arXiv subjects

Anabella T. Araudo

Publications and source records attributed to Anabella T. Araudo.

32 records · Page 2Linked to original sources

arXiv2011

Transient gamma-ray emission from Cygnus X-3

The high-mass microquasar Cygnus X-3 has been recently detected in a flaring state by the gamma-ray satellites Fermi and Agile. In the present contribution, we study the high-energy emission from Cygnus X-3 through a model based on the interaction of clumps from the Wolf-Rayet wind with the jet. The clumps inside the jet act as obstacles in which shocks are formed leading to particle acceleration and non-thermal emission. We model the high energy emission produced by the interaction of one clump with the jet and briefly discus the possibility of many clumps interacting with the jet. From the characteristics of the considered scenario, the produced emission could be flare-like due to discontinuous clump penetration, with the GeV long-term activity explained by changes in the wind properties.

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arXiv2010

Radiation from matter entrainment in astrophysical jets: the AGN case

Jets are found in a variety of astrophysical sources, from young stellar objects to active galactic nuclei. In all the cases the jet propagates with a supersonic velocity through the external medium, which can be inhomogeneous, and inhomogeneities could penetrate into the jet. The interaction of the jet material with an obstacle produces a bow shock in the jet in which particles can be accelerated up to relativistic energies and emit high-energy photons. In this work, we explore the active galactic nuclei scenario, focusing on the dynamical and radiative consequences of the interaction at different jet heights. We find that the produced high-energy emission could be detectable by the current gamma-ray telescopes. In general, the jet-clump interactions are a possible mechanism to produce (steady or flaring) high-energy emission in many astrophysical sources in which jets are present.

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arXiv2010

Non-thermal radiation from a runaway massive star

We present a study of the radio emission from a massive runaway star. The star forms a bow shock that is clearly observed in the infrared. We have performed VLA observations under the assumption that the reverse shock in the stellar wind might accelerate charged particles up to relativistic energies. Non-thermal radio emission of synchrotron origin has been detected, confirming the hypothesis. We have then modeled the system and we predict a spectral energy distribution that extends up to gamma-rays. Under some simplifying assumptions, we find that the intensity at high energies is too low to be detected by current instruments, but the future Cherenkov Telescope Array might detect the source.

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arXiv2010

Non-thermal radiation associated with astrophysical shock waves

The main goal of this thesis is to study the physical processes that can produce non-thermal emission at high energies in astrophysical objects capable to accelerate particles up to relativistic velocities. In particular, we have studied the gamma-ray emission produced in cosmic sources with different spatial scales, from young stellar objects to clusters of galaxies, going through microquasars and active galactic nuclei. In the former cases, we have modeled the gamma-ray emission using the radio data from the sources IRAS 16547-4247 and Abell 3376. In the latter, we have developed a specific radiation model based on the interaction of the inhomogeneities of the external medium with the jets generated by the compact object. Specifically, we have considered clumps of the massive stellar wind in microquasars, and clouds of the broad line region in active galactic nuclei, interacting with the jets of the sources. In all cases, the developed models allow us to make predictions testables with the new generation of instruments operating at high energies, such as the satellites Fermi and AGILE, and the Cherenkov telescopes HESS, MAGIC, and the forthcoming CTA.

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arXiv2010

Detection of nonthermal emission from the bow shock of a massive runaway star

The environs of massive, early-type stars have been inspected in recent years in the search for sites where particles can be accelerated up to relativistic energies. Wind regions of massive binaries that collide have already been established as sources of high-energy emission; however, there is a different scenario for massive stars where strong shocks can also be produced: the bow-shaped region of matter piled up by the action of the stellar strong wind of a runaway star interacting with the interstellar medium. We study the bow-shock region produced by a very massive runaway star, BD+43 3654, to look for nonthermal radio emission as evidence of a relativistic particle population. We observed the field of BD+43 3654 at two frequencies, 1.42 and 4.86 GHz, with the Very Large Array (VLA), and obtained a spectral index map of the radio emission. We have detected, for the first time, nonthermal radio emission from the bow shock of a massive runaway star. After analyzing the radiative mechanisms that can be at work, we conclude that the region under study could produce enough relativistic particles whose radiation might be detectable by forthcoming gamma-ray instruments, like CTA North.

