arXiv subjects

Gustavo E. Romero

Publications and source records attributed to Gustavo E. Romero.

At least 37 records · Page 2Linked to original sources

arXiv2023

NuSTAR and XMM-Newton observations of the binary 4FGL J1405.1-6119. A $γ$-ray emitting microquasar?

4FGL J1405.1-6119 is a high-mass $γ$-ray emitting binary that has been studied at several wavelengths. The nature of this type of binary is still under debate, with three possible scenarios usually invoked to explain the origin of the $γ$-ray emission: collisions between the winds of a rapidly rotating neutron star and its companion, collisions between the winds of two massive stars, and non-thermal emission from the jet of a microquasar. We analyze two pairs of simultaneous NuSTAR and XMM-Newton observations to investigate the origin of the radio, X-ray, and $γ$-ray emissions. We extracted light curves between 0.5-78 keV from two different epochs, named Epoch 1 and Epoch 2, respectively. We propose a scenario to explain the observations involving a parabolic, mildly relativistic, lepto-hadronic jet. This jet has a compact acceleration region that injects a hard spectrum of relativistic particles. The dominant non-thermal emission processes include synchrotron radiation of electrons, inverse Compton scattering of photons from the stellar radiation field, and the decay of neutral pions resulting from inelastic proton-proton collisions within the bulk matter of the jet. These estimates are in accordance with the values of a super-Eddington lepto-hadronic jet scenario. The compact object could be either a black hole or a neutron star with a low magnetic field. Most of the X-ray emission from the disk could be absorbed by the dense wind that is ejected from the same disk. We conclude that it is possible that the binary 4FGL J1405.1-6119 could be a supercritical microquasar like SS433.

astro-ph.HE↗

arXiv2023

Achromatic rapid flares in hard X-rays in the $γ$-ray binary LS I +61-303

We report on the presence of very rapid hard X-ray variability in the $γ$-ray binary LS I +61 303. The results were obtained by analysing NuSTAR data, which show two achromatic strong flares on ks time-scales before apastron. The Swift-BAT orbital X-ray light curve is also presented, and the NuSTAR data are put in the context of the system orbit. The spectrum and estimated physical conditions of the emitting region indicate that the radiation is synchrotron emission from relativistic electrons, likely produced in a shocked pulsar wind. The achromaticity suggests that losses are dominated by escape or adiabatic cooling in a relativistic flow, and the overall behaviour in hard X-rays can be explained by abrupt changes in the size of the emitting region and/or its motion relative to the line of sight, with Doppler boosting potentially being a prominent effect. The rapid changes of the emitter could be the result of different situations such as quick changes in the intra-binary shock, variations in the re-accelerated shocked pulsar wind outside the binary, or strong fluctuations in the location and size of the Coriolis shock region. Although future multi-wavelength observations are needed to further constrain the physical properties of the high-energy emitter, this work already provides important insight into the complex dynamics and radiation processes in LS I +61-303.

astro-ph.HE↗

arXiv2023

Study of variability in long-term multiwavelength optical lightcurves of blazar AO 0235+164

We present a long-term and intraday variability study on optical multiwaveband ($U\!BV\!RI$) data from the blazar AO 0235+164 collected by various telescopes for $\sim$44 years (1975--2019). The blazar was found to be significantly variable over the years in all wavebands with a variation of about six magnitudes between its low and active states. The variations in the different wavebands are highly correlated without any time lag. We did not observe any significant trend in color variation with time, but we observed a bluer-when-brighter trend between the $B-I$ color index and the $R$-magnitude. Optical $BV\!R$-band spectral energy distributions always show a convex shape. Significant intraday variability was frequently seen in the quasi-simultaneous observations of AO\,0235+164 made on 22 nights in $R$ and $V$-bands by the CASLEO and CAHA telescopes during 1999--2019. We also estimated the central supermassive black-hole mass of $7.9\times10^7 M_{\odot}$ by analyzing the broad Mg II emission line in AO 0235+164's spectrum. We briefly explore the probable physical scenarios responsible for the observed variability.

astro-ph.HE↗

arXiv2022

Nonthermal radiation from the central region of super-accreting active galactic nuclei

