arXiv subjects

Gustavo E. Romero

Publications and source records attributed to Gustavo E. Romero.

At least 55 records · Page 3Linked to original sources

arXiv2020

Primordial black hole evolution in two-fluid cosmology

Several processes in the early universe might lead to the formation of primordial black holes with different masses. These black holes would interact with the cosmic plasma through accretion and emission processes. Such interactions might have affected the dynamics of the universe and generated a considerable amount of entropy. In this paper we investigate the effects of the presence of primordial black holes on the evolution of the early universe. We adopt a two-fluid cosmological model with radiation and a primordial black hole gas. The latter is modelled with different initial mass functions taking into account the available constraints over the initial primordial black hole abundances. We find that certain populations with narrow initial mass functions are capable to produce significant changes in the scale factor and the entropy.

astro-ph.CO↗

arXiv2020

Astrophysical Constraints on Strong Modified Gravity

We offer a discussion on the strong field regime predictions of two families of theories that deviate from General Relativity in different aspects: $f(R)$-gravity and Scalar-Tensor-Vector Gravity (STVG). We discuss astrophysical effects in models based upon both matter and vacuum solutions of such theories. In particular, we analize neutron star structure and the constraints on the parameters of the theories introduced by the latest observations. We also review black hole solutions and several astrophysical consequences of them, including accretion disks and jets. Finally, we report on the implications of the detection of various gravitational wave events for these theories.

gr-qc↗

arXiv2019

Gamma Rays from Large-Scale Outflows in Starburst Galaxies

The combined effects of supernova explosions and stellar winds produce a hot bubble in the central regions of starburst galaxies. As the bubble expands, it can outbreak into the galactic halo driving a superwind that transports hot gas and fields to the intergalactic space. We present estimates of cosmic ray generation and gamma-ray emission in both this large-scale wind and the bow shocks created around the embedded clouds.

astro-ph.HE↗

arXiv2019

Exact cosmological black hole solutions in Scalar Tensor Vector Gravity

We find an exact solution of Scalar-Tensor-Vector Gravity field equations that represents a black hole embedded in an expanding universe. This is the first solution of the kind found in the theory. We analyze the properties of the apparent horizons as well as the essential singularities of the metric, and compare it with the McVittie spacetime of General Relativity. Depending on the cosmological model adopted and the value of the free parameter $α$ of the theory, the solution describes a cosmological black hole, an inhomogeneity in an expanding universe, or a naked singularity. We use the latter result to set further constraints on the free parameters of the theory.

gr-qc↗

arXiv2019

Gamma Rays from colliding winds in massive binaries. Status and prospects

Binary systems formed by early-type stars with strong winds are known to display variable non-thermal radio emission, thermal X-rays, and, at least in one case (Eta Carina), $γ$ rays. Some of these systems are quite eccentric and the conditions for efficient particle acceleration and $γ$-ray production might manifest only occasionally. In this paper I briefly review the physics of colliding wind binaries with emphasis on the observational signatures of non-thermal particle acceleration. I discuss, in particular, the case of the system HD 93129A which is made up by an O2 If* star (the primary) and an O3.5 V star (the secondary). The primary is among the earliest, hottest, most massive, luminous, and windy O stars in the Galaxy. The periastron passage during 2018 will offer an outstanding observational window that will be exploited by an international multi-wavelength campaign.

astro-ph.HE↗

arXiv2019

Electromagnetic fields and charges in expanding universes

We analyze the properties of the electric and magnetic fields in different reference frames within a cosmological background space-time. First, we investigate the conformal properties of the electromagnetic fields and charge currents, discussing how the spatial curvature of the universe affects the field on different scales. Then, we analyze the effects of the expansion of the universe on local electromagnetic sources using Fermi coordinates. In particular, we investigate the energy balance and Poynting flux in this locally defined reference frame. We show that a charge following the Hubble flow in an accelerated FLRW universe is accelerated as seen by the local inertial frame, leading to non-null radiation.

gr-qc↗

arXiv2019

A model of high-energy emission from jets of microquasars of Population III

Current research indicates that the radiation of the first stars alone would not have been suficient to ionize the intergalactic medium on long distances because of the high columnar density of the clouds in which they formed. It becomes necessary, then, to explore other alternatives for the reionization. The study of the contribution of the first accreting binary systems to the reionization and heating of the intergalactic medium requires the formulation of a concrete model for the first microquasars. We aim at constructing a consistent model for jets of microquasars where the donor star is of Population III. We calculate the spectral energy distribution of the radiation produced by the relativistic particles in the jets, within the framework of a lepto-hadronic model. In addition, we study the interaction between the jets and the early intergalactic medium. All calculations have been computed for a cosmological redshift value z = 10. We determine that jets of microquasars of Population III are sources of intense gamma radiation in regions of particle acceleration near the compact object and in the terminal jets.