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arXiv2010

Gamma rays from cloud penetration at the base of AGN jets

Dense and cold clouds seem to populate the broad line region surrounding the central black hole in AGNs. These clouds could interact with the AGN jet base and this could have observational consequences. We want to study the gamma-ray emission produced by these jet-cloud interactions, and explore under which conditions this radiation would be detectable. We investigate the hydrodynamical properties of jet-cloud interactions and the resulting shocks, and develop a model to compute the spectral energy distribution of the emission generated by the particles accelerated in these shocks. We discuss our model in the context of radio-loud AGNs, with applications to two representative cases, the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet power can be channelled to gamma-rays, which would be likely dominated by synchrotron self-Compton radiation, and show typical variability timescales similar to the cloud lifetime within the jet, which is longer than several hours. Many clouds can interact with the jet simultaneously leading to fluxes significantly higher than in one interaction, but then variability will be smoothed out. Jet-cloud interactions may produce detectable gamma-rays in non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A, and steady in the case of powerful objects of FR II type.

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arXiv2010

High-energy emission from jet-cloud interactions in AGNs

Active galactic nuclei present continuum and line emission. The emission lines are originated by gas located close to the central super-massive black hole. Some of these lines are broad, and would be produced in a small region called broad-line region. This region could be formed by clouds surrounding the central black hole. In this work, we study the interaction of such clouds with the base of the jets in active galactic nuclei, and we compute the produced high-energy emission. We focus on sources with low luminosities in the inner jet regions, to avoid strong gamma-ray absorption. We find that the resulting high-energy radiation may be significant in Centaurus A. Also, this phenomenon might be behind the variable gamma-ray emission detected in M87, if very large dark clouds are present. The detection of jet-cloud interactions in active galactic nuclei would give information on the properties of the jet base and the very central regions.

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arXiv2009

Jet-Cloud Interactions in AGNs

Active galactic nuclei present continuum and line emission. The former is produced by the accretion disk and the jets, whereas the latter is originated by gas located close to the super-massive black hole. The small region where the broad lines are emitted is called the broad-line region. The structure of this region is not well known, although it has been proposed that it may be formed by small and dense ionized clouds surrounding the supermassive black-hole. In this work, we study the interaction of one cloud from the broad line region with the jet of the active galactic nuclei. We explore the high-energy emission produced by this interaction close to the base of the jet. The resulting radiation may be detectable for nearby non-blazar sources as well as for powerful quasars, and its detection could give important information on the broad line region and the jet itself.

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arXiv2009

Gamma-rays from massive protostars

Massive protostars have associated bipolar outflows with velocities of hundreds of km/s. Such outflows produce strong shocks when interact with the ambient medium leading to regions of non-thermal radio emission. Under certain conditions, the population of relativistic particles accelerated at the terminal shocks of the protostellar jets can produce significant gamma-ray emission. We estimate the conditions necessary for high-energy emission in the non-thermal hot spots of jets associated with massive protostars embedded in dense molecular clouds. Our results show that particle-matter interactions can lead to the detection of molecular clouds hosting massive young stellar objects by the Fermi satellite at MeV-GeV energies and even by Cherenkov telescope arrays in the GeV-TeV range. Astronomy at gamma-rays can be used to probe the physical conditions in star forming regions and particle acceleration processes in the complex environment of massive molecular clouds.

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arXiv2009

High-energy flares from jet-clump interactions

High-mass microquasars are binary systems composed by a massive star and a compact object from which relativistic jets are launched. Regarding the companion star, observational evidence supports the idea that winds of hot stars are formed by clumps. Then, these inhomogeneities may interact with the jets producing a flaring activity. In the present contribution we study the interaction between a jet and a clump of the stellar wind in a high-mass microquasar. This interaction produces a shock in the jet, where particles may be accelerated up to relativistic energies. We calculate the spectral energy distributions of the dominant non-thermal processes: synchrotron radiation, inverse Compton scattering, and proton-proton collisions. Significant levels of X- and gamma-ray emission are predicted, with luminosities in the different domains up to ~ 10^{34} - 10^{35} erg/s on a timescale of about ~ 1 h. Finally, jet-clump interactions in high-mass microquasars could be detectable at high energies. These phenomena may be behind the fast TeV variability found in some high-mass X-ray binary systems, such as Cygnus X-1, LS 5039 and LS I+61 303. In addition, our model can help to derive information on the properties of jets and clumpy winds.

