1,721,525 research outputs found
Particle acceleration by rotating magnetospheres in active galactic nuclei
No full textWe consider the centrifugal acceleration of charged test particles by rotating magnetospheres which are widely believed to be responsible for the relativistic jet phenomenon in active galactic nuclei (AGN). Based on an analysis of forces the equation for the radial accelerated motion is derived and an analytical solution presented under the assumption of an idealized spherical magnetosphere. We show that the rotational energy gain of charged particles moving outwards along rotating magnetic field lines is limited in general by (i) inverse-Compton losses in the radiation field of the disk in which the magnetosphere is anchored and (ii) the breakdown of the bead-on-the-wire approximation which occurs in the vicinity of the light cylinder. The corresponding maximum Lorentz factor for electrons is of the order of a few hundred for the sub-Eddington conditions regarded to be typical for BL Lacs. In AGN with enhanced accretion rate the acceleration mechanism seems to be almost inefficient due to increasing inverse-Compton losses
Implications of a possible 23 day periodicity for binary black hole models in Mkn 501
No full textWe investigate the implications of a massive binary system in the centre of the gamma-ray blazar Mkn 501 and show that the periodical behaviour recently observed in the TeV and X-ray lightcurves may possibly be related to the orbital motion of the relativistic jet emerging from the less massive black hale. For the special relativistic jet properties inferred from emission models, we derive an intrinsic orbital period of (6-14) yrs and a centre-of-mass distance of (2.0-3.5) x 10(16) cm. If the binary is very close with a separation of the order of that for which gravitational radiation becomes dominant, we find a maximum primary mass of similar to 10(8)M(circle dot) and a corresponding secondary mass in the range of similar to (4-42) x 10(6)M(circle dot) depending on the intrinsic jet properties. Such values are in line with the black hole masses expected from merger scenarios
Particle acceleration by rotating magnetospheres in active galactic nuclei
No full textWe consider the centrifugal acceleration of charged test particles by rotating magnetospheres which are widely believed to be responsible for the relativistic jet phenomenon in active galactic nuclei (AGN). Based on an analysis of forces the equation for the radial accelerated motion is derived and an analytical solution presented under the assumption of an idealized spherical magnetosphere. We show that the rotational energy gain of charged particles moving outwards along rotating magnetic field lines is limited in general by (i) inverse-Compton losses in the radiation field of the disk in which the magnetosphere is anchored and (ii) the breakdown of the bead-on-the-wire approximation which occurs in the vicinity of the light cylinder. The corresponding maximum Lorentz factor for electrons is of the order of a few hundred for the sub-Eddington conditions regarded to be typical for BL Lacs. In AGN with enhanced accretion rate the acceleration mechanism seems to be almost inefficient due to increasing inverse-Compton losses
Implications of a possible 23 day periodicity for binary black hole models in Mkn 501
No full textWe investigate the implications of a massive binary system in the centre of the gamma-ray blazar Mkn 501 and show that the periodical behaviour recently observed in the TeV and X-ray lightcurves may possibly be related to the orbital motion of the relativistic jet emerging from the less massive black hale. For the special relativistic jet properties inferred from emission models, we derive an intrinsic orbital period of (6-14) yrs and a centre-of-mass distance of (2.0-3.5) x 10(16) cm. If the binary is very close with a separation of the order of that for which gravitational radiation becomes dominant, we find a maximum primary mass of similar to 10(8)M(circle dot) and a corresponding secondary mass in the range of similar to (4-42) x 10(6)M(circle dot) depending on the intrinsic jet properties. Such values are in line with the black hole masses expected from merger scenarios
On the central black hole mass in Mkn 501
Get PDFWe analyse the apparent disagreement between the mass estimates
of the central black hole(s) in Mkn 501 based on (i) the observations of
the host galaxy, (ii) the high energy (HE) emission mechanism, and (iii)
the modulation of the beamed radiation by a black hole (BH) binary system.
While method (i) seems to imply a central mass \ga
, method (ii) suggests a BH mass less than
. We critically discuss the estimates inferred
from (i) showing that current uncertainties may permit a central mass
as low as (2–.
We demonstrate that in this case the estimates (i) and (ii) might be
brought into agreement by assuming a binary BH system where the jet
dominating the HE emission originates from the less massive (secondary)
BH as suggested by method (iii). On the other hand, if Mkn 501 has in
fact a high central BH mass of order , a change of
fundamental assumptions seems to be required in the context of several
HE emission models. We show, that in this case a binary scenario following
(iii) may be still possible if the jet which dominates the emission
emerges from the more massive (primary) BH and if the binary evolution
passes through phases of super-Eddington accretion and/or decreased
conversion efficiency.
Particle acceleration in rotating and shearing jets from AGN
No full textWe model the acceleration of energetic particles due to shear and
centrifugal effects in rotating astrophysical jets.
The appropriate equation describing the diffusive transport of energetic
particles in a collisionless, rotating background flow is derived and
analytical steady state solutions are discussed. In particular, by
considering velocity profiles from rigid, over flat to Keplerian rotation,
the effects of centrifugal and shear acceleration of particles scattered
by magnetic inhomogeneities are distinguished. In the case where shear
acceleration dominates, it is confirmed that power law particle momentum
solutions exist, if the mean scattering time
is an increasing function of momentum.
We show that for a more complex interplay between shear and centrifugal
acceleration, the recovered power law momentum spectra might be
significantly steeper but flatten with increasing azimuthal velocity
due to the increasing centrifugal effects.
The possible relevance of shear and centrifugal acceleration for the
observed extended emission in AGN is demonstrated for the case of the
jet in the quasar 3C273
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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