma in an accretion disc has the potential to become magnetically polarized when it interacts with the magnetic field. In this paper we study the effect of this magnetic susceptibility, parameterized by χ m , on the MRI, specifically within the context of black hole accretion. We find from a linear analysis within the Newtonian limit that the minimum wavelength of the first unstable mode and the wavelength of the fastest growing mode are shorter in paramagnetic (χ m > 0) than in diamagnetic (χ m < 0) discs, all other parameters being equal. Furthermore, the magnetization parameter (ratio of gas to magnetic pressure) in the saturated state should be smaller when the magnetic susceptibility is positive than when it is negative. We confirm this latter prediction through a set of numerical simulations of magnetically polarized black hole accretion discs. We additionally find that the vertically integrated stress and mass accretion rate are somewhat larger when the disc is paramagnetic than when it is diamagnetic. If astrophysical discs are able to become magnetically polarized to any significant degree, then our results would be relevant to properly interpreting observations.
Monthly Notices of the Royal Astronomical Society, Nov 18, 2022
Magnetic fields in black hole accretion disks are associated with processes of mass accretion and... more Magnetic fields in black hole accretion disks are associated with processes of mass accretion and energy amplification. The contribution of the magnetic field due to the magnetic polarization of the material induces effects on the physical properties of the medium that have repercussions on the radiation coming from the accretion disks. Hence, from observations, it could be possible to infer the "fingerprint" left by the magnetic polarization of the material and establish the properties of the spacetime itself. As the first step in this purpose, we use numerical simulations to systematically analyze the possible observable effects produced by the magnetic properties of an accretion disk around a Kerr black hole. We found that under the synchrotron radiation power-law model the effects of the magnetic polarization are negligible when the plasma is gas pressure-dominated. Nevertheless, as beta-plasma decreases, the emission becomes more intense for magnetic pressure-dominated disks. In particular, we found that paramagnetic disks emit the highest intensity value independent of the beta-plasma parameter in this regime. By contrast, the emitted flux decreases with the increase of beta-plasma due to the dependence of the magnetic field on the emission and absorption coefficients. Moreover, the disk morphology changes with the magnetic susceptibility: paramagnetic disks are more compact than diamagnetic ones. This fact leads to diamagnetic disks emitting a greater flux because each photon has a more optical path to travel inside the disk.
The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic... more The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic plasma orbiting close to the black hole event horizon, where strong gravitational effects are relevant. The observational data of such systems can be compared with theoretical models to infer the black hole and plasma properties. In the comparison process, ray-tracing algorithms are essential to computing the trajectories followed by the photons from the source to our telescopes. In this paper, we present OSIRIS: a new stable FORTRAN code capable of efficiently computing nullgeodesics around compact objects, including general relativistic effects such as gravitational lensing, redshift, and relativistic boosting. The algorithm is based on the Hamiltonian formulation and uses different integration schemes to evolve null-geodesics while tracking the error in the Hamiltonian constrain to ensure physical results. We found from an error analysis that the integration schemes are all stable, and the best one maintains an error below 10 −11. Particularly, to test the robustness and ability of the code to evolve geodesics in curved space-time, we compute the shadow and Einstein rings of a Kerr black hole with different rotation parameters and obtain the image of a thin Keplerian accretion disk around a Schwarzschild black hole. Although OSIRIS is parallelized neither with MPI nor with CUDA, the computation times are of the same order as those reported by other codes with these types of parallel computing platforms.
Monthly Notices of the Royal Astronomical Society, Oct 12, 2019
The origin and strength of the magnetic field in some systems like active galactic nuclei or gamm... more The origin and strength of the magnetic field in some systems like active galactic nuclei or gamma-ray bursts is still an open question in astrophysics. A possible mechanism to explain the magnetic field amplification is the Kelvin-Helmholtz instability, since it is able to transform the kinetic energy in a shear flow into magnetic energy. Through the present work, we investigate the linear and non linear effects produced by the magnetic susceptibility in the development of the Kelvin-Helmholtz instability in a relativistic plasma. The system under study consists of a plane interface separating two uniform fluids that move with opposite velocities. The magnetic field in the system is parallel to the flows and the susceptibility is assumed to be homogeneous, constant in time, and equal in both fluids. In particular, we analyze the instability in three different cases, when the fluids are diamagnetic, paramagnetic, and when the susceptibility is zero. We compute the dispersion relation in the linear regime and found that the interface between diamagnetic fluids is more stable than between paramagnetic ones. We check the analytical results with numerical simulations, and explore the effect of the magnetic polarization in the non linear regime. We find that the magnetic field is more amplified in paramagnetic fluids than in diamagnetic ones. Surprisingly, the effect of the susceptibility in the amplification is stronger when the magnetization parameter is smaller. The results of our work make this instability a more efficient and effective amplification mechanism of seed magnetic fields when considering the susceptibility of matter.