The radio emission mechanism in active galactic nuclei (AGN) with high accretion rates is unclear. It has been suggested that low-power jets may explain the observed radiation at sub-parsec scales. The mechanisms for jet formation at super-Eddington rates, however, are not well understood. On the same scale, clouds from the broad-line region (BLR) propagating with supersonic velocities in the wind launched by the accretion disk may lead to the production of nonthermal radiation. We aim to characterize the nonthermal emission produced by the propagation of clouds through the wind of the accretion disk in super-accreting AGNs and to estimate the relevance of such contribution to the radio band of the electromagnetic spectrum. We determine the conditions under which the BLR clouds are not destroyed by shocks or hydrodynamic instabilities when immersed in the powerful wind of the accretion disk. These clouds form bowshocks that are suitable sites for particle acceleration. We develop a semianalytical model to calculate the distribution of relativistic particles in these bowshocks and the associated spectral energy distribution (SED) of the emitted radiation. For typical parameters of super-accreting AGNs, we find that the cloud-wind interactions can produce nonthermal emission from radio up to a few tens of TeV, with slight absorption effects, if the processes occur outside the wind photosphere. Radio emission in AGNs without jets can be explained if the accretion rate is super-Eddington and there is a broad-line region at sub-parsec scales around the central black hole. The accretion rate must not be extremely high so most of the clouds orbit outside of the wind photosphere and the radiation can escape to the observer.

astro-ph.HE↗

arXiv2022

Survival of black holes through a cosmological bounce

We analyze whether a black hole can exist and survive in a universe that goes through a cosmological bounce. To this end, we investigate a central inhomogeneity embedded in a bouncing cosmological background modeled by the comoving generalized McVittie metric. Contrary to other dynamical metrics available in the literature, this solution allows for the interaction of the central object with the cosmological fluid. We show that the horizons associated with this metric change with cosmic time because they are coupled to the cosmic evolution as the mass of the central object is always proportional to the scale factor: it decreases during contraction and increases during expansion phases. After a full analysis of the causal structure of this spacetime, we determine that a dynamical black hole persists during the contraction, bounce, and expansion of the universe. This result implies that there is a class of bouncing models that admits black holes at all cosmological epochs. If these models are correct approximation to the real universe, then black holes surviving a cosmic collapse could play some role in the subsequent expanding phase.

gr-qc↗

arXiv2022

Detection of a quasi-periodic oscillation in the optical light curve of the remarkable blazar AO 0235+164

We present a long term optical $R$ band light curve analysis of the gravitationally lensed blazar AO 0235+164 in the time span 1982 - 2019. Several methods of analysis lead to the result that there is a periodicity of ~8.13 years present in these data. In addition, each of these five major flares are apparently double-peaked, with the secondary peak following the primary one by ~2 years. Along with the well known system, OJ 287, our finding constitutes one of the most secure cases of long term quasi-periodic optical behaviour in a blazar ever found. A binary supermassive black hole system appears to provide a good explanation for these results.

astro-ph.HE↗

arXiv2021

The content of astrophysical jets

Jets, collimated outflows of particles and fields, are observed in a wide variety of astrophysical systems, including Active Galactic Nuclei of various types, microquasars, gamma-ray bursts, and young stellar objects. Despite intensive efforts along several decades of observational and theoretical research, there are still many uncertainties and open questions related to how jets are produced and what is their composition. In this review I offer an outline of some current views on the content and basic properties of astrophysical jets.

astro-ph.HE↗

arXiv2021

Nonthermal processes in hot accretion flows onto supermassive black holes: An inhomogeneous model

Context. Many low-redshift active galactic nuclei harbor a supermassive black hole accreting matter at low or medium rates. At such rates, the accretion flow usually consists of a cold optically thick disk, plus a hot, low density, collisionless corona. In the latter component, charged particles can be accelerated to high energies by various mechanisms. Aims. We aim to investigate, in detail, nonthermal processes in hot accretion flows onto supermassive black holes, covering a wide range of accretion rates and luminosities. Methods. We developed a model consisting of a thin Shakura-Sunyaev disk plus an inner hot accretion flow or corona, modeled as a radiatively inefficient accretion flow, where nonthermal processes take place. We solved the transport equations for relativistic particles and estimated the spectral energy distributions resulting from nonthermal interactions between the various particle species and the fields in the source. Results. We covered a variety of scenarios, from low accretion rates up to 10% of the Eddington limit, and identified the relevant cooling mechanisms in each case. The presence of hadrons in the hot flow is decisive for the spectral shape, giving rise to secondary particles and gamma-ray cascades. We applied our model to the source IC 4329A, confirming earlier results that showed evidence of nonthermal particles in the corona.