astro-ph.HE↗

arXiv2019

A note on geodesics in inhomogeneous expanding spacetimes

There are several solutions of Einstein field equations that describe an inhomogeneity in an expanding universe. Among these solutions, the McVittie metric and its generalizations have been investigated through decades, though a full understanding of them is still lacking. In this note, we explore the trajectories of photons and massive particles in generalized McVittie spacetimes. In the case of massless particles, we show that no circular orbits are possible for those models that admit cosmological singularities. We also analyze the trajectory of particles for a specific generalized McVittie spacetime that is conformal to the Schwarzschild metric. By integrating the equations of motion in the Newtonian approximation, we show that particles behave in quite distinctive ways in different cosmological black hole solutions. We conclude that the analysis of the geodetic motion in inhomogeneous expanding metrics can help to discriminate those solutions that represent real cosmological black holes in the universe.

gr-qc↗

arXiv2018

Population III microquasars

We present the first results obtained in the elaboration of a complete model of a microquasar where the donor star is from Population III. These stars do not produce stellar winds so we consider that the mass loss is due exclusively to matter overflowing the Roche lobe towards the compact object, a maximally rotating black hole. The rate of accretion is extremely super-Eddington, with an intense mass loss from the system in the form of winds and jets. We calculate the relativistic particle content of the jet and the corresponding spectral energy distribution (SED) considering a lepto-hadronic model. Prospects for the cosmological implications of these objects are briefly discussed.

astro-ph.HE↗

arXiv2018

Particle acceleration in the superwinds of starburst galaxies

Starbursts are galaxies undergoing massive episodes of star formation. The combined effect of stellar winds from hot stars and supernova explosions creates a high-temperature cavity in the nuclear region of these objects. The very hot gas expands adiabatically and escapes from the galaxy creating a superwind which sweeps matter from the galactic disk. The superwind region in the halo is filled with a multi-phase gas with hot, warm, cool, and relativistic components. The shocks associated with the superwind of starbursts and the turbulent gas region of the bubble inflated by them might accelerate cosmic rays up to high energies. In this work we calculate the cosmic ray production associated with the superwind using parameters that correspond to the nearby southern starburst galaxy NGC 253, which has been suggested as a potential accelerator of ultra-high energy cosmic rays. We evaluate the efficiency of both diffusive shock acceleration (DSA) and stochastic diffusive acceleration (SDA) in the superwind of NGC 253. We estimate the distribution of both hadrons and leptons and calculate the corresponding spectral energy distributions of photons. We find that the strong mass load of the superwind, recently determined through ALMA observations, strongly attenuates the efficiency of DSA in NGC 253, whereas SDA is constrained by the age of the starburst. We conclude that NGC 253 and similar starbursts can only accelerate iron nuclei beyond $\sim10^{18}$ eV under very special conditions. If the central region of the galaxy harbors a starved supermassive black hole of $\sim10^6$ $M_{\odot}$, as suggested by some recent observations, a contribution in the range $10^{18}-10^{19}$ eV can be present for accretion rates $\dot{m}\sim10^{-3}$ in Eddington units. Only if very strong magnetic field amplification occurs close to the superwind shock energies of the order of 100 EeV might be achieved.

astro-ph.HE↗

arXiv2017

High-energy radiation from collisions of high velocity clouds and the Galactic disk

High-velocity clouds (HVCs) are interstellar clouds of atomic hydrogen that do not partake of the Galactic rotation and have velocities of a several hundred kilometers per second. A considerable number of these clouds are falling down towards the Galactic disk. HVCs form large and massive complexes, so their collisions with the disk must release a great amount of energy into the interstellar medium. The cloud-disk interaction produces two shocks, one propagates through the cloud and the other through the disk; the properties of these shocks depend mainly on the cloud velocity and the disk-cloud density ratio. In this work we study the conditions necessary for these shocks to accelerate particles by diffusive shock acceleration and the produced non-thermal radiation. We analyze particle acceleration in both the cloud and disk shocks. Solving a time-dependent 2-D transport equation for both relativistic electrons and protons we obtain particle distributions and non-thermal spectral energy distributions. In a shocked cloud significant synchrotron radio emission is produced along with soft gamma rays. In the case of acceleration in the shocked disk, the non-thermal radiation is stronger; the gamma rays, of leptonic origin, might be detectable with current instruments. A large number of protons are injected into the Galactic interstellar medium, and locally exceed the cosmic-ray background. We conclude that under adequate conditions the contribution from HVC-disk collisions to the galactic population of relativistic particles and the associated extended non-thermal radiation might be important.

astro-ph.HE↗

arXiv2017

The Highly Collimated Radio Jet of HH 80-81: Structure and Non-Thermal Emission

Radio emission from protostellar jets is usually dominated by free-free emission from thermal electrons. However, in some cases, it has been proposed that non-thermal emission could also be present. This additional contribution from non-thermal emission has been inferred through negative spectral indices at centimeter wavelengths in some regions of the radio jets. In the case of HH 80-81, one of the most powerful protostellar jets known, linearly polarized emission has also been detected, revealing that the non-thermal emission is of synchrotron nature from a population of relativistic particles in the jet. This result implies that an acceleration mechanism should be taking place in some parts of the jet. Here, we present new high sensitivity and high angular resolution radio observations at several wavelengths (in the 3-20 cm range) of the HH80-81 radio jet. These new observations represent an improvement in sensitivity and angular resolution by a factor of $\sim$10 with respect to previous observations. This allows us to resolve the morphology of the radio jet, and to study the different emission mechanisms involved through spectral index maps. We conclude that synchrotron emission in this jet arises from an extended component detected at low frequencies and from the termination points of the jet, where strong shocks against the ambient medium can produce efficient particle acceleration.

