Colonia Elisa, Argentina
Colonia Elisa, Argentina

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Pepe C.,Institute Astronomia y Fisica del Espacio | Pepe C.,CONICET | Pellizza L.J.,CONICET | Pellizza L.J.,Instituto Argentino Of Radioastronomia
Monthly Notices of the Royal Astronomical Society | Year: 2016

During the last decades, much effort has been devoted to explain the discrepancy between the amount of intracluster medium (ICM) estimated from stellar evolution theories and that emerging from observations in globular clusters (GCs). One possible scenario is the accretion of this medium by an intermediate-mass black hole (IMBH) at the centre of the cluster. In this work, we aim at modelling the cluster colour-excess profile as a tracer of the ICM density, both with and without an IMBH. Comparing the profiles with observations allows us to test the existence of IMBHs and their possible role in the cleansing of the ICM. We derive the intracluster density profiles from hydrodynamical models of accretion on to a central IMBH in a GC and we determine the corresponding dust density. This model is applied to a list of 25 Galactic GCs. We find that central IMBHs decrease the ICM by several orders of magnitude. In a subset of nine clusters, the absence of the black hole combined with a low-ICM temperature would be at odds with present gas mass content estimations. As a result, we conclude that IMBHs are an effective cleansing mechanism of the ICM of GCs. We construct synthetic extinction maps for M 62 and ωCen, two clusters in the small subset of nine with observed 2D extinction maps. We find that under reasonable assumptions regarding the model parameters, if the gas temperature in M 62 is close to 8000 K, an IMBH needs to be invoked. Further ICM observations regarding both the gas and dust in GCs could help to settle this issue. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Romero G.E.,Instituto Argentino Of Radioastronomia | Thomas R.,University Paris - Sud | Perez D.,Instituto Argentino Of Radioastronomia
International Journal of Theoretical Physics | Year: 2012

Pure thermodynamical considerations to describe the entropic evolution of the universe seem to violate the Second Law of Thermodynamics. This suggests that the gravitational field itself has entropy. In this paper we expand recent work done by Rudjord, Grøn and Sigbjørn where they suggested a method to calculate the gravitational entropy in black holes based on the so-called 'Weyl curvature conjecture'. We study the formulation of an estimator for the gravitational entropy of Reissner-Nordström, Kerr, Kerr-Newman black holes, and a simple case of wormhole. We calculate in each case the entropy for both horizons and the interior entropy density. Then, we analyze whether the functions obtained have the expected behavior for an appropriate description of the gravitational entropy density. © 2011 Springer Science+Business Media, LLC.


Pepe C.,Institute Astronomia y Fisica del Espacio | Pepe C.,CONICET | Pellizza L.J.,Institute Astronomia y Fisica del Espacio | Pellizza L.J.,CONICET | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

In this work we investigate the accretion of cosmological fluids on to an intermediate-mass black hole at the centre of a globular cluster, focusing on the influence of the parent stellar system on the accretion flow. We show that the accretion of cosmic background radiation and the so-called dark energy on to an intermediate-mass black hole is negligible. On the other hand, if cold dark matter has a non-vanishing pressure, the accretion of dark matter is large enough to increase the black hole mass well beyond the present observed upper limits. We conclude that either intermediate-mass black holes do not exist, or dark matter does not exist, or it is not strictly collisionless. In the latter case, we set a lower limit for the parameter of the cold dark matter equation of state. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Del Valle M.V.,Instituto Argentino Of Radioastronomia | Lazarian A.,University of Wisconsin - Madison | Santos-Lima R.,University of Sao Paulo
Monthly Notices of the Royal Astronomical Society | Year: 2016

Galactic cosmic rays are believed to be mostly accelerated at supernova shocks. However, the interstellar magnetic field is too weak to efficiently accelerate galactic cosmic rays up to the highest energies, i.e. 1015 eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic ray non-resonant streaming instability has been claimed to be responsible for the amplification of precursor magnetic fields. However, an alternative mechanism has been proposed in which the cosmic ray pressure gradient forms the shock precursor and drives turbulence, amplifying the magnetic field via the small-scale dynamo. Key ingredients for the mechanism to operate are the inhomogeneities present in the interstellar medium. These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different interstellar medium conditions through 3D magnetohydrodynamic numerical simulations. © 2016 The Authors.


