402 research outputs found
Causal connection in parsec-scale relativistic jets: results from the MOJAVE VLBI survey
We report that active galactic nucleus (AGN) jets are causally connected on
parsec scales, based on 15 GHz Very Long Baseline Array (VLBA) data from a
sample of 133 AGN jets. This result is achieved through a new method for
measuring the product of the jet Lorentz factor and the intrinsic opening angle
Gamma*theta_j from measured apparent opening angles in flux density limited
samples of AGN jets. The Gamma*theta_j parameter is important for jet physics
because it is related to the jet-frame sidewise expansion speed and causal
connection between the jet edges and its symmetry axis. Most importantly, the
standard model of jet production requires that the jet be causally connected
with its symmetry axis, implying that Gamma*theta_j < 1. When we apply our
method to the MOJAVE flux density limited sample of radio loud objects, we find
Gamma*theta_j = 0.2, implying that AGN jets are causally connected. We also
find evidence that AGN jets viewed very close to the line of sight effectively
have smaller intrinsic opening angles compared with jets viewed more off-axis,
which is consistent with Doppler beaming and a fast inner spine/slow outer
sheath velocity field. Notably, gamma-ray burst (GRB) jets have a typical
Gamma*theta_j that is two orders of magnitude higher, suggesting that different
physical mechanisms are at work in GRB jets compared to AGN jets. A useful
application of our result is that a jet's beaming parameters can be derived.
Assuming Gamma*theta_j is approximately constant in the AGN jet population, an
individual jet's Doppler factor and Lorentz factor (and therefore also its
viewing angle) can be determined using two observable quantities: apparent jet
opening angle and the apparent speed of jet components.Comment: 9 pages, 4 figure
Microstructural strain energy of α-uranium determined by calorimetry and neutron diffractometry
The microstructural contribution to the heat capacity of α-uranium was determined by measuring the heat-capacity difference between polycrystalline and single-crystal samples from 77 to 320 K. When cooled to 77 K and then heated to about 280 K, the uranium microstructure released (3±1) J/mol of strain energy. On further heating to 300 K, the microstructure absorbed energy as it began to redevelop microstrains. Anisotropic strain-broadening parameters were extracted from neutron-diffraction measurements on polycrystals. Combining the strain-broadening parameters with anisotropic elastic constants from the literature, the microstructural strain energy is predicted in the two limiting cases of statistically isotropic stress and statistically isotropic strain. The result calculated in the limit of statistically isotropic stress was (3.7±0.5) J/mol K at 77 K and (1±0.5) J/mol at room temperature. In the limit of statistically isotropic strain, the values were (7.8±0.5) J/mol K at 77 K and (4.5±0.5) J/mol at room temperature. In both cases the changes in the microstructural strain energy showed good agreement with the calorimetry
Optical Polarization and Spectral Variability in the M87 Jet
During the last decade, M87's jet has been the site of an extraordinary
variability event, with one knot (HST-1) increasing by over a factor 100 in
brightness. Variability was also seen on timescales of months in the nuclear
flux. Here we discuss the optical-UV polarization and spectral variability of
these components, which show vastly different behavior. HST-1 shows a highly
significant correlation between flux and polarization, with P increasing from
at minimum to >40% at maximum, while the orientation of its electric
vector stayed constant. HST-1's optical-UV spectrum is very hard
(, ), and displays "hard lags"
during epochs 2004.9-2005.5, including the peak of the flare, with soft lags at
later epochs. We interpret the behavior of HST-1 as enhanced particle
acceleration in a shock, with cooling from both particle aging and the
relaxation of the compression. We set 2 upper limits of
parsecs and 1.02 on the size and advance speed of the flaring region. The
slight deviation of the electric vector orientation from the jet PA, makes it
likely that on smaller scales the flaring region has either a double or twisted
structure. By contrast, the nucleus displays much more rapid variability, with
a highly variable electric vector orientation and 'looping' in the
plane. The nucleus has a much steeper spectrum () but
does not show UV-optical spectral variability. Its behavior can be interpreted
as either a helical distortion to a steady jet or a shock propagating through a
helical jet.Comment: 14 pages, 7 figures, ApJ, in pres
Analysing the Transverse Structure of the Relativistic Jets of AGN