astro-ph.HE↗

arXiv2017

Effects of Scalar-Tensor-Vector Gravity on relativistic jets

Scalar-Tensor-Vector Gravity (STVG) is a theory that does not require dark components to describe astrophysical data. We aim at constraining the free parameters of STVG based on recent observations of the jet in M87. We derive the equations of motion for particles in STVG-Kerr spacetime, we develop a numerical code that integrates such equations, and apply it to the jet of M87. We find that STVG deviates from GR and we set new upper limits for the free parameters of the former. We conclude that STVG is not contradicted by the observational data of M87, and may help to explain jet formation.

astro-ph.HE↗

arXiv2017

Gravitational energy and radiation of a charged black hole

We analyze the energy configuration of a charged black hole in the Teleparallel Framework of General Relativity. We obtain the energy-momentum tensor of the gravitational field in a stationary frame, and we calculate its contribution to the total energy of the system. We study the same gravitational field measured by an accelerated frame and we analyze how the energy-momentum tensor is transformed. We found that in the accelerated frame, a Poyinting-like flux appears for the gravitational field but not for the electromagnetic field.

gr-qc↗

arXiv2017

Bunge on gravitational waves

I discuss the recent claims made by Mario Bunge on the philosophical implications of the discovery of gravitational waves. I think that Bunge is right when he points out that the detection implies the materiality of spacetime, but I reject his identification of spacetime with the gravitational field. I show that Bunge's analysis of the spacetime inside a hollow sphere is defective, but this in no way affects his main claim.

gr-qc↗

arXiv2017

Is Teleparallel Gravity really equivalent to General Relativity?

An axiomatization of the so-called Teleparallel Equivalent to General Relativity is presented. A set of formal and semantic postulates are elaborated from where the physical meaning of various key concepts of the theory are clarified. These concepts include those of inertia, Lorentz and diffeomorphism invariance, and reference frame. It is shown that Teleparallel Gravity admits a wider representation of space-time than General Relativity, allowing to define properties of the gravitational field such as energy and momentum that are usually considered problematic. In this sense, although the dynamical equations of both theories are equivalent, their inequivalence from a physical point of view is demonstrated. Finally, the axiomatic formulation is used to compare Teleparallel Gravity with other theories of gravity based on absolute parallelism such as non-local and f(T) gravity.

gr-qc↗

arXiv2017

Accretion disks around black holes in Scalar-Tensor-Vector Gravity

Scalar Tensor Vector Gravity (STVG) is an alternative theory of gravitation that has successfully explained the rotation curves of nearby galaxies, the dynamics of galactic clusters, and cosmological data without dark matter, but has hardly been tested in the strong gravity regime. In this work, we aim at building radiative models of thin accretion disks for both Schwarzschild and Kerr black holes in STVG theory. In particular, we study stable circular equatorial orbits around stellar and supermassive black holes in Schwarzschild and Kerr STVG spacetimes. We also calculate the temperature and luminosity distributions of accretion disks around these objects. We find that accretion disks in STVG around stellar and supermassive black holes are colder and less luminous than in GR. The spectral energy distributions obtained do not contradict current astronomical observations.

astro-ph.HE↗

arXiv2017

A model for the repeating FRB 121102 in the AGN scenario

Fast radio bursts, or FRBs, are transient sources of unknown origin. Recent radio and optical observations have provided strong evidence for an extragalactic origin of the phenomenon and the precise localization of the repeating FRB 121102. Observations using the Karl G. Jansky Very Large Array (VLA) and very-long-baseline interferometry (VLBI) have revealed the existence of a continuum non-thermal radio source consistent with the location of the bursts in a dwarf galaxy. All these new data rule out several models that were previously proposed, and impose stringent constraints to new models. We aim to model FRB 121102 in light of the new observational results in the active galactic nucleus (AGN) scenario. We propose a model for repeating FRBs in which a non-steady relativistic $e^\pm$-beam, accelerated by an impulsive magnetohydrodynamic (MHD)-driven mechanism, interacts with a cloud at the centre of a star-forming dwarf galaxy. The interaction generates regions of high electrostatic field called cavitons in the plasma cloud. Turbulence is also produced in the beam. These processes, plus particle isotropization, the interaction scale, and light retardation effects, provide the necessary ingredients for short-lived, bright coherent radiation bursts. The mechanism studied in this work explains the general properties of FRB 121102, and may also be applied to other repetitive FRBs. Coherent emission from electrons and positrons accelerated in cavitons provides a plausible explanation of FRBs.

astro-ph.HE↗