astro-ph.HE↗

arXiv2021

Black hole in asymmetric cosmological bounce

We determine the causal structure of the McVittie spacetime for a cosmological model with an asymmetric bounce. The analysis includes the computation of trapping horizons, regular, trapped, and anti-trapped regions, and the integration of the trajectories of radial null geodesics before, during, and after the bounce. We find a trapped region since the beginning of the contracting phase up to shortly before the bounce, thus showing the existence of a black hole. When the universe reaches a certain minimum scale in the contracting phase, the trapping horizons disappear and the central singularity becomes naked. These results suggest that neither a contracting nor an expanding universe can accommodate a black hole at all times.

gr-qc↗

arXiv2021

Dynamical black hole in a bouncing universe

We analyze the causal structure of McVittie spacetime for a classical bouncing cosmological model. In particular, we compute the trapping horizons of the metric and integrate the trajectories of radial null geodesics before, during, and after the bounce takes place. In the contracting phase up to the occurrence of the bounce, a dynamical black hole is present. When the universe reaches a certain minimum scale, the trapping horizons disappear and the black hole ceases to exist. After the bounce, the central weak singularity becomes naked. In the expanding phase, for large positive values of the cosmic time, the behaviour of null geodesics indicates that the solution contains a black hole. These results suggest that neither a contracting nor an expanding universe can accommodate a black hole at all times.

gr-qc↗

arXiv2021

Neutrino production in Population III microquasars

Microquasars (MQs) are binary systems composed by a star feeding mass to a compact object through an accretion disk. The compact object, usually a black hole, launches oppositely directed jets which are typically observed in our galaxy through their broadband electromagnetic emission. These jets are considered potential galactic neutrino sources. MQs can also have been formed by the first generations of stars in the universe, i.e., Population III (Pop III) stars, which are considered essential contributors to the ionization processes that took place during the period of 'cosmic reionization'. In the present work, we develop a model that accounts for the main particle processes occurring within Pop III MQ jets, with the aim to obtain the diffuse neutrino flux at the Earth [...] We find that, for a range of parameters suitable for Pop III MQ jets, the most relevant site for neutrino production in the jets is the base of the inner conical jet. Additionally, if protons accelerated at the forward shock formed at terminal jet region can escape from the outer shell, they would produce further neutrinos via $pγ$ interactions with the cosmic microwave background (CMB). The latter contribution to the diffuse neutrino flux turns out to be dominant in the range $10^7 {\rm GeV} \lesssim E_ν \lesssim 10^9 {\rm GeV}$, while the neutrinos produced in the inner jet could only account for a small fraction of the IceCube flux for $E_ν\sim 10^5$ GeV. The co-produced multiwavelength photon background is also computed and it is checked to be in agreement with observations.

astro-ph.HE↗

arXiv2020

Equatorial outflows driven by jets in Population III microquasars

Binary systems of Population III can evolve to microquasars when one of the stars collapses into a black hole. When the compact object accretes matter at a rate greater than the Eddington rate, powerful jets and winds driven by strong radiation pressure should form. We investigate the structure of the jet-wind system for a model of Population III microquasar on scales beyond the jet-wind formation region. Using relativistic hydrodynamic simulations we find that the ratio of kinetic power between the jet and the disk wind determines the configuration of the system. When the power is dominated by the wind, the jet fills a narrow channel, collimated by the dense outflow. When the jet dominates the power of the system, part of its energy is diverted turning the wind into a quasi-equatorial flow, while the jet widens. From the results of our simulations, we implement semi-analytical calculations of the impact of the quasiequatorial wind on scales of the order of the size of the binary system. Our results indicate that Population III microquasars might inject gamma rays and relativistic particles into the early intergalactic medium, contributing to its reionization at large distances from the binary system.

astro-ph.HE↗

arXiv2020

Large Latin American Millimeter Array

The Large Latin American Millimeter Array (LLAMA) is a multipurpose single-dish 12 m radiotelescope with VLBI capability under construction in the Puna de Atacama desert in the Province of Salta, Argentina. In this paper I review the project, the instrument, the current status, and the scientific goals of this astronomical collaboration between Argentina and Brazil.

astro-ph.IM↗

arXiv2020

Relativistic rigid systems and the cosmic expansion

We analyze the necessary conditions for a body to remain rigid in an expanding cosmological Universe. First, we establish the main theorems and definitions for having a rigid body in a general spacetime as well as the new concept of quasilocal rigidity. We apply the obtained results to a homogeneous universe exploring the differences with flat spacetime. We discuss how the concept of rigid body helps to understand the expansion of space in cosmology. Finally, using a rigid system as a reference frame, we calculate the gravitational energy, and we compare it with previous results in the literature.