Magnetic fields in black hole accretion discs are associated with processes of mass accretion and... more Magnetic fields in black hole accretion discs are associated with processes of mass accretion and energy amplification. The contribution of the magnetic field due to the magnetic polarization of the material induces effects on the physical properties of the medium that have repercussions on the radiation coming from the accretion discs. Hence, from observations, it could be possible to infer the ‘fingerprint’ left by the magnetic polarization of the material and establish the properties of the space–time itself. As the first step in this purpose, we use numerical simulations to systematically analyse the possible observable effects produced by the magnetic properties of an accretion disc around a Kerr black hole. We found that under the synchrotron radiation power-law model the effects of the magnetic polarization are negligible when the plasma is gas pressure-dominated. Nevertheless, as beta-plasma decreases, the emission becomes more intense for magnetic pressure-dominated discs. ...
The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic... more The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic plasma orbiting close to the black hole event horizon, where strong gravitational effects are relevant. The observational data of such systems can be compared with theoretical models to infer the black hole and plasma properties. In the comparison process, ray-tracing algorithms are essential to computing the trajectories followed by the photons from the source to our telescopes. In this paper, we present : a new stable FORTRAN code capable of efficiently computing null-geodesics around compact objects, including general relativistic effects such as gravitational lensing, redshift, and relativistic boosting. The algorithm is based on the Hamiltonian formulation and uses different integration schemes to evolve null-geodesics while tracking the error in the Hamiltonian constrain to ensure physical results. We found from an error analysis that the integration schemes are all stable, and th...
One crucial problem in relativistic astrophysics is that of the nature of black hole candidates. ... more One crucial problem in relativistic astrophysics is that of the nature of black hole candidates. It is usually assumed that astrophysical black holes are described by the Schwarzschild or Kerr space–times; however, there is no direct evidence to assert this. Moreover, there are various solutions in general relativity that can be alternatives to black holes, usually called black hole mimickers. In this work, we study the shadow produced by a compact object described by the q-metric, which is the simplest static and axially symmetric solution of Einstein equations with a non-vanishing quadrupole moment. This particular space–time has the property of containing an independent parameter q, which is related to the compact object deformation. The solution corresponds to naked singularities for some specific values of this parameter. Additionally, we analyze the eigenvalues of the Riemann tensor using the SO(3, C) representation, which allows us to find, in an invariant way, regions where ...
A new family of three-dimensional Newtonian models for galaxies is constructed. The models descri... more A new family of three-dimensional Newtonian models for galaxies is constructed. The models describe a thin disk and a matter halo, whose gravitational potentials satisfies the equation (2) presented in Gonzalez & Pimentel (2016, Phys. Rev. D, 93, 044034), and therefore, they satisfy the energy conditions for a gravitational system. The expressions for the potential of the disk and the halo are obtained by applying the ”displace, cut, and reflect” method to the solution of the Laplace equation in cylindrical coordinates. Analytical expressions that describe the rotation curves and the mass distributions in the disk and in the halo are computed for the first three models of the family of solutions. It is shown that the mass densities of the disks and the haloes present a maximum at the center of the system and go to zero at infinity. Finally, for some values of the free parameters, the obtained rotation curves present a flat region for larger values of the radial coordinate. The model...
