1,721,152 research outputs found
Relativistic large scale jets and minimum power requirements
No full textThe recent discovery, by the Chandra satellite, that jets of blazars are strong X-ray emitters at large scales (0.1-1 Mpc) bears support to the hypothesis that (also) on these scales the emitting plasma is moving at highly relativistic speeds. In this case in fact the emission via inverse Compton scattering off cosmic background photons is enhanced and the resulting predicted X-ray spectrum well accounts for the otherwise puzzling observations. Here we point out another reason to favor relativistic large scale jets, based on a minimum power argument: by estimating the Poynting flux and bulk kinetic powers corresponding to, at least, the relativistic particles and magnetic field responsible for the emission, one can derive the value of the bulk Lorentz factor for which the total power is minimized. It is found that both the inner and extended parts of the jet of PKS 0637-752 satisfy such condition
POLARIZATION PROPERTIES OF SYNCHROTRON SELF-COMPTON EMISSION
No full textWe study the polarization properties of synchrotron self-Compton (SSC) emission for an isotropic electron distribution in an axisymmetric magnetic field, as a function of the electron energy and the viewing angle. Despite the opposite contributions of the unpolarized and polarized components of synchrotron radiation, a large fraction of the synchrotron polarization can be preserved. If the magnetic field is uniform, the degree of SSC polarization can be very high, reaching values up to 40 per cent. Such a high polarization is within the measurement capabilities of the present generation of X-ray polarimeters. The polarization vector is found to be parallel to the synchrotron one, which in turn is perpendicular or parallel to the magnetic field axis, depending on the magnetic field distribution. Simultaneous, multiwavelength polarization measurements can therefore provide an important test for the SSC model
A fireworks model for gamma-ray bursts
No full textThe energetics of the long duration GRB phenomenon is compared with models of a rotating Black Hole (BH) in a strong magnetic field generated by an accreting torus. A rough estimate of the energy extracted from a rotating BH with the Blandford-Znajek mechanism is obtained with a very simple assumption: an inelastic collision between the rotating BH and the torus. The GRB energy emission is attributed to an high magnetic field that breaks down the vacuum around the BH and gives origin to a e+- fireball. Its subsequent evolution is hypothesized, in analogy with the in-flight decay of an elementary particle, to evolve in two distinct phases. The first one occurs close to the engine and is responsible of energizing and collimating the shells. The second one consists of a radiation dominated expansion, which correspondingly accelerates the relativistic photon--particle fluid and ends at the transparency time. This mechanism simply predicts that the observed Lorentz factor is determined by the product of the Lorentz factor of the shell close to the engine and the Lorentz factor derived by the expansion. An anisotropy in the fireball propagation is thus naturally produced, whose degree depends on the bulk Lorentz factor at the end of the collimation phase
Toward measuring the spin of obscured supermassive black holes: A critical assessment with disk megamasers
Full text linkContext. Mass and spin are two fundamental properties of astrophysical black holes. While some established indirect methods are adopted to measure both these properties of active galactic nuclei (AGN) when viewed relatively face-on, very few suggested methods exist to measure these properties when AGN are highly inclined and potentially obscured by large amounts of gas. Aims. In this context we explore the accuracy and performance of a recently proposed method to estimate the spin of AGN by fitting the accretion disk spectral energy distribution, when adapted for highly inclined and obscured systems, and in particular to a sample of six local water megamasers. For these sources the accretion rate and inclination angle are both known, allowing us to rely only on the AGN bolometric luminosity to infer their spin. Methods. Using the bolometric luminosity as a proxy for the accretion disk peak luminosity, we derived the expected bolometric luminosity as a function of spin. Then, we measured the bolometric luminosity of each source through X-ray spectroscopy, and compared it with the expected value to constrain the spin of the AGN. Results. The quality of the constraints depend critically on the accuracy of the measured bolometric luminosity, which is difficult to estimate in heavily obscured systems. Three out of six sources do not show consistency between the expected and measured bolometric luminosities, while the other three (four, when considering the [OIII] line as tracer of the bolometric luminosity) are formally consistent with high spin values. Conclusions. Our results suggest that this method, although promising (and possibly considered as a future calibrator for other methods) needs better observational data and further theoretical modeling to be successfully applied to obscured AGN and to infer robust results
The BL Lac heart of Centaurus A
No full textEmission from the nucleus of the closest radio galaxy, Centaurus A, is observed from the radio to the gamma ray band. We build, for the first time, its overall Spectral Energy Distribution (SED) that appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ~0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac and provide strong support in favour of the unification scheme for low luminosity radio-loud AGNs. Modeling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-pc scale jets
Magnetic flares in accretion disc coronae and the spectral states of black hole candidates: the case of GX 339-4
No full textWe examine the constraints that the observations of different spectral states displayed by Galactic black hole candidates impose on the properties of magnetic flares resulting from the reconnection of flux tubes that rise from the accretion disc into a corona because of the magnetic buoyancy (Parker) instability. Using observations of one of the best-studied examples, GX339-4, we identify the geometry and physical conditions characterizing each of these states. We find that, if in the soft state flaring occurs at small scaleheights above the accretion disc, a soft thermal-like spectrum, characteristic of this state, can result from the heating and consequent reradiation of the hard X-rays produced by such flares. The hard tail can then be produced by Comptonization of the soft radiation. Conversely, the hard state may result from a phase in which flares are triggered high above the underlying accretion disc and produce X-rays via Comptonization of either internal cyclo-synchrotron radiation or soft disc photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disc the system is photon-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disc greatly enhances the local soft-photon field with the result that the spectrum softens. We interpret these two states as being related to two different phases of magnetic energy dissipation. We speculate that, in the soft state. Parker instability in the disc may favour the emergence of large numbers of relatively low-magnetic-field flux tubes. In the hard state, only intense magnetic fields become buoyant and magnetic loops are able to rise and expand in the coronal atmosphere. This possibility can also qualitatively account for the observed short time-scale variability and the characteristics of the X-ray-reflected component of the hard state
Pair dominated Active Galactic Nuclei and the origin of the X-ray background
No full textWe discuss the possibility that the major energy contribution to the hard X-ray background (XRB) is the integrated emission from a mixed population of obscured and unobscured active galactic nuclei (AGN), as predicted by the Seyfert unification scheme. The model assumes that AGN are compact and in thermal pair equilibrium, and that the source compactness (i.e. the luminosity-to-size ratio) undergoes evolution following the X-ray luminosity evolution. Owing to the process of pair production, the model predicts evolution in the spectral properties, which are self-consistently taken into account. The XRB emission can be accounted for by a population with a relatively limited range of temperatures, around a few hundred keV. Finally, we estimate the number counts predicted in the LR band and the contribution of this population of AGN to the diffuse IR background, as a result of re-emission of the absorbed high-energy radiation
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
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