gr-qc↗

arXiv2020

The Origin of Matter at the Base of Relativistic Jets in Active Galactic Nuclei

The generation of relativistic jets in active sources such as blazars is a complex problem with many aspects, most of them still not fully understood. Relativistic jets are likely produced by the accretion of matter and magnetic fields onto spinning black holes. Ergospheric dragging effects launch a Poynting-dominated outflow in the polar directions of these systems. Observations with very high resolution of the jet in the nearby radio galaxy M87 and evidence of extremely fast variability in the non-thermal radiation of several other objects indicate that charged particles produce synchrotron emission and gamma rays very close to the base of the jet. How these particles are injected into the magnetically shielded outflow is a mystery. Here we explore the effects of various processes in the hot accretion inflow close to the black hole that might result in the copious production of neutral particles which, through annihilation and decay in the jet's funnel, might load the outflow with mass and charged particles on scales of a few Schwarzschild radii.

astro-ph.HE↗

arXiv2020

High-energy processes in starburst-driven winds

Starburst galaxies generate large-scale winds powered by the activity in the star-forming regions located in the galactic disks. Fragmentation of the disk produced by the outbreak of the wind results in the formation of clouds. Bowshocks caused by the supersonic outflow appear around such clouds. In this paper we discuss the acceleration of relativistic particles and the production of non-thermal radiation in such scenario. Cosmic rays accelerated at the bowshocks do not reach the highest energies, although the high-energy luminosity generated is significant. We show that up to ~10% of the gamma-ray emission in starbursts might come from these sources outside the galactic disks. Discrete X-ray sources with a power-law component are also expected.

astro-ph.HE↗

arXiv2020

Radiation from the impact of broad-line region clouds onto AGN accretion disks

Active galactic nuclei are supermassive black holes surrounded by an accretion disk, two populations of clouds, bipolar jets, and a dusty torus. The clouds move in Keplerian orbits at high velocities. In particular, the broad-line region (BLR) clouds have velocities ranging from $1000$ to $10000$ km s$^{-1}$. Given the extreme proximity of these clouds to the supermassive black hole, frequent collisions with the accretion disk should occur. The impact of BLR clouds onto the accretion disk can produce strong shock waves where particles might be accelerated. The goal of this work is to investigate the production of relativistic particles, and the associated non-thermal radiation in these events. In particular, we apply the model we develop to the Seyfert galaxy NGC 1068. We analyze the efficiency of diffusive shock acceleration in the shock of colliding clouds of the BLR with the accretion disk. We calculate the spectral energy distribution of photons generated by the relativistic particles and estimate the number of simultaneous impacts needed to explain the gamma radiation observed by the Fermi satellite in Seyfert galaxies. We find that is possible to understand the measured gamma emission in terms of the interaction of clouds with the disk if the hard X-ray emission of the source is at least obscured between $20\%$ and $40\%$. The total number of clouds contained in the BLR region might be between $3\times10^{8}$ and $6\times10^{8}$, which are values in good agreement with the observational evidence. The maximum energy achieved by the protons ($\sim$ PeV) in this context allows the production of neutrinos in the observing range of IceCube.

astro-ph.HE↗

arXiv2020

Cosmic rays from the nearby starburst galaxy NGC 253: the effect of a low luminosity active galactic nucleus

NGC 253 is a nearby starburst galaxy in the Sculptor group located at a distance of 3.5 Mpc that has been suggested by some authors as a potential site for cosmic-ray acceleration up to ultra-high energies. Its nuclear region is heavily obscured by gas and dust, which prevents establishing whether or not the galaxy harbours a supermassive black hole coexisting with the starburst. Some sources have been proposed in the literature as candidates for an active nucleus. In this work, we aim at determining the implications that the presence of a supermassive black hole at the nucleus of NGC 253 might have on cosmic ray acceleration. With this aim, we model the accretion flow on to the putative active nucleus, and we evaluate the feasibility of particle acceleration by the black hole dynamo mechanism. As a by-product, we explore the potential contribution from non-thermal particles in the accretion flow to the high-energy emission of the galaxy. We found that in the three most plausible nucleus candidates, the emission of the accretion flow would inhibit the black hole dynamo mechanism. To rule out completely the influence that a putative nucleus in NGC 253 might have in cosmic-ray acceleration, a better clarification concerning the true nature of the nucleus is needed.

astro-ph.HE↗