arXiv subjects

Alejandro Cruz-Osorio

Publications and source records attributed to Alejandro Cruz-Osorio.

49 records · Page 3Linked to original sources

arXiv2023

Probing spacetime and accretion model for the Galactic Center: Comparison of Kerr and dilaton black hole shadows

In the vicinity of black holes, the influence of strong gravity, plasma physics, and emission processes govern the behavior of the system. Since observations such as those carried out by the EHT are not yet able to unambiguously constrain models for astrophysical and gravitational properties, it is imperative to explore the accretion models, particle distribution function, and description of the spacetime geometry. Our current understanding of these properties is often based on the assumption that the spacetime is well-described by by the Kerr solution to general relativity, combined with basic emission and accretion models. We explore alternative models for each property performing general relativistic magnetohydrodynamic and radiative transfer simulations. By choosing a Kerr solution to general relativity and a dilaton solution to Einstein-Maxwell-dilaton-axion gravity as exemplary black hole background spacetimes, we aim to investigate the influence of accretion and emission models on the ability to distinguish black holes in two theories of gravity. We carry out three-dimensional general relativistic magnetohydrodynamics simulations of both black holes, matched at their innermost stable circular orbit, in two distinct accretion scenarios. Using general-relativistic radiative transfer calculations, we model the thermal synchrotron emission and in the next step apply a non-thermal electron distribution function, exploring representative parameters to compare with multiwavelength observations. We further consider Kerr and dilaton black holes matched at their unstable circular photon orbits, as well as their event horizons. From multiwavelength emission and spectral index analysis, we find that accretion model and spacetime have only a small impact on the spectra compared to the choice of emission model.

astro-ph.HE↗

arXiv2022

The Photon Ring in M87*

We report measurements of the gravitationally lensed secondary image -- the first in an infinite series of so-called "photon rings" -- around the supermassive black hole M87* via simultaneous modeling and imaging of the 2017 Event Horizon Telescope (EHT) observations. The inferred ring size remains constant across the seven days of the 2017 EHT observing campaign and is consistent with theoretical expectations, providing clear evidence that such measurements probe spacetime and a striking confirmation of the models underlying the first set of EHT results. The residual diffuse emission evolves on timescales comparable to one week. We are able to detect with high significance a southwestern extension consistent with that expected from the base of a jet that is rapidly rotating in the clockwise direction. This result adds further support to the identification of the jet in M87* with a black hole spin-driven outflow, launched via the Blandford-Znajek process. We present three revised estimates for the mass of M87* based on identifying the modeled thin ring component with the bright ringlike features seen in simulated images, one of which is only weakly sensitive to the astrophysics of the emission region. All three estimates agree with each other and previously reported values. Our strongest mass constraint combines information from both the ring and the diffuse emission region, which together imply a mass-to-distance ratio of $4.20^{+0.12}_{-0.06}~μ{\rm as}$ and a corresponding black hole mass of $(7.13\pm0.39)\times10^9M_\odot$, where the error on the latter is now dominated by the systematic uncertainty arising from the uncertain distance to M87*.

astro-ph.HE↗

arXiv2021

Event Horizon Telescope observations of the jet launching and collimation in Centaurus A

Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimeter wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to $10-100$ gravitational radii ($r_g=GM/c^2$) scales in nearby sources. Centaurus A is the closest radio-loud source to Earth. It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our galactic center. A large southern declination of $-43^{\circ}$ has however prevented VLBI imaging of Centaurus A below $λ1$cm thus far. Here, we show the millimeter VLBI image of the source, which we obtained with the Event Horizon Telescope at $228$GHz. Compared to previous observations, we image Centaurus A's jet at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly-collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that Centaurus A's source structure resembles the jet in Messier 87 on ${\sim}500r_g$ scales remarkably well. Furthermore, we identify the location of Centaurus A's SMBH with respect to its resolved jet core at $λ1.3$mm and conclude that the source's event horizon shadow should be visible at THz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses.

astro-ph.GA↗

arXiv2021

State-of-the-art energetic and morphological modelling of the launching site of the M87 jet

M87 has been the target of numerous astronomical observations across the electromagnetic spectrum and Very Long Baseline Interferometry (VLBI) resolved an edge-brightened jet. However, the origin and formation of its jets remain unclear. In our current understand black holes (BH) are the driving engine of jet formation, and indeed the recent Event Horizon Telescope (EHT) observations revealed a ring-like structure in agreement with theoretical models of accretion onto a rotating Kerr BH. In addition to the spin of the BH being a potential source of energy for the launching mechanism, magnetic fields are believed to play a key role in the formation of relativistic jets. A priori, the spin, $a_\star$, of BH in M87* is unknown, however, when accounting for the estimates on the X-ray luminosity and jet power, values $\left |a_\star \right| \gtrsim 0.5$ appear favoured. Besides the properties of the accretion flow and the BH spin, the radiation microphysics including the particle distribution (thermal and non-thermal) as well as the particle acceleration mechanism play a crucial role. We show that general-relativistic magnetohydrodynamics simulations and general-relativistic radiative transfer calculations can reproduce the broadband spectrum from the radio to the near-infrared regime and simultaneously match the observed collimation profile of M87, thus allowing us to set rough constraints on the dimensionless spin of M87* to be $0.5\lesssim a_{\star}\lesssim 1.0$, with higher spins being possibly favoured.

