1,354,210 research outputs found
The structure of accretion flow at the base of jets in AGN
No full textDonea, Alina-C; Biermann, Peter
How relevant is the torus activity/geometry for the TeV gamma-rays emitted in the jet of M87?
No full textCopyright © 2003. Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Submitted to Cornell University’s online archive www.arXiv.org in 2003 by Donea A.-C. Post-print sourced from www.arxiv.org.Motivated by unification schemes of active galactic nuclei, we review evidence for the existence of a small-scale dust torus in M87, a Fanaroff-Riley Class I radio galaxy. Since there is no direct evidence of any thermal emission from its torus we consider indirect evidence, such as BLR activity and ageing arguments to model the cold dust structure of M87. In the context of the jet -- accretion disk -- torus symbiosis we discuss the interactions of GeV and TeV gamma-rays produced in the jet of M87 with the infrared radiation fields external to the jet, produced by a less active torus. A thin and cold torus with less defined outer boundaries could still posses problems to some of the TeV emission from the jet.A.-C. Donea, R. J. Prothero
Gamma ray and infrared emission from the M87 jet and torus
No full textCopyright © 2003. Progress of Theoretical Physics 2003 All rights reserved. Printed in U.S.A. Submitted to Cornell University’s online archive www.arXiv.org in 2003 by Donea A.-C. Post-print sourced from www.arxiv.org.The existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic. M87, the nearest such object, is puzzling in that although it has very massive central black hole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have a rather low infrared luminosity. Using arguments from unification theory of active galactic nuclei, we have earlier suggested that the inner parsec-scale region of M87 could still harbour a small torus sufficiently cold such that its infrared emission is dwarfed by the jet emission. The infrared emission from even a small cold torus could affect through photon-photon pair production interactions the escape of 100 GeV to TeV energy gamma rays from the central parsec of M87. /par The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactions of gamma-rays produced in the inner jet of M87 with the weak infrared radiation expected from a possible dusty small-scale torus, and show that the HEGRA detection shows that the temperature of any torus surrounding the gamma-ray emission region must be cooler than about 250 K. We suggest that if no gamma-rays are in future detected during extreme flaring activity in M87 at other wavelength, this may be expected because of torus heating.Alina C. Donea and Raymond J. Prothero
Radiation fields in blazars - a possible extension of the small scale symbiosis (disk/jet) into a large scale (dust/dust) symbiosis
No full text© CSIRO 2002In blazar models both protons and electrons may be efficiently accelerated in jets and produce γ-rays. Here we discuss the interactions of these γ-rays with different radiation fields. The external radiation fields within a few parsecs from the black hole involved in such interactions could be the direct radiation from the accretion disk coupled with the jet, the infrared radiation from a dusty torus, and the emission line radiation from the broad line region surrounding the accretion disk. The optical thickness for absorption of γ-ray photons in the external radiation fields is analysed for blazars and quasars. Based on the unification theory of active galactic nuclei we briefly review the evidence for the existence of small scale dust tori in blazars/FR I. We propose that the existing jet–accretion disk symbiosis extrapolates to a large scale symbiosis between other important dusty constituents of the blazar/FR I family.Alina-C. Donea and Raymond J. Prothero
Multi-wavelength analysis in flare seismology and the role of magnetic field dynamics in the seismicity of solar active regions
No full textSolar flares are one of the most catastrophic and energetic processes in the solar system, converting vast amounts of magnetic energy into kinetic and thermal energies. Some flares are believed to drive seismic events on the solar surface, but in general the majority of flares are seismically inactive. They raise several questions about sunquakes and its generation: How it is possible to determine which flares may induce sunquakes? What is the role of dynamic field changes in the generation of sunquakes? Can the magnetic field be a driver of seismic waves? Notwithstanding the strong theoretical and observational progress of the last decade, these questions still remain open. Since the first discovery of a sunquake by Kosovichev and Zharkova (1998) many other works have been written about the detection of flare generated seismic waves in the Sun (Kosovichev, 2006b; Zharkova and Zharkov, 2007; Donea et al., 1999; Donea and Lindsey, 2005; Donea et al., 2006b; Besliu-Ionescu et al., 2006a; Moradi et al., 2007; Martínez-Oliveros et al., 2007, 2008c; Martínez-Oliveros and Donea, 2009). Sunquakes are defined as the observed expanding waves on the solar photosphere induced by solar flares. It is believed that sunquakes are produced by the momentum or energy transfer of high energy particles, waves and radiation from flares into the solar photosphere. The momentum induces the generation of internal waves that travel in the solar interior where they are refracted back to the surface. They are observed as close to circular photospheric ripples, detected as changes in the mean velocity amplitude of the photosphere in Dopplergrams. The work presented here starts with multifrequency observations of seismically active flares from microwaves and hard X-rays, which give us information about the electron acceleration process taking place in the solar corona, to the much lower frequencies associated with seismic waves. As the importance of particle interactions with the solar atmosphere has been already revealed (Zharkova and Zharkov, 2007; Donea et al., 2006b; Moradi et al., 2007; Martínez-Oliveros et al., 2007), the study of their acceleration, propagation and precipitation is a key area in understanding solar seismicity. Therefore, studying of the flare emission at different wavelengths can give us important information about the physical conditions under which a sunquake can be generated. The magnetic field rules the majority of the processes in the solar corona, including