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All articles by
Romero D. A. Roa | Romero Gustavo E. | Romero J. L. | Romero C. | Romero G. E. | Romero G. | Romero José Luis | Romero Juan M. | Romero Aldo H. | Romero A. H.

Gustavo Frédéric | Gustavo Appignanesi | Gustavo Baume | Gustavo Breno | Gustavo F. | Gustavo Lopez | Gustavo Luis | Gustavo Mata

E Weinan | E Yiwen | e | E Qiang | E Sandhya | E Shijia | E Sivasankar | e A. Torres-Forn' | e A. Vicer' | e Alain Togb\'

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Large Latin American Millimeter Array

Romero Gustavo E.
01 Oct 2020 astro-ph.IM arxiv.org/abs/2010.00738

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.

Relativistic rigid systems and the cosmic expansion

Combi Luciano, Romero Gustavo E.
26 Aug 2020 gr-qc arxiv.org/abs/2008.11836

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.

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

Romero Gustavo E.,
19 Jul 2020 astro-ph.HE arxiv.org/abs/2007.09717

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.

High-energy processes in starburst-driven winds

, Romero Gustavo E., Roth Markus
22 Jun 2020 astro-ph.HE arxiv.org/abs/2006.12259

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.

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

, Romero Gustavo E.
27 Mar 2020 astro-ph.HE arxiv.org/abs/2003.12438

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.

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

, Romero Gustavo E., Vieyro Florencia L.
20 Mar 2020 astro-ph.HE astro-ph.GA arxiv.org/abs/2003.09410

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.

Astrophysical Constraints on Strong Modified Gravity

, Romero Gustavo E.
03 Jan 2020 gr-qc astro-ph.CO astro-ph.GA astro-ph.HE arxiv.org/abs/2001.00863

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.

Gamma Rays from Large-Scale Outflows in Starburst Galaxies

Romero Gustavo E.,
17 Dec 2019 astro-ph.HE arxiv.org/abs/1912.07969

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.

Exact cosmological black hole solutions in Scalar Tensor Vector Gravity

, Romero Gustavo E.
20 May 2019 gr-qc astro-ph.CO arxiv.org/abs/1905.08274

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 $\alpha $ 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.

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

Romero Gustavo E.
06 Mar 2019 astro-ph.HE arxiv.org/abs/1903.02496

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), $\gamma$ rays. Some of these systems are quite eccentric and the conditions for efficient particle acceleration and $\gamma$-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.