arXiv subjects

Gabriel Gómez

Publications and source records attributed to Gabriel Gómez.

12 recordsLinked to original sources

arXiv2026

Unimodular Gravity with Arbitrary Diffusion Function: A Dynamical System Reconstruction Approach

We investigate cosmological diffusion models in unimodular gravity within a dynamical systems reconstruction framework. By treating the logarithmic slope of the diffusion sector as an invertible dynamical variable, the diffusion function can be systematically reconstructed from the phase-space structure of the cosmological evolution. Under these conditions, we determine the physically admissible fixed points of the system, identifying novel matter--diffusion scaling solutions associated with power-law diffusion sectors, as well as purely diffusion-dominated configurations capable of driving late-time accelerated expansion without requiring an explicit cosmological constant term. The local behavior around the fixed points is then extended to the full cosmological evolution, providing a framework to explore the global implications of diffusion cosmologies. Beyond the asymptotic fixed-point structure, we further develop a reconstruction formalism based on the dynamical evolution of the diffusion slope, allowing for trajectories interpolating between different diffusion regimes during the cosmic history. Our results establish a systematic framework for constructing and classifying viable diffusion cosmologies in unimodular gravity directly from the phase-space dynamics.

gr-qc↗

arXiv2026

Strong-lensing degeneracies of black holes embedded in self-interacting scalar field dark matter halos

In this paper, we explore the strong gravitational lensing properties of black holes embedded in self-interacting scalar field dark matter halos, together with NFW-type configurations for comparison. The corresponding spacetime geometry is reconstructed numerically through the Einstein cluster formalism, allowing us to study how the surrounding dark matter distribution affects the propagation of photons near the black hole. We first analyze the effective function governing photon trajectories and calculate the corresponding photon sphere radius and critical impact parameter. We then investigate different strong-lensing observables, including relativistic Einstein rings, finite-order image positions, image separations, magnifications, and time delays, with particular attention to the supermassive black holes M87* and Sgr A*. Our results show that the considered halo configurations produce only small deviations with respect to the Schwarzschild case, typically at the level of $\mathcal{O}(10^{-3})$ or smaller, leading to a strong observational degeneracy among the models. Nevertheless, small but systematic differences remain present, especially in the time delay between relativistic images, which provides the clearest amplification of the halo-induced corrections for very massive black holes. These results suggest that, although standard strong-lensing observables remain highly robust against the considered halo environments, time-domain signatures may offer a more promising way to probe the effect of dark matter surrounding black holes.

gr-qc↗

arXiv2024

Quasinormal modes and shadow in Einstein Maxwell power-Yang-Mills black hole

In the present paper, we investigate the quasinormal modes of an Einstein-Maxwell power-Yang-Mills black hole in four dimensions, considering a specific value of the power parameter $p = 1/2$. This particular case represents a black hole with both Abelian and Non-Abelian charges and is asymptotically non-flat. We begin by deriving the effective potential for both a neutral massless particle and a neutral Dirac particle using the aforementioned black hole solution. Subsequently, employing the sixth-order WKB approximation method, we calculate the (scalar) quasinormal modes. Our numerical analysis indicates that these modes are stable within the considered parameter range. This result is also confirmed using the eikonal approximation. Furthermore, we calculate the shadow radius for this class of BH and derive constraints on the electric and Yang-Mills charges ($Q, Q_{\rm YM}$) by using imaging observational data for Sgr A${^\star}$, provided by the Event Horizon Telescope Collaboration. We observe that as the electric charge $Q$ increases, the allowed range shifts towards negative values of $Q_{\rm YM}$. For instance, for the maximum value $Q\approx 1.1$ obtained, the allowed range becomes $-0.171 \lesssim Q_{\rm YM} \lesssim -0.087$ consistent with KECK and VLTI data, while still retaining a non-vanishing horizon.

