3,953 research outputs found
A small RNA response at DNA ends in Drosophila
Small RNAs have been implicated in numerous cellular processes, including effects on chromatin structure and the repression of transposons. We describe the generation of a small RNA response at DNA ends in Drosophila that is analogous to the recently reported double-strand break (DSB)-induced RNAs or Dicer- and Drosha-dependent small RNAs in Arabidopsis and vertebrates. Active transcription in the vicinity of the break amplifies this small RNA response, demonstrating that the normal messenger RNA contributes to the endogenous small interfering RNAs precursor. The double-stranded RNA precursor forms with an antisense transcript that initiates at the DNA break. Breaks are thus sites of transcription initiation, a novel aspect of the cellular DSB response. This response is specific to a double-strand break since nicked DNA structures do not trigger small RNA production. The small RNAs are generated independently of the exact end structure (blunt, 3âČ- or 5âČ-overhang), can repress homologous sequences in trans and may thereforeâin addition to putative roles in repairâexert a quality control function by clearing potentially truncated messages from genes in the vicinity of the break
Implications of automatic photon quenching on compact gamma-ray sources
Aims: We investigate photon quenching in compact non-thermal sources. This
involves photon-photon annihilation and lepton synchrotron radiation in a
network that can become non-linear. As a result the gamma-ray luminosity of a
source cannot exceed a critical limit that depends only on the radius of the
source and on the magnetic field. Methods: We perform analytic and numerical
calculations that verify previous results and extend them so that the basic
properties of photon quenching are investigated. Results: We apply the above to
the 2006 TeV observations of quasar 3C279 and obtain the parameter space of
allowed values for the radius of the emitting source, its magnetic field
strength and the Doppler factor of the flow. We argue that the TeV observations
favour either a modest Doppler factor and a low magnetic field or a high
Doppler factor and a high magnetic field.Comment: 10 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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
Timing Signatures of the Internal-Shock Model for Blazars
We investigate the spectral and timing signatures of the internal-shock model
for blazars. For this purpose, we develop a semi-analytical model for the
time-dependent radiative output from internal shocks arising from colliding
relativistic shells in a blazar jet. The emission through synchrotron and
synchrotron-self Compton (SSC) radiation as well as Comptonization of an
isotropic external radiation field are taken into account. We evaluate the
discrete correlation function (DCF) of the model light curves in order to
evaluate features of photon-energy dependent time lags and the quality of the
correlation, represented by the peak value of the DCF. The almost completely
analytic nature of our approach allows us to study in detail the influence of
various model parameters on the resulting spectral and timing features. This
paper focuses on a range of parameters in which the gamma-ray production is
dominated by Comptonization of external radiation, most likely appropriate for
gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL
Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability
of the optical emission is highly correlated with the X-ray and high-energy
(HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the
optical variability with respect to the higher-energy bands by 1 - 2 hours and
of the HE gamma-rays before the X-rays by about 1 hour. We show that variations
of certain parameters may lead to changing signs of inter-band time lags,
potentially explaining the lack of persistent trends of time lags in most
blazars.Comment: Accepted for publication in Ap
Temporal signatures of leptohadronic feedback mechanisms in compact sources
The hadronic model of Active Galactic Nuclei and other compact high energy
astrophysical sources assumes that ultra-relativistic protons,
electron-positron pairs and photons interact via various hadronic and
electromagnetic processes inside a magnetized volume, producing the
multiwavelength spectra observed from these sources. A less studied property of
such systems is that they can exhibit a variety of temporal behaviours due to
the operation of different feedback mechanisms. We investigate the effects of
one possible feedback loop, where \gamma-rays produced by photopion processes
are being quenched whenever their compactness increases above a critical level.
This causes a spontaneous creation of soft photons in the system that result in
further proton cooling and more production of \gamma-rays, thus making the loop
operate. We perform an analytical study of a simplified set of equations
describing the system, in order to investigate the connection of its temporal
behaviour with key physical parameters. We also perform numerical integration
of the full set of kinetic equations verifying not only our analytical results
but also those of previous numerical studies. We find that once the system
becomes `supercritical', it can exhibit either a periodic behaviour or a damped
oscillatory one leading to a steady state. We briefly point out possible
implications of such a supercriticality on the parameter values used in Active
Galactic Nuclei spectral modelling, through an indicative fitting of the VHE
emission of blazar 3C 279.Comment: 19 pages, 20 figures, accepted for publication in MNRA
A New Model for the Hard Time Lags in Black Hole X-Ray Binaries
The time-dependent Comptonized output of a cool soft X-ray source drifting
inward through an inhomogeneous hot inner disk or corona is numerically
simulated. We propose that this scenario can explain from first principles the
observed trends in the hard time lags and power spectra of the rapid aperiodic
variability of the X-ray emission of Galactic black-hole candidates.Comment: 10 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted
for ApJ Letter
The long-term optical spectral variability of BL Lacertae
We present the results from a study of the long-term optical spectral
variations of BL Lacertae, using the long and well-sampled B and R-band light
curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time
intervals of 1 day. The relation between spectral slope and flux (the spectrum
gets bluer as the source flux increases) is well described by a power-law
model, although there is significant scatter around the best-fitting model
line. To some extent, this is due to the spectral evolution of the source
(along well-defined loop-like structures) during low-amplitude events, which
are superimposed on the major optical flares, and evolve on time scales of a
few days. The "bluer-when-brighter" mild chromatism of the long-term variations
of the source can be explained if the flux increases/decreases faster in the B
than in the R band. The B and R-band variations are well correlated, with no
significant, measurable delays larger than a few days. On the other hand, we
find that the spectral variations lead those in the flux light curves by ~ 4
days. Our results can be explained in terms of Doppler factor variations due to
changes in the viewing angle of a curved and inhomogeneous emitting jet.Comment: 7 pages, 5 figures, accepted for publication in A&
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