Monthly Notices of the Royal Astronomical Society, 2021
The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angul... more The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angular momentum and dissipation of energy within hot, ionized accretion discs. This instability is produced through the interactions of a differentially rotating plasma with an embedded magnetic field. Like all substances in nature, the plasma in an accretion disc has the potential to become magnetically polarized when it interacts with the magnetic field. In this paper we study the effect of this magnetic susceptibility, parameterized by χm, on the MRI, specifically within the context of black hole accretion. We find from a linear analysis within the Newtonian limit that the minimum wavelength of the first unstable mode and the wavelength of the fastest growing mode are shorter in paramagnetic (χm > 0) than in diamagnetic (χm < 0) discs, all other parameters being equal. Furthermore, the magnetization parameter (ratio of gas to magnetic pressure) in the saturated state should be small...
We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. ... more We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. The models were obtained in the test fluid approximation by assuming that the tori is a linear media, making it is possible to characterize the magnetic polarization of the fluid through the magnetic susceptibility χm. The magnetohydrodynamic (MHD) structure of the models was solved by following the Komissarov approach, but with the aim of including the magnetic polarization of the fluid, the integrability condition for the magnetic counterpart was modified. We build two kinds of magnetized tori depending on whether the magnetic susceptibility is constant in space or not. In the models with constant χm, we find that the paramagnetic tori ( χm > 0) are more dense and less magnetized than the diamagnetic ones ( χm < 0) in the region between the inner edge, rin, and the center of the disk, rc; however, we find the opposite behavior for r > rc. Now, in the models with non-constan...
The magnetically polarized matter in astrophysical systems may be relevant in some magnetically d... more The magnetically polarized matter in astrophysical systems may be relevant in some magnetically dominated regions. For instance, the funnel that is generated in some highly magnetized disks configurations whereby relativistic jets are thought to spread, or in pulsars where the fluids are subject to very intense magnetic fields. With the aim of dealing with magnetic media in the astrophysical context, we present for the first time the conservative form of the ideal general relativistic magnetohydrodynamics (GRMHD) equations with a non-zero magnetic polarization vector m µ. Then, we follow the Anile method to compute the eigenvalue structure in the case where the magnetic polarization is parallel to the magnetic field, and it is parametrized by the magnetic susceptibility χ m. This approximation allows us to describe diamagnetic fluids, for which χ m < 0, and paramagnetic fluids where χ m > 0. The theoretical results were implemented in the CAFE code to study the role of the magnetic polarization in some 1D Riemann problems. We found that independently of the initial condition, the first waves that appear in the numerical solutions are faster in diamagnetic materials than in paramagnetic ones. Moreover, the constant states between the waves change notably for different magnetic susceptibilities. All these effects are more appreciable if the magnetic pressure is much bigger than the fluid pressure. Additionally, with the aim of analysing a magnetic media in a strong gravitational field, we carry out for the first time the magnetized Michel accretion of a magnetically polarized fluid. With this test, we found that the numerical solution is effectively maintained over time (t > 4000), and that the global convergence of the code is 2 for χ m 0.005, for all the magnetic field strength β we considered. Finally, when χ m = 0.008 and β ≥ 10, the global convergence of the code is reduced to a value between first and second order.
Nowadays, the magnetic and radiation fields are very important to understand the matter accretion... more Nowadays, the magnetic and radiation fields are very important to understand the matter accretion into compact objects, the dynamics of binary systems, the equilibrium configurations of neutron stars, the photon diffusion, etc. The energy and the momentum associated to these fields, along with the matter one, need to satisfy some conditions that guarantee an appropriate physical behavior of the source and its gravitational field. Based on this fact, we present the energy conditions for a perfect fluid with magnetic and radiation field, in which the radiation part of the energy-momentum tensor is assumed to be approximately isotropic, in accordance with the optically thick regime. In order to find these conditions, the stress tensor of the system is written in an orthonormal basis in which it becomes diagonal, and the energy conditions are computed through contractions of the energy-momentum tensor with the four velocity vector of an arbitrary observer. Finally, the conditions for a magnetized fluid are presented as a particular case in which the radiation contribution is zero.
