1,721,071 research outputs found
Reprocessing of radiation by multi-phase gas in low-luminosity accretion flows
No full textWe discuss the role that magnetic fields in low-luminosity accretion flows can play in creating and maintaining a multi-phase medium, and show that small clouds or filaments of dense cold gas can dramatically reprocess the 'primary' radiation from tori. In particular, radio emission is suppressed by free-free absorption, and an extra tweak) component appears at optical wavelengths. This is expected to be a common process in various environments in the central regions of active galaxies, such as broad-line regions, accretion disc coronae and jets
RADIATIVE TRANSFER INA CLUMPY UNIVERSE: III. THE NATURE OF COSMOLOGICAL IONIZING SOURCES
No full textEvents in the life of a cocoon surrounding a light, collapsar jet
No full textAccording to the collapsar model, gamma-ray bursts are thought to be produced in shocks that occur after the relativistic jet has broken free from the stellar envelope. If the mass density of the collimated outflow is less than that of the stellar envelope, the jet will then be surrounded by a cocoon of relativistic plasma. This material would itself be able to escape along the direction of least resistance, which is likely to be the rotation axis of the stellar progenitor, and accelerate in approximately the same way as an impulsive fireball. We discuss how the properties of the stellar envelope have a decisive effect on the appearance of a cocoon propagating through it. The relativistic material that accumulated in the cocoon would have enough kinetic energy to substantially alter the structure of the relativistic outflow, if not in fact provide much of the observed explosive power. Shock waves within this plasma can produce gamma-ray and X-ray transients, in addition to the standard afterglow emission that would arise from the deceleration shock of the cocoon fireball
Limits from rapid TeV variability of Mrk 421
No full textThe extreme variability event in the TeV emission of Mrk 421, recently reported by the Whipple team, imposes the tightest limits on the typical size of the TeV emitting regions in active galactic nuclei (AGN). We examine the consequences that this imposes on the bulk Lorentz factor of the emitting plasma and on the radiation fields present in the central region of this active nucleus. No strong evidence is found for extreme Lorentz factors. However, energetics arguments suggest that any accretion in Mrk 421 has to take place at low rates, compatible with an advection-dominated regime
Compton-dragged gamma-ray bursts associated with supernovae
No full textIt is proposed that the gamma-ray photons that characterize the prompt emission of gamma-ray bursts are produced through the Compton-drag process, which is caused by the interaction of a relativistic fireball with a very dense soft photon bath. If gamma-ray bursts are indeed associated with supernovae, then the exploding star can provide enough soft photons for radiative drag to be effective. This model accounts for the basic properties of gamma-ray bursts, i.e,, the overall energetics, the peak frequency of the spectrum, and the fast variability, with an efficiency that can exceed 50%. In this scenario. there is no need for particle acceleration in relativistic collisionless shocks. Furthermore, although the Poynting flux may be important in accelerating the outflow, no magnetic field is required in the gamma-ray production. The drag also naturally limits the relativistic expansion of the fireball to Gamma less than or similar to 10(4)
Compton dragged gamma-ray bursts: the spectrum
No full textWe calculate the spectrum resulting from the interaction of a fireball with ambient soft photons. These photons are assumed to be produced by the walls of a funnel in a massive star. By parametrizing the radial dependence of the funnel temperature we calculate the deceleration of the fireball self-consistently, taking into account the absorption of high energy gamma-rays arising from interactions with the softer ambient photons. The resulting spectrum is peaked at energies that are in agreement with observations, has a nu(2) slope in the X-ray band and a steep power law high energy tail
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Dense thin clouds in the central regions of Active Galactic Nuclei
No full textSmall amounts of very dense matter in the central engine of an active galaxy have an important effect on the emitted spectrum at ultraviolet and longer wavelengths. If only 0.1 per cent of the total matter expected there is in the form of small dense clouds, then much of the infrared and optical spectrum is absorbed and reradiated in the ultraviolet. Such clouds thereby suppress much of the primary synchrotron radiation emitted by relativistic electrons in the strong magnetic field which is responsible for accelerating them. The magnetic field supports and confines the dense clouds in a manner analogous to prominences above the solar photosphere. In this paper we discuss the properties of the clouds and show that they are a likely constituent of the central engine
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