the acceleration and precipitation of high energy particles which are associated with changes in the coronal magnetic field configuration of the active regions. The proposal of the magnetic field as a direct driver of seismicity is a new idea to be explored (Hudson et al., 2008). Variations of the magnetic field do work on the solar surface, and may induce waves in the interior of the Sun, eventually generating sunquakes. The study of solar seismicity is then expanded in this thesis, by analysing the temporal and spatial properties of the magnetic field in seismically active flares. Lastly, dynamic changes of the magnetic field during and after the flare may facilitate the precipitation of particles into the photosphere. This idea is contemplated in the trapping plus precipitation models, in which the amount of particles reaching the chromosphere is a function of the altitude of the loop and the loss cone. Using a stochastic acceleration model we study the changes and behaviour of particle distribution for various configurations of magnetic field loops. These studies reveal that changes in the height of the loop may have serious consequences in the energetic release in the chromosphere by high energy particles accelerated in the solar corona
Tori and TeV gamma-ray emission in AGN
No full textThe absorption of TeV gamma-rays in active galactic nuclei by photon-photon pair production on infrared radiation from a parsec scale torus at temperature ˜ 1000 K surrounding the accretion disk/base of jet was discussed by Protheroe and Biermann (Astropart. Phys., 6, 293, 1997). Here we briefly review the evidence for the existence of dusty infrared tori in blazars, and construct torus models consistent with infrared and optical polarimetry data. This leads us to propose a symbiosis between large and small-scale dust features and broad line regions in AGN. We discuss the radiation fields of the accretion disk, broad line region and dusty torus as target photons for pion photoproduction by protons and photon-photon pair production by γ-rays. We use our results to constrain the sites of emission of TeV gamma rays.A.-C. Donea and R. J. Prothero
Cosmic Rays from the Nucleus of M87
No full text© 2003 by Universal Academy PressThe unresolved nuclear region of M87 emits strong non-thermal emission from radio to X-rays, and this has been interpreted as jet emission from a misaligned BL Lac Object in the context of the Synchrotron Proton Blazar (SPB) model (Protheroe et al 2003). In this model extragalactic cosmic rays are generated as neutrons produced in pion photoproduction interactions decaying into protons after escaping from the host galaxy. Because energetic protons are deflected by the intergalactic magnetic field, the protons from the decay of neutrons emitted in all directions, including along the jet axis where the Doppler factor and hence emitted neutron energies are higher, can contribute to the observed ultra-high energy cosmic rays. We consider the propagation of these cosmic ray protons to Earth and conclude that M87 could account for the observed flux if the extragalactic magnetic field topology were favourable.Alina-C. Donea, Raymond J. Protheroe and Anita Reime
TeV g-rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object
No full textCopyright © 2003 Elsevier Science B.V. All rights reserved.The unresolved nuclear region of M87 emits strong non-thermal emission from radio to X-rays. Assuming this emission to originate in the pc scale jet aligned at θ∼30° to the line of sight, we interpret this emission in the context of the synchrotron proton blazar model. We find the observed nuclear jet emission to be consistent with M87 being a mis-aligned BL Lac object and predict γ-ray emission extending up to at least 100 GeV at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS depending on whether it is high-frequency or low-frequency peaked. Predicted neutrino emission is below the sensitivity of existing and planned neutrino telescopes. Ultra-high-energy neutrons produced in pion photoproduction interactions decay into protons after escaping from the host galaxy. Because energetic protons are deflected by the intergalactic magnetic field, the protons from the decay of neutrons emitted in all directions, including along the jet axis where the Doppler factor and hence emitted neutron energies are higher, can contribute to the observed ultra-high-energy cosmic rays. We consider the propagation of these cosmic ray protons to Earth and conclude that M87 could account for the observed flux if the extragalactic magnetic field topology were favourable. © 2003 Elsevier Science B.V. All rights reserved.R. J. Protheroe, A. -C. Donea and A. Reimerhttp://www.elsevier.com/wps/find/journaldescription.cws_home/523319/description#descriptio
Radiation fields of disk, BLR and torus in quasars and blazars: implications for gamma-ray absorption
No full textThe radiation fields external to the jets and originating from within a few parsecs from the black hole, are discussed in this paper. They are the direct radiation from an accretion disk in symbiosis with jets, the radiation field from the broad line region (BLR) surrounding the accretion disk, and the infrared radiation from a dusty torus. The jet/disk symbiosis modifies the energetics in the central parsec of AGN such that for a given accretion rate, a powerful jet would occur at the expense of the disk luminosity, and consequently the disk would less efficiently ionize the BRL clouds or heat the dust in the torus, thereby affecting potentially important target photon fields for interactions of gamma-rays, accelerated electrons and protons along the jet. Motivated by unification schemes of active galactic nuclei, we briefly review the evidence for the existence of broad line regions and small-scale dust tori in BL Lacs and Fanaroff-Riley Class I (FR-I) radio galaxies. We propose that an existing jet-accretion disk symbiosis can be extrapolated to provide a large scale-symbiosis between other important dusty constituents of the blazar/FR-I family. In the present paper, we discuss in the context of this symbiosis interactions of GeV and TeV gamma-rays produced in the jet with the various radiation fields external to the jet in quasars and blazars, taking account the anisotropy of the radiation.Alina-C. Donea, and R. J. Prothero
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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