This paper describes a method of fitting total intensity and polarization profiles in VLBI images of astrophysical jets to profiles predicted by a theoretical model. As an example, the method is used to fit profiles of the jet in the Active Galactic Nucleus Mrk501 with profiles predicted by a model in which a cylindrical jet of synchrotron plasma is threaded by a magnetic field with helical and disordered components. This fitting yields model Stokes Q profiles that agree with the observed profiles to within the 1-2 \sigma uncertainties; the I model and observed profiles are overall not in such good agreement, with the model I profiles being generally more symmetrical than the observed profiles. Consistent fitting results are obtained for profiles derived from 6cm VLBI images at two distances from the core, and also for profiles obtained for different wavelengths at a single location in the VLBI jet. The most striking success of the model is its ability to reproduce the spine-sheath polarization structure observed across the jet. Using the derived viewing angle in the jet rest frame, \delta' approximately 83 degrees, together with a superluminal speed reported in the literature, \beta apparent = 3.3, yields a solution for the viewing angle and velocity of the jet in the observer's frame \delta degrees and \beta approximately 0.96. Although these results for Mrk501 must be considered tentative, the combined analysis of polarization profiles and apparent component speeds holds promise as a means of further elucidating the magnetic field structures and other parameters of parsec-scale AGN jets
Magnetic field structure of relativistic jets without current sheets
We present an analytical class of equilibrium solutions for the structure of
relativistic sheared and rotating magnetized jets that contain no boundary
current sheets. We demonstrate the overall dynamical stability of these
solutions and, most importantly, a better numerical resistive stability than
the commonly employed force-free structures which inevitably require the
presence of dissipative surface currents. The jet is volumetrically confined by
the external pressure, with no pressure gradient on the surface. We calculate
the expected observed properties of such jets. Given the simplicity of these
solution we suggest them as useful initial conditions for relativistic jet
simulations.Comment: 13 pages, 13 figures, Accepted by MNRA
Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?
A leading formation scenario for R Coronae Borealis (RCB) stars invokes the
merger of degenerate He and CO white dwarfs (WD) in a binary. The observed
ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the
solar value of ~500. In this paper, we investigate whether such a low ratio can
be obtained in simulations of the merger of a CO and a He white dwarf. We
present the results of five 3-dimensional hydrodynamic simulations of the
merger of a double white dwarf system where the total mass is 0.9 Mdot and the
initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations
with a feature around the merged stars where the temperatures
and densities are suitable for forming 18O. However, more 16O is being
dredged-up from the C- and O-rich accretor during the merger than the amount of
18O that is produced. Therefore, on a dynamical time scale over which our
hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is
found. If the conditions found in the hydrodynamic simulations persist for 10^6
seconds the oxygen ratio drops to 16 in one case studied, while in a hundred
years it drops to ~4 in another case studied, consistent with the observed
values in RCB stars. Therefore, the merger of two white dwarfs remains a strong
candidate for the formation of these enigmatic stars.Comment: 42 pages, 19 figures. Accepted for publication in the Astrophysical
Journa
Nested radiations and the pulse of angiosperm diversification: increased diversification rates often follow whole genome duplications
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111924/1/nph13491-sup-0001-FigS1-TableS1-S2.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111924/2/nph13491.pd
Signatures of large-scale magnetic fields in AGN jets: transverse asymmetries
We investigate the emission properties that a large-scale helical magnetic
field imprints on AGN jet synchrotron radiation. A cylindrically symmetric
relativistic jet and large-scale helical magnetic field produce significant
asymmetrical features in transverse profiles of fractional linear polarization,
intensity, Faraday rotation, and spectral index. The asymmetrical features of
these transverse profiles correlate with one another in ways specified by the
handedness of the helical field, the jet viewing angle (theta_ob), and the bulk
Lorentz factor of the flow (Gamma). Thus, these correlations may be used to
determine the structure of the magnetic field in the jet. In the case of radio
galaxies (theta_ob~1 radian) and a subclass of blazars with particularly small
viewing angles (theta_ob << 1/Gamma), we find an edge-brightened intensity
profile that is similar to that observed in the radio galaxy M87. We present
observations of the AGNs 3C 78 and NRAO 140 that display the type of transverse
asymmetries that may be produced by large-scale helical magnetic fields.Comment: accepted by MNRAS, added reference
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