gr-qc↗

arXiv2024

Scalar Field Dark Matter Around Charged Black Holes

In this paper, we investigate the behavior of a massive scalar field dark matter scenarios in the large mass limit around a central Reissner-Nordström black hole. This study is motivated by observations from the Event Horizon Telescope collaboration, which does not exclude the possibility of the existence of such black holes. Through these inquiries, we uncover that the electric charge may significantly impact the scalar field profile and the density profile in the vecinty of the black hole. For the maximum electric charge allowed by the constraints of the Event Horizon Telescope, the maximum accretion rate decreases by $\thicksim$ 50 \% compared to the Schwarszchild case for marginally bound orbits. The maximum accretion rate of the massive scalar field is approximately $\dot M_{\text{SFDM}} \thicksim 10^{-8} M_{\odot} \;\text{yr}^{-1}$, which is significantly lower than the typical baryonic accretion rate commonly found in the literature. This implies that the scalar cloud located at the center of galaxies may have survived untill present times.

astro-ph.CO↗

arXiv2024

Constraining Self-interacting Scalar Field Dark Matter From the Black Hole Shadow of the Event Horizon Telescope

An exciting possibility to constrain dark matter (DM) scenarios is to search for their gravitational imprints on Black Hole (BH) observations. In this paper, we investigate the impact of self-interacting scalar field DM on the shadow radius of a Schwarzschild BH. We implement a self-consistent formulation, paying attention to the enhancement of the DM density due to the BH gravitational influence and the accretion flow onto the BH. First, we calculate the first-order correction to the shadow radius caused by a general DM environment. Then, we apply this perturbative method to the case of self-interacting scalar field DM and derive analytical expressions for the critical impact parameter. We find that self-consistency requirements, involving the lifetime and the mass of the central DM soliton, or the mass and the size of the extended virialized DM halo, ensure that the impact of the DM environment on the shadow radius is below the observational upper bound. This emphasizes the importance of taking into account the self-consistency constraints of the underlying DM scenario, which can strongly limit the range of possible DM density profiles and their impact on the shadow radius.

astro-ph.CO↗

arXiv2023

Black Holes with Abelian and Non-Abelian Charges and Their Impact on Matter Accretion Flows

We study the black hole spacetime structure of a model consisting of the standard Maxwell theory and a $p$-power-Yang-Mills term. This non-linear contribution introduces a non-Abelian charge into the global solution, resulting in a modified structure of the standard Reissner-Nordström black hole. Specifically, we focus on the model with $p=1/2$, which gives rise to a new type of modified Reissner-Nordström black hole. For this class of black holes, we compute the event horizon, the innermost stable circular orbit, and the conditions to preserve the weak cosmic censorship conjecture. The latter condition sets a well-established relation between the electric and the Yang-Mills charges. As a first astrophysical implication, the accretion properties of spherical steady flows are investigated in detail. Extensive numerical examples of how the Yang-Mills charge affects the accretion process of an isothermal fluid in comparison to the standard Reissner-Nordström and Schwarzschild black holes are displayed. Finally, analytical solutions in the fully relativistic regime, along with numerical computations, of the mass accretion rate for a polytropic fluid in terms of the electric and Yang-Mills charges are obtained. As a main result, the mass accretion rate efficiency is considerably improved, with respect to the standard Reissner-Nordström and Schwarzschild solutions, for negative values of the Yang-Mills charge.

gr-qc↗

arXiv2023

Accretion of Self-interacting Scalar Field Dark Matter Onto a Reissner-Nordström Black Hole