We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. ... more We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. The models were obtained in the test fluid approximation by assuming that the tori is a linear media, making it is possible to characterize the magnetic polarization of the fluid through the magnetic susceptibility χ m. The magnetohydrodynamic (MHD) structure of the models was solved by following the Komissarov approach, but with the aim of including the magnetic polarization of the fluid, the integrability condition for the magnetic counterpart was modified. We build two kinds of magnetized tori depending on whether the magnetic susceptibility is constant in space or not. In the models with constant χ m , we find that the paramagnetic tori (χ m > 0) are more dense and less magnetized than the diamagnetic ones (χ m < 0) in the region between the inner edge, r in , and the center of the disk, r c ; however, we find the opposite behavior for r > r c. Now, in the models with non-constant χ m , the tori become more magnetized than the Komissarov solution in the region where ∂χ m /∂r < 0, and less magnetized when ∂χ m /∂r > 0. Nevertheless, it is worth mentioning that in all solutions presented in this paper the magnetic pressure is greater than the hydrodynamic pressure. These new equilibrium tori can be useful for studying the accretion of a magnetic media onto a rotating black hole.
Two new families of exact solutions to the Einstein equations for a conformastatic spacetime with... more Two new families of exact solutions to the Einstein equations for a conformastatic spacetime with axial symmetry are presented which describe thin disks of dust immersed in a spheroidal halo. The solutions are obtained by expressing the metric function in terms of an auxiliary function which satisfies the Laplace equation, a characteristic property of the conformastatic spacetimes. The first family of solutions is obtained from the displacement, cut and reflexion method, which introduces a discontinuity in the first z-derivate of the metric tensor across the plane of the disk. The second family of solutions is obtained by using the oblate spheroidal coordinates because they adapt to the shape of the source and introduce naturally a cutting radius for the disk. The energy densities of the disk and the halo are everywhere positive and well behaved and their energy-momentum tensor agrees with all the energy conditions. Some particular solutions for the energy density of the disk and the halo are presented and the rotational curves are obtained by solving the geodesic equation for a particle that moves in circular orbits in the plane of the disk.
In astronomical surveys, such as the Zwicky Transient Facility, supernovae (SNe) are relatively u... more In astronomical surveys, such as the Zwicky Transient Facility, supernovae (SNe) are relatively uncommon objects compared to other classes of variable events. Along with this scarcity, the processing of multi-band light-curves is a challenging task due to the highly irregular cadence, long time gaps, missing-values, few observations, etc. These issues are particularly detrimental to the analysis of transient events: SN-like light-curves. We offer three main contributions: 1) Based on temporal modulation and attention mechanisms, we propose a Deep attention model (TimeModAttn) to classify multi-band light-curves of different SN types, avoiding photometric or hand-crafted feature computations, missing-value assumptions, and explicit imputation/interpolation methods. 2) We propose a model for the synthetic generation of SN multi-band light-curves based on the Supernova Parametric Model, allowing us to increase the number of samples and the diversity of cadence. Thus, the TimeModAttn model is first pre-trained using synthetic light-curves. Then, a fine-tuning process is performed. The TimeModAttn model outperformed other Deep Learning models, based on Recurrent Neural Networks, in two scenarios: late-classification and early-classification. Also, the TimeModAttn model outperformed a Balanced Random Forest (BRF) classifier (trained with real data), increasing the balanced-F 1 score from ≈ .525 to ≈ .596. When training the BRF with synthetic data, this model achieved similar performance to the TimeModAttn model proposed while still maintaining extra advantages. 3) We conducted interpretability experiments. High attention scores were obtained for observations earlier than and close to the SN brightness peaks. This also correlated with an early highly variability of the learned temporal modulation.
Considering the growing interest of the astrophysicist community in the study of dissipative flui... more Considering the growing interest of the astrophysicist community in the study of dissipative fluids with the aim of getting a more realistic description of the universe, we present in this paper a physical analysis of the energy-momentum tensor of a viscous fluid with heat flux. We introduce the general form of this tensor and, using the approximation of small velocity gradients, we relate the stresses of the fluid with the viscosity coefficients, the shear tensor and the expansion factor. Exploiting these relations, we can write the stresses in terms of the extrinsic curvature of the normal surface to the 4-velocity vector of the fluid, and we can also establish a connection between the perfect fluid and the symmetries of the spacetime. On the other hand, we calculate the energy conditions for a dissipative fluid through contractions of the energy-momentum tensor with the 4-velocity vector of an arbitrary observer. This method is interesting because it allows us to compute the conditions in a reasonably easy way and without considering any approximation or restriction on the energy-momentum tensor.
