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 the best one maintains an error below $$10^{-11}$$ 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 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.

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 there may be repulsive effects. Furthermore, we numerically solve the motion equations to show the shadow, the Einstein ring, and the gravitational lensing to establish a possible signature of such repulsive effects. We found that as q is smaller, the Einstein ring decreases, but the shape is the same as the Schwarzschild black hole case. However, for values of q lower or equal than −0.5, repulsive gravitational effects appear in the gravitational lensing close to the compact object, where a strong dependence of the system to the initial conditions seems to take place.

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.

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