Self-interacting scalar field dark matter can be seen as an extension of the free case known as Fuzzy dark matter. The interactive case is capable of reproducing the positive features of the free case at both astrophysical and cosmological scales. On the other hand, current imaging black holes (BHs) observations provided by the Event Horizon Telescope (EHT) collaboration cannot rule out the possibility that BHs can carry some amount of charge. Motivated by these aspects, and by the possibility of detecting dark matter through its gravitational imprints on BH observations, in this paper, we extend previous studies of accretion of self-interacting scalar field dark matter to the charged BH case. Our analysis is based on the assumption on spherically symmetric flow and employs a test fluid approximation. All analytical expressions are derived from the ground up in Schwarzschild coordinates. Concretely, we implement analytical and numerical approaches to investigate the impact of the charge on the energy flux. From this analysis, we notice that the mass accretion rate efficiency is reduced up to $\sim 20\%$ for the maximum allowed charge. Additionally, considering the mass accretion rate of M87$^{\star}$ inferred from Polarization data of the EHT, we infer the conservative bound $ λ_4 > (1.49-10.2)( m / 1 \rm {eV} )^4$ based on the simple criterion that ensures the mass accretion rate caused by DM remains subdominant compared to the baryonic component.

astro-ph.CO↗

arXiv2023

A new Parametrization for Bulk Viscosity Cosmology as Extension of the $Λ$CDM Model

Bulk viscosity in cold dark matter is an appealing feature that introduces distinctive phenomenological effects in the cosmological setting as compared to the $Λ$CDM model. Under this view, we propose a general parametrization of the bulk viscosity of the form $ξ\sim H^{1-2s} ρ_{m}^{s}$, that covers intriguingly some well-known cases in the Eckart's theory. Some advantages of this novel parametrization are: first, it allows to write the resulting equations of cosmological evolution in the form of an autonomous system for any value of $s$, so a general treatment of the fixed points and stability can be done, and second, the bulk viscosity effect is consistently handled so that it naturally turns off when matter density vanishes. As a main result we find, based on detailed dynamical system analysis, one-parameter family of de-Sitter-like asymptotic solutions with non-vanishing bulk viscosity coefficient during different cosmological periods. Numerical computations are performed jointly along with analytical phase space analysis in order to assess more quantitatively the bulk viscosity effect on the cosmological background evolution. Finally, as a first contact with observation we derive constraints on the free parameters of some bulk viscosity models with specific $s$-exponents from Supernovae Ia and observations of the Hubble parameter, by performing a Bayesian statistical analysis thought the Markov Chain Monte Carlo method.

gr-qc↗

arXiv2023

Conformally and Disformally Coupled Vector field Models of Dark Energy

Scalar fields coupled to dark matter by conformal or disformal transformations give rise to a general class of scalar-tensor theories which leads to a rich phenomenology in a cosmological setting. While this possibility has been studied comprehensively in the literature for scalar fields, the vector case has been hardly treated. We build hence models based on vector fields conformally and disformally coupled to dark matter and derive explicitly the general covariant form of the interaction term in an independent way of the gravity theory, whereby this result can be applied to general vector-tensor theories. For concreteness, the standard Proca theory with a vector exponential potential is taken to describe the vector-tensor sector, and some specific coupling functions are assumed to study the cosmological background dynamics by dynamical system techniques. Despite of choosing such a minimalist form for the underlying theory, the parameter space is considerably enriched compared to the uncoupled case due to the novel interactions, leading to new branches of solutions for the vector equation of motion. Thus, different trajectories can exist in phase space depending on the coupling parameters associated to the conformal and disformal functions. From here, new emerging vector-dark matter scaling solutions, and renewed stable attractor points are found to drive the late-time accelerated expansion of the universe. As a first examination about instabilities issues, we derive general conditions to avoid classical instabilities in a more general setup of the theory. Numerical calculations are performed as well to investigate more quantitatively the impact of the conformal and disformal couplings on the cosmological background evolution. These effects depend essentially on the strength on the coupling parameters and, in some specific cases, on their associated signs.

gr-qc↗

arXiv2021

A survey of IRAS young stellar object candidates. Searching for large-scale Herbig-Haro objects

