3,225 research outputs found
Conversion of relativistic pair energy into radiation in the jets of active galactic nuclei
It is generally accepted that relativistic jet outflows power the nonthermal
emission from active galactic nuclei (AGN). The composition of these jets --
leptonic versus hadronic -- is still under debate. We investigate the
microphysical details of the conversion process of the kinetic energy in
collimated relativistic pair outflows into radiation through interactions with
the ambient interstellar medium. Viewed from the coordinate system comoving
with the pair outflow, the interstellar protons and electrons represent a
proton-electron beam propagating with relativistic speed in the pair plasma. We
demonstrate that the beam excites both electrostatic and low-frequency
magnetohydrodynamic Alfven-type waves via a two-stream instability in the pair
background plasma, and we calculate the time evolution of the distribution
functions of the beam particles and the generated plasma wave turbulence power
spectra. For standard AGN jet outflow and environment parameters we show that
the initial beam distributions of interstellar protons and electrons quickly
relax to plateau-distributions in parallel momentum, transferring thereby
one-half of the initial energy density of the beam particles to electric field
fluctuations of the generated electrostatic turbulence. On considerably longer
time scales, the plateaued interstellar electrons and protons will isotropise
by their self-generated transverse turbulence and thus be picked-up in the
outflow pair plasma. These longer time scales are also characteristic for the
development of transverse hydromagnetic turbulence from the plateaued electrons
and protons. This hydromagnetic turbulence upstream and downstream is crucial
for diffusive shock acceleration to operate at external or internal shocks
associated with pair outflows.Comment: A&A in pres
Analysis of Accordion DNA Stretching Revealed by The Gold Cluster Ruler
A promising new method for measuring intramolecular distances in solution
uses small-angle X-ray scattering interference between gold nanocrystal labels
(Mathew-Fenn et al, Science, 322, 446 (2008)). When applied to double stranded
DNA, it revealed that the DNA length fluctuations are strikingly strong and
correlated over at least 80 base pair steps. In other words, the DNA behaves as
accordion bellows, with distant fragments stretching and shrinking concertedly.
This hypothesis, however, disagrees with earlier experimental and computational
observations. This Letter shows that the discrepancy can be rationalized by
taking into account the cluster exclusion volume and assuming a moderate
long-range repulsion between them. The long-range interaction can originate
from an ion exclusion effect and cluster polarization in close proximity to the
DNA surface.Comment: 9 pages, 4 figures, to appear in Phys. Rev.
Absolute velocity measurements in sunspot umbrae
In sunspot umbrae, convection is largely suppressed by the strong magnetic
field. Previous measurements reported on negligible convective flows in umbral
cores. Based on this, numerous studies have taken the umbra as zero reference
to calculate Doppler velocities of the ambient active region. To clarify the
amount of convective motion in the darkest part of umbrae, we directly measured
Doppler velocities with an unprecedented accuracy and precision. We performed
spectroscopic observations of sunspot umbrae with the Laser Absolute Reference
Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency
comb enabled the calibration of the high-resolution spectrograph and absolute
wavelength positions. A thorough spectral calibration, including the
measurement of the reference wavelength, yielded Doppler shifts of the spectral
line Ti i 5713.9 {\AA} with an uncertainty of around 5 m s-1. The measured
Doppler shifts are a composition of umbral convection and magneto-acoustic
waves. For the analysis of convective shifts, we temporally average each
sequence to reduce the superimposed wave signal. Compared to convective
blueshifts of up to -350 m s-1 in the quiet Sun, sunspot umbrae yield a
strongly reduced convective blueshifts around -30 m s-1. {W}e find that the
velocity in a sunspot umbra correlates significantly with the magnetic field
strength, but also with the umbral temperature defining the depth of the
titanium line. The vertical upward motion decreases with increasing field
strength. Extrapolating the linear approximation to zero magnetic field
reproduces the measured quiet Sun blueshift. Simply taking the sunspot umbra as
a zero velocity reference for the calculation of photospheric Dopplergrams can
imply a systematic velocity error.Comment: 10 pages, 7 figures, 2 tables, Appendix with 5 figure
Feller Processes: The Next Generation in Modeling. Brownian Motion, L\'evy Processes and Beyond
We present a simple construction method for Feller processes and a framework
for the generation of sample paths of Feller processes. The construction is
based on state space dependent mixing of L\'evy processes.
Brownian Motion is one of the most frequently used continuous time Markov
processes in applications. In recent years also L\'evy processes, of which
Brownian Motion is a special case, have become increasingly popular.
L\'evy processes are spatially homogeneous, but empirical data often suggest
the use of spatially inhomogeneous processes. Thus it seems necessary to go to
the next level of generalization: Feller processes. These include L\'evy
processes and in particular Brownian motion as special cases but allow spatial
inhomogeneities.