The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angul... more The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angular momentum and dissipation of energy within hot, ionized accretion discs. This instability is produced through the interactions of a differentially rotating plasma with an embedded magnetic field. Like all substances in nature, the plasma in an accretion disc has the potential to become magnetically polarized when it interacts with the magnetic field. In this paper we study the effect of this magnetic susceptibility, parameterized by χ m , on the MRI, specifically within the context of black hole accretion. We find from a linear analysis within the Newtonian limit that the minimum wavelength of the first unstable mode and the wavelength of the fastest growing mode are shorter in paramagnetic (χ m > 0) than in diamagnetic (χ m < 0) discs, all other parameters being equal. Furthermore, the magnetization parameter (ratio of gas to magnetic pressure) in the saturated state should be smaller when the magnetic susceptibility is positive than when it is negative. We confirm this latter prediction through a set of numerical simulations of magnetically polarized black hole accretion discs. We additionally find that the vertically integrated stress and mass accretion rate are somewhat larger when the disc is paramagnetic than when it is diamagnetic. If astrophysical discs are able to become magnetically polarized to any significant degree, then our results would be relevant to properly interpreting observations.
Monthly Notices of the Royal Astronomical Society, Nov 18, 2022
Magnetic fields in black hole accretion disks are associated with processes of mass accretion and... more Magnetic fields in black hole accretion disks are associated with processes of mass accretion and energy amplification. The contribution of the magnetic field due to the magnetic polarization of the material induces effects on the physical properties of the medium that have repercussions on the radiation coming from the accretion disks. Hence, from observations, it could be possible to infer the "fingerprint" left by the magnetic polarization of the material and establish the properties of the spacetime itself. As the first step in this purpose, we use numerical simulations to systematically analyze the possible observable effects produced by the magnetic properties of an accretion disk around a Kerr black hole. We found that under the synchrotron radiation power-law model the effects of the magnetic polarization are negligible when the plasma is gas pressure-dominated. Nevertheless, as beta-plasma decreases, the emission becomes more intense for magnetic pressure-dominated disks. In particular, we found that paramagnetic disks emit the highest intensity value independent of the beta-plasma parameter in this regime. By contrast, the emitted flux decreases with the increase of beta-plasma due to the dependence of the magnetic field on the emission and absorption coefficients. Moreover, the disk morphology changes with the magnetic susceptibility: paramagnetic disks are more compact than diamagnetic ones. This fact leads to diamagnetic disks emitting a greater flux because each photon has a more optical path to travel inside the disk.
The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic... more The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic plasma orbiting close to the black hole event horizon, where strong gravitational effects are relevant. The observational data of such systems can be compared with theoretical models to infer the black hole and plasma properties. In the comparison process, ray-tracing algorithms are essential to computing the trajectories followed by the photons from the source to our telescopes. In this paper, we present OSIRIS: a new stable FORTRAN code capable of efficiently computing nullgeodesics around compact objects, including general relativistic effects such as gravitational lensing, redshift, and relativistic boosting. The algorithm is based on the Hamiltonian formulation and uses different integration schemes to evolve null-geodesics while tracking the error in the Hamiltonian constrain to ensure physical results. We found from an error analysis that the integration schemes are all stable, and the best one maintains an error below 10 −11. Particularly, to test the robustness and ability of the code to evolve geodesics in curved space-time, we compute the shadow and Einstein rings of a Kerr black hole with different rotation parameters and obtain the image of a thin Keplerian accretion disk around a Schwarzschild black hole. Although OSIRIS is parallelized neither with MPI nor with CUDA, the computation times are of the same order as those reported by other codes with these types of parallel computing platforms.