Jets and outflows are associated with young stellar objects across the stellar mass spectrum, from brown dwarf protostars to massive, Ae/Be stars. Frequently, the jet morphology is spatially discontinuous because of the temporal variability of the ejection from the driving source. Images covering a wide field of view around the jet driving-source are useful to map the large-scale jet emission and to explore the mass ejection history. The aim of this work was to search for large-scale optical Herbig-Haro (HH) objects lying in a wide field around a sample of IRAS sources, candidates to trace young stellar objects. Deep, narrow-band images through the H$α$ and [SII] emission lines, and through an off-line continuum filter, covering a wide ($\sim15'$) field around the IRAS targets were acquired. The images in the three filters were analyzed to identify shock-excited line emission (i.e., HH) in contrast to scattered line emission. New images of a sample of fifteen IRAS sources, obtained in an homogeneous way are presented. HH emission was detected in six fields, and the astrometry of the knot features is given. The nature of the extended emission as scattered emission around nine of the IRAS targets is confirmed. For seven IRAS sources, with unclear optical counterpart, a more plausible counterpart is proposed. A refined value of the source distance is reported for seven targets. An update of the main data available for each of the sampled fields, including images from public data archives, is also presented.

astro-ph.SR↗

arXiv2020

On the Usage of Generative Models for Network Anomaly Detection in Multivariate Time-Series

Despite the many attempts and approaches for anomaly detection explored over the years, the automatic detection of rare events in data communication networks remains a complex problem. In this paper we introduce Net-GAN, a novel approach to network anomaly detection in time-series, using recurrent neural networks (RNNs) and generative adversarial networks (GAN). Different from the state of the art, which traditionally focuses on univariate measurements, Net-GAN detects anomalies in multivariate time-series, exploiting temporal dependencies through RNNs. Net-GAN discovers the underlying distribution of the baseline, multivariate data, without making any assumptions on its nature, offering a powerful approach to detect anomalies in complex, difficult to model network monitoring data. We further exploit the concepts behind generative models to conceive Net-VAE, a complementary approach to Net-GAN for network anomaly detection, based on variational auto-encoders (VAE). We evaluate Net-GAN and Net-VAE in different monitoring scenarios, including anomaly detection in IoT sensor data, and intrusion detection in network measurements. Generative models represent a promising approach for network anomaly detection, especially when considering the complexity and ever-growing number of time-series to monitor in operational networks.

cs.AI↗

arXiv2012

The counterjet of HH 30: new light on its binary driving source

We present new [SII] images of the HH 30 jet and counterjet observed in 2006, 2007, and 2010 that allowed us to measure with improved accuracy the positions and proper motions of the jet and counterjet knots. Our results show that the motion of the knots is essentially ballistic, with the exception of the farthest knots, which trace the large scale C-shape bending of the jet. The observed bending of the jet can be produced by a relative motion of the HH 30 star with respect to its surrounding environment, caused either by a possible proper motion of the HH 30 star, or by the entrainment of environment gas by the red lobe of the nearby L1551-IRS 5 outflow. Alternatively, the bending can be produced by the stellar wind from a nearby CTTS, identified in the 2MASS catalog. The proper motion velocities of the knots of the counterjet show more variations than those of the jet. In particular, we identify two knots of the counterjet that have the same kinematic age but whose velocities differ by almost a factor of two. Thus, it appears that counterjet knots launched simultaneously can be ejected with very different velocities. We confirm that the observed wiggling of the jet and counterjet arises from the orbital motion of the jet source in a binary system. Precession is of secondary importance in shaping the jet. We derive an orbital period $τ_o=114\pm2$ yr and a mass function $mμ_c^3=0.014\pm0.006$ $M_\odot$. For a mass of the system of $m=0.45\pm0.04$ $M_\odot$ (the value inferred from the disk kinematics) we obtain a mass $m_j=0.31\pm0.04$ $M_\odot$ for the jet source, a mass $m_c=0.14\pm0.03$ $M_\odot$ for the companion, and a binary separation of $a=18.0\pm0.6$ AU. This binary separation coincides with the value required to account for the size of the inner hole observed in the disk, attributed to tidal truncation in a binary system.

astro-ph.GA↗