Many properties of Feller processes are known, but proving the very existence
is, in general, very technical. Moreover, an applicable framework for the
generation of sample paths of a Feller process was missing. We explain, with
practitioners in mind, how to overcome both of these obstacles. In particular
our simulation technique allows to apply Monte Carlo methods to Feller
processes.Comment: 22 pages, including 4 figures and 8 pages of source code for the
generation of sample paths of Feller processe
Equation of Motion for the Solvent Polarization Apparent Charges in the Polarizable Continuum Model: Application to Time-Dependent CI
The dynamics of the electrons for a molecule in solution is coupled to the
dynamics of its polarizable environment, i.e., the solvent. To theoretically
investigate such electronic dynamics, we have recently developed equations of
motion (EOM) for the apparent solvent polarization charges that generate the
reaction field in the Polarizable Continuum Model (PCM) for solvation and we
have coupled them to a real-time time-dependent density functional theory (RT
TDDFT) description of the solute [Corni et al. J. Phys. Chem. A 119, 5405
(2014)]. Here we present an extension of the EOM-PCM approach to a
Time-Dependent Configuration Interaction (TD CI) description of the solute
dynamics, which is free from the qualitative artifacts of RT TDDFT in the
adiabatic approximation. As tests of the developed approach, we investigate the
solvent Debye relaxation after an electronic excitation of the solute obtained
either by a pulse of light or by assuming the idealized sudden promotion
to the excited state. Moreover, we present EOM for the Onsager solvation model
and we compare the results with PCM. The developed approach provides
qualitatively correct real-time evolutions and is promising as a general tool
to investigate the electron dynamics elicited by external electromagnetic
fields for molecules in solution.Comment: This is the final peer-reviewed manuscript accepted for publication
in The Journal of Chemical Physics. Copyright by AIP, the final published
version can be found at
http://scitation.aip.org/content/aip/journal/jcp/146/6/10.1063/1.497562
A forceful connection: mechanoregulation of oncogenic YAP
The Yes-associated protein (YAP) is an important transcriptional co-activator that mediates the cellular response to mechanical and cytoskeletal cues. In two recent papers published in The EMBO Journal, Dae-Sik Lim and colleagues show how YAP activity affects cancer formation and metastasis via a crosstalk with myocardin-related transcription factors (MRTFs; Kim et al, 2017) and SKP2-dependent cell cycle progressio
Interpretation of increased energetic particle flux measurements by SEPT aboard the STEREO spacecraft and contamination
Context. Interplanetary (IP) shocks are known to be accelerators of energetic
charged particles observed in-situ in the heliosphere. However, the
acceleration of near-relativistic electrons by shocks in the interplanetary
medium is often questioned. On 9 August 2011 a Corotating Interaction Region
(CIR) passed STEREO B (STB) that resulted in a flux increase in the electron
and ion channels of the Solar Electron and Proton Telescope (SEPT). Because
electron measurements in the few keV to several 100 keV range rely on the
so-called magnet foil technique, which is utilized by SEPT, ions can contribute
to the electron channels. Aims. We aim to investigate whether the flux increase
in the electron channels of SEPT during the CIR event on 9 August 2011 is
caused by ion contamination only. Methods. We compute the SEPT response
functions for protons and helium utilizing an updated GEANT4 model of SEPT. The
CIR energetic particle ion spectra for protons and helium are assumed to follow
a Band function in energy per nucleon with a constant helium to proton ratio.
Results. Our analysis leads to a helium to proton ratio of 16.9% and a proton
flux following a Band function with the parameters /
(cm2 s sr MeV/nuc.), keV/nuc. and spectral indices of and which are in good agreement with measurements by
the Suprathermal Ion Telescope (SIT) aboard STB. Conclusions. Since our results
explain the SEPT measurements, we conclude that no significant amount of
electrons were accelerated between keV and keV by the CIR
A new ordering parameter of spectral energy distributions from synchrotron-self-Compton emitting blazars
The broadband SEDs of blazars exhibit two broad spectral components, which in
leptonic emission models are attributed to synchrotron radiation and
synchrotron self-Compton (SSC) radiation of relativistic electrons. During high
state phases, the high-frequency SSC component often dominates the
low-frequency synchrotron component, implying that the inverse Compton SSC
losses of electrons are at least equal to or greater than the synchrotron
losses of electrons. We calculate from the analytical solution of the kinetic
equation of relativistic electrons, subject to the combined linear synchrotron
and nonlinear synchrotron self-Compton cooling, for monoenergetic injection the
time-integrated total synchrotron and SSC radiation fluences and spectral
energy distributions (SED). Depending on the ratio of the initial cooling
terms, displayed by the injection parameter , we find for , implying complete linear cooling, that the synchrotron peak dominates the
inverse Compton peak and the usual results of the spectra are recovered. For
the SSC peak dominates the synchrotron peak, proving our
assumption that in such a case the cooling becomes initially non-linear. The
spectra also show some unique features, which can be attributed directly to the
non-linear cooling. To show the potential of the model, we apply it to
outbursts of 3C 279 and 3C 454.3, successfully reproducing the SEDs. The
results of our analysis are promising, and we argue that this non-equilibrium
model should be considered in future modeling attempts for blazar flares.Comment: accepted by MNRAS, 32 pages (single column), 7 figure
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