astro-ph.HE↗

arXiv2021

Impact of non-thermal particles on the spectral and structural properties of M87

The recent 230 GHz observations of the Event Horizon Telescope (EHT) are able to image the innermost structure of the M87 and show a ring-like structure which is in agreement with thermal synchrotron emission generated in a torus surrounding a supermassive black hole. However, at lower frequencies M87 is characterised by a large-scale and edge-brightened jet with clear signatures of non-thermal emission. In order to bridge the gap between these scales and to provide a theoretical interpretation of these observations we perform general relativistic magnetohydrodynamic simulations of accretion on to black holes and jet launching. M87 has been the target for multiple observations across the entire electromagnetic spectrum. Among these VLBI observations provide unique details on the collimation profile of the jet down to several gravitational radii. In this work we aim to model the observed broad-band spectrum of M87 from the radio to the NIR regime and at the same time fit the jet structure as observed with Global mm-VLBI at 86 GHz. We use general relativistic magnetohydrodynamics and simulate the accretion of the magnetised plasma onto Kerr-black holes in 3D. The radiative signatures of these simulations are computed taking different electron distribution functions into account and a detailed parameter survey is performed in order to match the observations. The results of our simulations show that magnetically arrested disks around fast spinning black holes ($a_\star\geq0.5$) together with a mixture of thermal and non-thermal particle distributions are able to model simultaneously the broad-band spectrum and the innermost jet structure of M87

astro-ph.HE↗

arXiv2021

GW190521 formation scenarios via relativistic accretion

The recent gravitational wave transient GW190521 has been interpreted by the LIGO-Virgo collaboration (LVC) as sourced by a binary black hole (BH) merger. According to the LVC parameter estimation, at least one of these progenitors falls into the so-called pair-instability supernova mass gap. This raises the important question of how and when these progenitors formed. In this paper we use an accretion model with super-Eddington mass accretion rate obtained from General Relativity hydrodynamics simulations to analyse the scenario wherein the GW190521 original progenitors (OPs) formed at lower masses (and spins) and grew to their estimated LVC parameters by relativistic accretion. We consider that the environment wherein the binary is immersed has density gradients as well as a dependence on the Mach number of the gas. Taking the LVC parameter estimation at $z=0.82$ as the endpoint of the accretion evolution, we estimate the initial masses and spins of the OPs at three different red-shifts $z=100, \ 50$, and $20$. We found three distinct possible types of OPs: $(i)$ $10^{-4} M_{\odot} - 3 M_{\odot}$ almost non-rotating (with Kerr spin parameter $a_{\star}< 10^{-2}$) primordial BHs; $(ii)$ $3 M_{\odot} - 40M_{\odot}$ slowly rotating ($ 10^{-2} < a_{\star} < 0.5$) stellar mass BHs; $(iii)$ $40M_{\odot} - 70M_{\odot}$ BHs with a moderate spin parameter $a_{\star}\sim 0.5$, which could originate from the collapse of high mass Pop III stars. The mass spread is due to varying the density gradient and the relativistic Mach number of the cosmic plasma; the variation of the masses due to the origin at different red-shifts, on the other hand, is negligible, $\sim 2\%$ ...

astro-ph.HE↗

arXiv2021

Magnetized discs and photon rings around Yukawa-like black holes

We present stationary solutions of geometrically thick discs (or tori) endowed with a self-consistent toroidal magnetic field distribution surrounding a non-rotating black hole in an analytical, static, spherically-symmetric $f(R)$-gravity background. These $f(R)$-gravity models introduce a Yukawa-like modification to the Newtonian potential, encoded in a single parameter $δ$ which controls the strength of the modified potential and whose specific values affect the disc configurations when compared to the general relativistic case. Our models span different magnetic field strengths, from purely hydrodynamical discs to highly magnetized tori. The characteristics of the solutions are identified by analyzing the central density, mass, geometrical size, angular size, and the black hole metric deviations from the Schwarzschild space-time. In the general relativistic limit ($δ=0$) our models reproduce previous results for a Schwarzschild black hole. For small values of the $δ$ parameter, corresponding to $\sim 10\%$ deviations from general relativity, we find variations of $\sim 2 \%$ in the event horizon size, a $\sim 5\%$ shift in the location of the inner edge and center of the disc, while the outer edge increases by $\sim 10\%$. Our analysis for $|δ|>0.1$, however, reveals notable changes in the black hole space-time solution which have a major impact in the morphological and thermodynamical properties of the discs. The comparison with general relativity is further investigated by computing the size of the photon ring produced by a source located at infinity. This allows us to place constraints on the parameters of the $f(R)$-gravity model based on the Event Horizon Telescope observations of the size of the light ring in M87 and SgrA$^*$.