Perez D.,Instituto Argentino Of Radioastronomia | Romero G.E.,Instituto Argentino Of Radioastronomia | Romero G.E.,National University of La Plata | Perez Bergliaffa S.E.,State University of Rio de Janeiro
Astronomy and Astrophysics | Year: 2013

Context. Stellar-mass black holes offer what is perhaps the best scenario to test theories of gravity in the strong-field regime. In particular, f(R) theories, which have been widely discus in a cosmological context, can be constrained through realistic astrophysical models of phenomena around black holes. Aims. We aim at building radiative models of thin accretion disks for both Schwarzschild and Kerr black holes in f(R) gravity. Methods. We study particle motion in f(R)-Schwarzschild and Kerr space-times. Results. We present the spectral energy distribution of the accretion disk around constant Ricci scalar f(R) black holes, and constrain specific f(R) prescriptions using features of these systems. Conclusions. A precise determination of both the spin and accretion rate onto black holes along with X-ray observations of their thermal spectrum might allow to identify deviations of gravity from general relativity. We use recent data on the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters of a class of f(R) models. © ESO, 2013.


Araudo A.T.,National Autonomous University of Mexico | Bosch-Ramon V.,University of Barcelona | Romero G.E.,Instituto Argentino Of Radioastronomia | Romero G.E.,National University of La Plata
Monthly Notices of the Royal Astronomical Society | Year: 2013

Dense populations of stars surround the nuclear regions of galaxies. In active galactic nuclei, these stars can interact with the relativistic jets launched by the supermassive black hole. In this work, we study the interaction of early-type stars with relativistic jets in active galactic nuclei. A bow-shaped double-shock structure is formed as a consequence of the interaction of the jet and the stellar wind of each early-type star. Particles can be accelerated up to relativistic energies in these shocks and emit high-energy radiation. We compute, considering different stellar densities of the galactic core, the gamma-ray emission produced by nonthermal radiative processes. This radiation may be significant in some cases, and its detection might yield valuable information on the properties of the stellar population in the galaxy nucleus, as well as on the relativistic jet. This emission is expected to be particularly relevant for nearby non-blazar sources. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Del Valle M.V.,Instituto Argentino Of Radioastronomia | Del Valle M.V.,National University of La Plata | Romero G.E.,Instituto Argentino Of Radioastronomia | Romero G.E.,National University of La Plata | De Becker M.,University of Liège
Astronomy and Astrophysics | Year: 2013

Context. HD 195592 is an O-type super-giant star, known as a well-established runaway. Recently, a Fermi γ-ray source (2FGL J2030.7+4417) with a position compatible with that of HD 195592 has been reported. Aims. Our goal is to explore the scenario where HD 195592 is the counterpart of the Fermi γ-ray source, modeling the non-thermal emission produced in the bowshock of the runaway star. Methods. We calculated the spectral energy distribution of the radiation produced in the bowshock of HD 195592 and we compared it with Fermi observations of the 2FGL J2030.7+4417. Results. We present relativistic particle losses and the resulting radiation of the bowshock of HD 195592 and show that the latter is compatible with the detected γ-ray emission. Conclusions. We conclude that the Fermi source 2FGL J2030.7+4417 might be produced, under some energetic assumptions, by inverse Compton up-scattering of photons from the heated dust in the bowshock of the runaway star. Therefore, HD 195592 might be the very first object detected belonging to the category of γ-ray emitting runaway massive stars, whose existence has been recently predicted. © ESO 2013.