Monthly Notices of the Royal Astronomical Society, Oct 12, 2019
The origin and strength of the magnetic field in some systems like active galactic nuclei or gamm... more The origin and strength of the magnetic field in some systems like active galactic nuclei or gamma-ray bursts is still an open question in astrophysics. A possible mechanism to explain the magnetic field amplification is the Kelvin-Helmholtz instability, since it is able to transform the kinetic energy in a shear flow into magnetic energy. Through the present work, we investigate the linear and non linear effects produced by the magnetic susceptibility in the development of the Kelvin-Helmholtz instability in a relativistic plasma. The system under study consists of a plane interface separating two uniform fluids that move with opposite velocities. The magnetic field in the system is parallel to the flows and the susceptibility is assumed to be homogeneous, constant in time, and equal in both fluids. In particular, we analyze the instability in three different cases, when the fluids are diamagnetic, paramagnetic, and when the susceptibility is zero. We compute the dispersion relation in the linear regime and found that the interface between diamagnetic fluids is more stable than between paramagnetic ones. We check the analytical results with numerical simulations, and explore the effect of the magnetic polarization in the non linear regime. We find that the magnetic field is more amplified in paramagnetic fluids than in diamagnetic ones. Surprisingly, the effect of the susceptibility in the amplification is stronger when the magnetization parameter is smaller. The results of our work make this instability a more efficient and effective amplification mechanism of seed magnetic fields when considering the susceptibility of matter.
Magnetic fields in black hole accretion discs are associated with processes of mass accretion and... more Magnetic fields in black hole accretion discs are associated with processes of mass accretion and energy amplification. The contribution of the magnetic field due to the magnetic polarization of the material induces effects on the physical properties of the medium that have repercussions on the radiation coming from the accretion discs. Hence, from observations, it could be possible to infer the ‘fingerprint’ left by the magnetic polarization of the material and establish the properties of the space–time itself. As the first step in this purpose, we use numerical simulations to systematically analyse the possible observable effects produced by the magnetic properties of an accretion disc around a Kerr black hole. We found that under the synchrotron radiation power-law model the effects of the magnetic polarization are negligible when the plasma is gas pressure-dominated. Nevertheless, as beta-plasma decreases, the emission becomes more intense for magnetic pressure-dominated discs. ...
The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic... more The radiation observed in quasars and active galactic nuclei is mainly produced by a relativistic plasma orbiting close to the black hole event horizon, where strong gravitational effects are relevant. The observational data of such systems can be compared with theoretical models to infer the black hole and plasma properties. In the comparison process, ray-tracing algorithms are essential to computing the trajectories followed by the photons from the source to our telescopes. In this paper, we present : a new stable FORTRAN code capable of efficiently computing null-geodesics around compact objects, including general relativistic effects such as gravitational lensing, redshift, and relativistic boosting. The algorithm is based on the Hamiltonian formulation and uses different integration schemes to evolve null-geodesics while tracking the error in the Hamiltonian constrain to ensure physical results. We found from an error analysis that the integration schemes are all stable, and th...
One crucial problem in relativistic astrophysics is that of the nature of black hole candidates. ... more One crucial problem in relativistic astrophysics is that of the nature of black hole candidates. It is usually assumed that astrophysical black holes are described by the Schwarzschild or Kerr space–times; however, there is no direct evidence to assert this. Moreover, there are various solutions in general relativity that can be alternatives to black holes, usually called black hole mimickers. In this work, we study the shadow produced by a compact object described by the q-metric, which is the simplest static and axially symmetric solution of Einstein equations with a non-vanishing quadrupole moment. This particular space–time has the property of containing an independent parameter q, which is related to the compact object deformation. The solution corresponds to naked singularities for some specific values of this parameter. Additionally, we analyze the eigenvalues of the Riemann tensor using the SO(3, C) representation, which allows us to find, in an invariant way, regions where ...
A new family of three-dimensional Newtonian models for galaxies is constructed. The models descri... more A new family of three-dimensional Newtonian models for galaxies is constructed. The models describe a thin disk and a matter halo, whose gravitational potentials satisfies the equation (2) presented in Gonzalez & Pimentel (2016, Phys. Rev. D, 93, 044034), and therefore, they satisfy the energy conditions for a gravitational system. The expressions for the potential of the disk and the halo are obtained by applying the ”displace, cut, and reflect” method to the solution of the Laplace equation in cylindrical coordinates. Analytical expressions that describe the rotation curves and the mass distributions in the disk and in the halo are computed for the first three models of the family of solutions. It is shown that the mass densities of the disks and the haloes present a maximum at the center of the system and go to zero at infinity. Finally, for some values of the free parameters, the obtained rotation curves present a flat region for larger values of the radial coordinate. The model...
