3,046 research outputs found
Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?
We compute the effective values of apparent transverse velocity and flux
boosting factors for the VLBI radio knots of blazar jets, by integrating over
the angular distributions of these quantities across the widths of jets with
finite opening angles but constant velocities. For high bulk Lorentz factors
(Gamma > 10) variations across the jet can be quite large if the opening angle,
omega, is even a few degrees on sub-parsec scales. The resulting apparent
speeds are often much lower than those obtained from the usual analyses that
ignore the finite jet opening angles. We can thus reconcile the usually
observed subluminal or mildly superluminal speeds with the very high (>~ 20)
Gamma factors, required by the inverse Compton origin and rapid variability of
TeV fluxes, as well as by intraday radio variability. Thus it is possible to
associate the VLBI radio knots directly with shocks in the ultra-relativistic
main jet flow, without invoking very rapid jet deceleration on parsec scales,
or extremely unlikely viewing angles.Comment: 10 pages, 1 figure, to appear in ApJ Letters, Nov. 10 2004 issu
3-D General Relativistic MHD Simulations of Generating Jets
We have performed a first fully 3-D GRMHD simulation with Schwarzschild black
hole with a free falling corona. The initial simulation results show that a jet
is created as in previous axisymmetric simulations. However, the time to
generate the jet is longer than in the 2-D simulations. We expect that due to
the additional azimuthal dimension the dynamics of jet formation can be
modified.Comment: 4 pages Proc. Oxford Radio Galaxy Workshop ed. R. Laing & K. Blundell
(San Francisco: PASP) in press (revised
Radio Emission from 3D Relativistic Hydrodynamic Jets: Observational Evidence of Jet Stratification
We present the first radio emission simulations from high resolution three
dimensional relativistic hydrodynamic jets, which allow for a study of the
observational implications of the interaction between the jet and external
medium. This interaction gives rise to a stratification of the jet where a fast
spine is surrounded by a slow high energy shear layer. The stratification, and
in particular the large specific internal energy and slow flow in the shear
layer largely determines the emission from the jet. If the magnetic field in
the shear layer becomes helical (e.g., resulting from an initial toroidal field
and an aligned field component generated by shear) the emission shows a cross
section asymmetry, in which either the top or the bottom of the jet dominates
the emission. This, as well as limb or spine brightening, is a function of the
viewing angle and flow velocity, and the top/bottom jet emission predominance
can be reversed if the jet changes direction with respect to the observer, or
presents a change in velocity. The asymmetry is more prominent in the polarized
flux, because of field cancellation (or amplification) along the line of sight.
Recent observations of jet cross section emission asymmetries in the blazar
1055+018 can be explained assuming the existence of a shear layer with a
helical magnetic field.Comment: 6 pages, 5 figures, 1 latex style file, ApJL accepte
Optical-NIR spectroscopy of the puzzling gamma-ray source 3FGL 1603.9-4903/PMN J1603-4904 with X-shooter
The Fermi/LAT instrument has detected about two thousands Extragalactic High
Energy (E > 100 MeV) gamma-ray sources. One of the brightest is 3FGL
1603.9-4903, associated to the radio source PMN J1603-4904. Its nature is not
yet clear, it could be either a very peculiar BL Lac or a CSO (Compact
Symmetric Object) radio source, considered as the early stage of a radio
galaxy. The latter, if confirmed, would be the first detection in gamma-rays
for this class of objects. Recently a redshift z=0.18 +/- 0.01 has been claimed
on the basis of the detection of a single X-ray line at 5.44 +/- 0.05 keV
interpreted as a 6.4 keV (rest frame) fluorescent line. We aim to investigate
the nature of 3FGL 1603.9-4903/PMN J1603-4904 using optical to NIR
spectroscopy. We observed PMN J1603-4904 with the UV-NIR VLT/X-shooter
spectrograph for two hours. We extracted spectra in the VIS and NIR range that
we calibrated in flux and corrected for telluric absorption and we
systematically searched for absorption and emission features. The source was
detected starting from ~6300 Ang down to 24000 Ang with an intensity comparable
to the one of its 2MASS counterpart and a mostly featureless spectrum. The
continuum lacks absorption features and thus is non-stellar in origin and
likely non-thermal. On top of this spectrum we detected three emission lines
that we interpret as the Halpha-[NII] complex, the [SII] 6716,6731 doublet and
the [SIII] 9530 line, obtaining a redshift estimate of z= 0.2321 +/- 0.0004.
The equivalent width of the Halpha-[NII] complex implies that PMN J1603-4904
does not follow the observational definition of BL Lac, the line ratios suggest
that a LINER/Seyfert nucleus is powering the emission. This new redshift
measurement implies that the X-ray line previously detected should be
interpreted as a 6.7 keV line which is very peculiar.Comment: Published in Astronomy and Astrophysic
A further critique of the analytic strategy of adjusting for covariates to identify biologic mediation
BACKGROUND: Epidemiologic research is often devoted to etiologic investigation, and so techniques that may facilitate mechanistic inferences are attractive. Some of these techniques rely on rigid and/or unrealistic assumptions, making the biologic inferences tenuous. The methodology investigated here is effect decomposition: the contrast between effect measures estimated with and without adjustment for one or more variables hypothesized to lie on the pathway through which the exposure exerts its effect. This contrast is typically used to distinguish the exposure's indirect effect, through the specified intermediate variables, from its direct effect, transmitted via pathways that do not involve the specified intermediates. METHODS: We apply a causal framework based on latent potential response types to describe the limitations inherent in effect decomposition analysis. For simplicity, we assume three measured binary variables with monotonic effects and randomized exposure, and use difference contrasts as measures of causal effect. Previous authors showed that confounding between intermediate and the outcome threatens the validity of the decomposition strategy, even if exposure is randomized. We define exchangeability conditions for absence of confounding of causal effects of exposure and intermediate, and generate two example populations in which the no-confounding conditions are satisfied. In one population we impose an additional prohibition against unit-level interaction (synergism). We evaluate the performance of the decomposition strategy against true values of the causal effects, as defined by the proportions of latent potential response types in the two populations. RESULTS: We demonstrate that even when there is no confounding, partition of the total effect into direct and indirect effects is not reliably valid. Decomposition is valid only with the additional restriction that the population contain no units in which exposure and intermediate interact to cause the outcome. This restriction implies homogeneity of causal effects across strata of the intermediate. CONCLUSIONS: Reliable effect decomposition requires not only absence of confounding, but also absence of unit-level interaction and use of linear contrasts as measures of causal effect. Epidemiologists should be wary of etiologic inference based on adjusting for intermediates, especially when using ratio effect measures or when absence of interacting potential response types cannot be confidently asserted
Cristalización de (Sr,Ca)CO, en sistemas difusión-reacción
Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu
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