astro-ph.HE↗

arXiv2009

High-energy emission from jet-clump interactions in microquasars

High-mass microquasars are binary systems consisting of a massive star and an accreting compact object from which relativistic jets are launched. There is considerable observational evidence that winds of massive stars are clumpy. Individual clumps may interact with the jets in high-mass microquasars to produce outbursts of high-energy emission. Gamma-ray flares have been detected in some high-mass X-ray binaries, such as Cygnus X-1, and probably in LS 5039 and LS I+61 303. We predict the high-energy emission produced by the interaction between a jet and a clump of the stellar wind in a high-mass microquasar. Assuming a hydrodynamic scenario for the jet-clump interaction, we calculate the spectral energy distributions produced by the dominant non-thermal processes: relativistic bremsstrahlung, synchrotron and inverse Compton radiation, for leptons, and for hadrons, proton-proton collisions. Significant levels of emission in X-rays (synchrotron), high-energy gamma rays (inverse Compton), and very high-energy gamma rays (from the decay of neutral pions) are predicted, with luminosities in the different domains in the range ~ 10^{32}-10^{35} erg/s. The spectral energy distributions vary strongly depending on the specific conditions. Jet-clump interactions may be detectable at high and very high energies, and provide an explanation for the fast TeV variability found in some high-mass X-ray binary systems. Our model can help to infer information about the properties of jets and clumpy winds by means of high-sensitivity gamma-ray astronomy.

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arXiv2008

Non-thermal processes in the cluster of galaxies Abell 3376

We model the high-energy emission that results from the interaction of relativistic particles with photons and matter in the cluster of galaxies Abell 3376. The presence of relativistic particles is inferred from the recently found radio relics in this cluster, being one of the most prominent examples of double opposite, giant ringlike radio structures. Assuming that diffusive shock acceleration takes place in the cluster regions where radio relics are observed, we calculate the spectral energy distribution resulting from the most relevant non-thermal processes, which are synchrotron radiation, inverse Compton scattering, relativistic Bremsstrahlung, and inelastic proton-proton collisions. In the context of our model, the major radiative component at high energies is inverse Compton scattering, which could reach luminosities L ~ 9x10^{41} erg/s in the energy range between ~ 1 MeV and 10 TeV. Hadronic interactions would yield a minor contribution to the overall non-thermal emission, but would dominate at ultra-high energies. The cluster Abell 3376 might be detectable at gamma-rays by HESS, GLAST satellite and future planned Cherenkov arrays.

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arXiv2008

Non-thermal emission from massive YSOs. Exploring the spectrum at high energies

Thermal radio and X-ray emission has been traditionally associated with the formation of stars. However, in recent years, non-thermal radiation from massive star forming regions has been detected. Synchrotron radio emission and non-thermal X-rays from the outflows of massive young stellar objects (YSOs) provide evidence of the presence of relativistic particles in these sources. In YSOs, the acceleration of particles is likely produced where the thermal jet impacts on the surrounding medium and a shock wave is formed. Thus, particles might be accelerated up to relativistic energies through a Fermi-I type mechanism. Relativistic electrons and protons can interact with thermal particles and photons, producing then gamma-rays. These energetic photons could be detected by the new generation of instruments, making massive YSOs a new population of gamma-ray surces. In the present contribution we briefly describe some massive star forming regions from which non-thermal radio emission has been detected. In addition, we present a general model for high-energy radiation from the massive YSOs embedded in these regions. We take into account both leptonic and hadronic interactions of particles accelerated at the termination points of the collimated outflows ejected from the protostar.

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arXiv2006

Extremely violent optical microvariability in blazars: fact or fiction?

Variability amplitudes larger than 1 magnitude over time-scales of a few tens of minutes have recently been reported in the optical light-curves of several blazars. In order to independently verify the real occurrence of such extremely violent events, we undertook an observational study of a selected sample of three blazars: PKS 0048-097, PKS 0754+100, and PKS 1510-089. Possible systematic error sources during data acquisition and reduction were carefully evaluated. We indeed found flux variability at intra-night time-scales in all three sources, although no extremely violent behaviour, as reported by other authors, was detected. We show that an incorrect choice of the stars used for differential photometry will, under fairly normal conditions, lead to spurious variability with large amplitudes on short time-scales. Wrong results of this kind can be avoided with the use of simple error-control techniques.

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