Monthly Notices of the Royal Astronomical Society, 2021
The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angul... more The magneto-rotational instability (MRI) is the most likely mechanism for transportation of angular momentum and dissipation of energy within hot, ionized accretion discs. This instability is produced through the interactions of a differentially rotating plasma with an embedded magnetic field. Like all substances in nature, the plasma in an accretion disc has the potential to become magnetically polarized when it interacts with the magnetic field. In this paper we study the effect of this magnetic susceptibility, parameterized by χm, on the MRI, specifically within the context of black hole accretion. We find from a linear analysis within the Newtonian limit that the minimum wavelength of the first unstable mode and the wavelength of the fastest growing mode are shorter in paramagnetic (χm > 0) than in diamagnetic (χm < 0) discs, all other parameters being equal. Furthermore, the magnetization parameter (ratio of gas to magnetic pressure) in the saturated state should be small...
We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. ... more We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. The models were obtained in the test fluid approximation by assuming that the tori is a linear media, making it is possible to characterize the magnetic polarization of the fluid through the magnetic susceptibility χm. The magnetohydrodynamic (MHD) structure of the models was solved by following the Komissarov approach, but with the aim of including the magnetic polarization of the fluid, the integrability condition for the magnetic counterpart was modified. We build two kinds of magnetized tori depending on whether the magnetic susceptibility is constant in space or not. In the models with constant χm, we find that the paramagnetic tori ( χm > 0) are more dense and less magnetized than the diamagnetic ones ( χm < 0) in the region between the inner edge, rin, and the center of the disk, rc; however, we find the opposite behavior for r > rc. Now, in the models with non-constan...
The magnetically polarized matter in astrophysical systems may be relevant in some magnetically d... more The magnetically polarized matter in astrophysical systems may be relevant in some magnetically dominated regions. For instance, the funnel that is generated in some highly magnetized disks configurations whereby relativistic jets are thought to spread, or in pulsars where the fluids are subject to very intense magnetic fields. With the aim of dealing with magnetic media in the astrophysical context, we present for the first time the conservative form of the ideal general relativistic magnetohydrodynamics (GRMHD) equations with a non-zero magnetic polarization vector m µ. Then, we follow the Anile method to compute the eigenvalue structure in the case where the magnetic polarization is parallel to the magnetic field, and it is parametrized by the magnetic susceptibility χ m. This approximation allows us to describe diamagnetic fluids, for which χ m < 0, and paramagnetic fluids where χ m > 0. The theoretical results were implemented in the CAFE code to study the role of the magnetic polarization in some 1D Riemann problems. We found that independently of the initial condition, the first waves that appear in the numerical solutions are faster in diamagnetic materials than in paramagnetic ones. Moreover, the constant states between the waves change notably for different magnetic susceptibilities. All these effects are more appreciable if the magnetic pressure is much bigger than the fluid pressure. Additionally, with the aim of analysing a magnetic media in a strong gravitational field, we carry out for the first time the magnetized Michel accretion of a magnetically polarized fluid. With this test, we found that the numerical solution is effectively maintained over time (t > 4000), and that the global convergence of the code is 2 for χ m 0.005, for all the magnetic field strength β we considered. Finally, when χ m = 0.008 and β ≥ 10, the global convergence of the code is reduced to a value between first and second order.
Nowadays, the magnetic and radiation fields are very important to understand the matter accretion... more Nowadays, the magnetic and radiation fields are very important to understand the matter accretion into compact objects, the dynamics of binary systems, the equilibrium configurations of neutron stars, the photon diffusion, etc. The energy and the momentum associated to these fields, along with the matter one, need to satisfy some conditions that guarantee an appropriate physical behavior of the source and its gravitational field. Based on this fact, we present the energy conditions for a perfect fluid with magnetic and radiation field, in which the radiation part of the energy-momentum tensor is assumed to be approximately isotropic, in accordance with the optically thick regime. In order to find these conditions, the stress tensor of the system is written in an orthonormal basis in which it becomes diagonal, and the energy conditions are computed through contractions of the energy-momentum tensor with the four velocity vector of an arbitrary observer. Finally, the conditions for a magnetized fluid are presented as a particular case in which the radiation contribution is zero.
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Papers by Oscar Pimentel