astro-ph.HE↗

arXiv2021

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes.

gr-qc↗

arXiv2021

The Polarized Image of a Synchrotron Emitting Ring of Gas Orbiting a Black Hole

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov (2002) and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images.

astro-ph.HE↗

arXiv2020

Common-envelope Dynamics of a Stellar-mass Black Hole: General Relativistic Simulations

With the goal of providing more accurate and realistic estimates of the secular behavior of the mass accretion and drag rates in the "common-envelope" scenario encountered when a black hole or a neutron star moves in the stellar envelope of a red supergiant star, we have carried out the first general relativistic simulations of the accretion flow onto a nonrotating black hole moving supersonically in a medium with regular but different density gradients. The simulations reveal that the supersonic motion always rapidly reaches a stationary state and it produces a shock cone in the downstream part of the flow. In the absence of density gradients we recover the phenomenology already observed in the well-known Bondi--Hoyle--Lyttleton accretion problem, with super-Eddington mass accretion rate and a shock cone whose axis is stably aligned with the direction of motion. However, as the density gradient is made stronger, the accretion rate also increases and the shock cone is progressively and stably dragged toward the direction of motion. With sufficiently large gradients, the shock-cone axis can become orthogonal to the direction, or even move in the upstream region of the flow in the case of the largest density gradient. Together with the phenomenological aspects of the accretion flow, we have also quantified the rates of accretion of mass and momentum onto the black hole. Simple analytic expressions have been found for the rates of accretion of mass, momentum, drag force, and bremsstrahlung luminosity, all of which have been employed in the astrophysical modelling of the secular evolution of a binary system experiencing a common-envelope evolution...

gr-qc↗

arXiv2020

Neutron and quark stars: constraining the parameters for simple EoS using the GW170817

It is well known that the equation of state (EoS) of compact objects like neutron and quark stars is not determined despite there are several sophisticated models to describe it. From the electromagnetic observations, summarized in \cite{Lattimer01}, and the recent observation of gravitational waves from binary neutron star inspiral GW170817 \cite{Abbott2017_etal} and GW190425 \cite{Abbott2019}, it is possible to make an estimation of the range of masses and so constraint the mass of the neutron and quark stars, determining not only the best approximation for the EoS, but which kind of stars we would be observing. In this paper we explore several configurations of neutron stars assuming a simple polytropic equation of state, using a single layer model without crust. In particular, when the EoS depends on the mass rest density, $p=K ρ_{0}^Γ$, and when it depends on the energy density $p=K ρ^Γ$, considerable differences in the mass-radius relationships are found. On the other hand, we also explore quark stars models using the MIT bag EoS for different values of the vacuum energy density $B$.

gr-qc↗

arXiv2020

Non-linear evolutions of magnetised thick discs around black holes: dependence on the initial data

We build equilibrium solutions of magnetised thick discs around a highly spinning Kerr black hole and evolve these initial data up to a final time of about 100 orbital periods. The numerical simulations reported in this paper solve the general relativistic magnetohydrodynamics equations using the BHAC code and are performed in axisymmetry. Our study assumes non-self-gravitating, polytropic, constant angular momentum discs endowed with a purely toroidal magnetic field. In order to build the initial data we consider three approaches, two of which incorporate the magnetic field in a self-consistent way and a third approach in which the magnetic field is included as a perturbation on to an otherwise purely hydrodynamical solution. To test the dependence of the evolution on the initial data, we explore four representative values of the magnetisation parameter spanning from almost hydrodynamical discs to very strongly magnetised tori. The initial data are perturbed to allow for mass and angular momentum accretion on to the black hole. Notable differences are found in the long-term evolutions of the initial data. In particular, our study reveals that highly magnetised discs are unstable, and hence prone to be fully accreted and expelled, unless the magnetic field is incorporated into the initial data in a self-consistent way.

gr-qc↗

arXiv2019

Anisotropic Quark Stars with an Interacting Quark Equation of State

A deep exploration of the parameter space that relates the interacting equation of state with the bag constant B, and the interaction parameter a, is fundamental for the construction of diverse models of quark stars. In particular, the anisotropy of quark stars with a well-motivated quantum chromodynamics (QCD) equation of state is presented here. The contribution of the fourth order corrections parameter ($\mathrm{a}$) of the QCD perturbation on the radial and tangential pressure generate significant effects on the mass-radius relation and the stability of the quark star. An adequate set of solutions for several values of the bag factor and the interaction parameter are used in order to calculate the relation between the mass, radius, density, compactness, and consequently the maximum masses and the stability. Therefore, while the more interactive quark solution leads to higher masses, the weak interaction among quarks give solutions similar to the widely known MIT bag model.

gr-qc↗