Araudo A.T.,University of Oxford | del Valle M.V.,Instituto Argentino Of Radioastronomia
Nuclear Physics B - Proceedings Supplements | Year: 2014

In addition to efficient particle accelerators in the Galaxy, such as supernova remnants and pulsar wind nebulae, other sources can accelerate particles up to very high energies. We present the case of protostellar jets and runaway stars, where strong shocks act upon electrons and protons through a diffusive process. The detection of synchrotron radio emission from these sources indicates that at least electrons are accelerated in these systems. If protons are accelerated as well, both population of relativistic particles can produce high-energy radiation through a variety of processes. We show results from models of the non-thermal emission, from radio to gamma rays, in these kind of sources. High-energy photons from both protostellar jets and massive runaways might be detectable with the current facilities in the GeV domain, or by the forthcoming Cherenkov arrays. A clear detection would prove that young stars are also cosmic ray accelerators. © 2014 Elsevier B.V.


Del Valle M.V.,Instituto Argentino Of Radioastronomia | Del Valle M.V.,National University of La Plata | Romero G.E.,Instituto Argentino Of Radioastronomia | Romero G.E.,National University of La Plata
Astronomy and Astrophysics | Year: 2012

Context. Runaway massive stars are O-and B-type stars with high spatial velocities with respect to the interstellar medium. These stars can produce bowshocks in the surrounding gas. Bowshocks develop as arc-shaped structures, with bows pointing to the same direction as the stellar velocity, while the star moves supersonically through the interstellar gas. The piled-up shocked matter emits thermal radiation and a population of locally accelerated relativistic particles is expected to produce non-thermal emission over a wide range of energies. Aims. We aim to model the non-thermal radiation produced in these sources. Methods. Under some assumptions, we computed the non-thermal emission produced by the relativistic particles and the thermal radiation caused by free-free interactions, for O4I and O9I stars. We applied our model to ζ Oph (HD 149757), an intensively studied massive star seen from the northern hemisphere. This star has spectral type O9.5V and is a well-known runaway. Results. Spectral energy distributions of massive runaways are predicted for the whole electromagnetic spectrum. Conclusions. We conclude that the non-thermal radiation might be detectable at various energy bands for relatively nearby runaway stars, especially at high-energy gamma rays. Inverse Compton scattering with photons from the heated dust gives the most important contribution to the high-energy spectrum. This emission approaches Fermi sensitivities in the case of ζ Oph. © 2012 ESO.


Del Valle M.V.,Instituto Argentino Of Radioastronomia | Del Valle M.V.,National University of La Plata | Romero G.E.,Instituto Argentino Of Radioastronomia | Romero G.E.,National University of La Plata
Astronomy and Astrophysics | Year: 2014

Context. Runaway stars are ejected from their formation sites well within molecular cores in giant dark clouds. Eventually, these stars can travel through the molecular clouds, which are highly inhomogeneous. The powerful winds of massive runaway stars interact with the medium forming bowshocks. Recent observations and theoretical modelling suggest that these bowshocks emit non-thermal radiation. As the massive stars move through the inhomogeneous ambient gas the physical properties of the bowshocks are modified, producing changes in the non-thermal emission. Aims. We aim to compute the non-thermal radiation produced in the bowshocks of runaway massive stars when travelling through a molecular cloud. Methods. We calculate the non-thermal emission and absorption for two types of massive runaway stars, an O9I and an O4I, as they move through a density gradient. Results. We present the spectral energy distributions for the runaway stars modelled. Additionally, we obtain light curves at different energy ranges. We find significant variations in the emission over timescales of ~1 yr. Conclusions. We conclude that bowshocks of massive runaway stars, under some assumptions, might be variable gamma-ray sources, with variability timescales that depend on the medium density profile. These objects might constitute a population of galactic gamma-ray sources turning on and off within years. © 2014 ESO.

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