2,234 research outputs found
Discovery of a tight correlation for gamma ray burst afterglows with `canonical' light curves
Gamma Ray Bursts (GRB) observed up to redshifts are fascinating objects
to study due to their still unexplained relativistic outburst mechanisms and a
possible use to test cosmological models. Our analysis of 77 GRB afterglows
with known redshifts revealed a physical subsample of long GRBs with canonical
{\it plateau breaking to power-law} light curves with a significant {\it
luminosity - break time } correlation in the GRB rest frame.
This subsample forms approximately the {\it upper envelope} of the studied
distribution. We have also found a similar relation for a small sample of GRB
afterglows that belong to the intermediate class (IC) between the short and the
long ones. It proves that within the full sample of afterglows there exist
physical subclasses revealed here by tight correlations of their afterglow
properties. The afterglows with regular (`canonical') light curves obey not
only a mentioned tight physical scaling, but -- for a given -- the more
regular progenitor explosions lead to preferentially brighter afterglows.Comment: 15 pages, 5 figures accepted to ApJ
The X-ray Jet in Centaurus A: Clues on the Jet Structure and Particle Acceleration
We report detailed studies of the X-ray emission from the kpc scale jet in
the nearest active galaxy, Cen A. 41 compact sources were found within the jet,
13 of which were newly identified. We construct the luminosity function for the
detected jet-knots and argue that the remaining emission is most likely to be
truly diffuse, rather than resulting from the pile-up of unresolved faint
knots. The transverse jet profile reveals that the extended emission has the
intensity peak at the jet boundaries. We note that limb-brightened jet
morphologies have been observed previously at radio frequencies in some jet
sources, but never so clearly at higher photon energies. Our result therefore
supports a stratified jet model, consisting of a relativistic outflow including
a boundary layer with a velocity shear. In addition, we found that the X-ray
spectrum of the diffuse component is almost uniform across and along the jet.
We discuss this spectral behavior within a framework of shock and stochastic
particle acceleration processes. We note some evidence for a possible spectral
hardening at the outer sheath of the jet. Due to the limited photon statistics
of the present data, further deep observations of Cen A are required to
determine the reality of this finding, however we note that the existence of
the hard X-ray features at outer jet boundaries would provide an important
challenge to theories for the evolution of ultra-relativistic particles within
the jets.Comment: 27page, 8 figures, ver2, accepted for publication in the Ap
Ras-mediated phosphorylation of a conserved threonine residue enhances the transactivation activities of c-Ets1 and c-Ets2
The Ras oncogene products regulate the expression of genes in transformed cells, and members of the Ets family of transcription factors have been implicated in this process. To determine which Ets factors are the targets of Ras signaling pathways, the abilities of several Ets factors to activate Ras-responsive enhancer (RRE) reporters in the presence of oncogenic Ras were examined. In transient transfection assay, reporters containing RREs composed of Ets-AP-1 binding sites could be activated 30-fold in NIH 3T3 fibroblasts and 80-fold in the macrophage-like line RAW264 by the combination of Ets1 or Ets2 and Ras but not by several other Ets factors that were tested in the assay. Ets2 and Ras also superactivated an RRE composed of Ets-Ets binding sites, but the Ets-responsive promoter of the c-fms gene was not superactivated. Mutation of a threonine residue to alanine in the conserved amino-terminal regions of Ets1 and Ets2 (threonine 38 and threonine 72, respectively) abrogated the ability of each of these proteins to superactivate reporter gene expression. Phosphoamino acid analysis of radiolabeled Ets2 revealed that Ras induced normally absent threonine-specific phosphorylation of the protein. The Ras-dependent increase in threonine phosphorylation was not observed in Ets2 proteins that had the conserved threonine 72 residue mutated to alanine or serine. These data indicate that Ets1 and Ets2 are specific nuclear targets of Ras signaling events and that phosphorylation of a conserved threonine residue is a necessary molecular component of Ras-mediated activation of these transcription factors
X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet-knots, hotspots and radio lobes
recently observed with Chandra and ASCA. This report will discuss the origin of
their X-ray emissions and investigate the dynamics of the jets. The data was
compiled at well sampled radio (5GHz) and X-ray frequencies (1keV) for more
than 40 radio galaxies. We examined three models for the X-ray production:
synchrotron (SYN), synchrotron self-Compton (SSC) and external Compton on CMB
photons (EC). For the SYN sources -- mostly jet-knots in nearby low-luminosity
radio galaxies -- X-ray photons are produced by ultrarelativistic electrons
with energies 10-100 TeV that must be accelerated in situ. For the other
objects, conservatively classified as SSC or EC sources, a simple formulation
of calculating the ``expected'' X-ray fluxes under an equipartition hypothesis
is presented. We confirmed that the observed X-ray fluxes are close to the
expected ones for non-relativistic emitting plasma velocities in the case of
radio lobes and majority of hotspots, whereas considerable fraction of
jet-knots is too bright at X-rays to be explained in this way. We examined two
possibilities to account for the discrepancy in a framework of the
inverse-Compton model: (1) magnetic field is much smaller than the
equipartition value, and (2) the jets are highly relativistic on kpc/Mpc
scales. We concluded, that if the inverse-Compton model is the case, the X-ray
bright jet-knots are most likely far from the minimum-power condition. We also
briefly discuss the other possibility, namely that the observed X-ray emission
from all of the jet-knots is synchrotron in origin.Comment: 20 pages, 10 figures, accepted for publication in the Astrophysical
Journal, vol.62
Signatures of the disk-jet coupling in the Broad-line Radio Quasar 4C+74.26
Here we explore the disk-jet connection in the broad-line radio quasar
4C+74.26, utilizing the results of the multiwavelength monitoring of the
source. The target is unique in that its radiative output at radio wavelengths
is dominated by a moderately-beamed nuclear jet, at optical frequencies by the
accretion disk, and in the hard X-ray range by the disk corona. Our analysis
reveals a correlation (local and global significance of 96\% and 98\%,
respectively) between the optical and radio bands, with the disk lagging behind
the jet by days. We discuss the possible explanation for this,
speculating that the observed disk and the jet flux changes are generated by
magnetic fluctuations originating within the innermost parts of a truncated
disk, and that the lag is related to a delayed radiative response of the disk
when compared with the propagation timescale of magnetic perturbations along
relativistic outflow. This scenario is supported by the re-analysis of the
NuSTAR data, modelled in terms of a relativistic reflection from the disk
illuminated by the coronal emission, which returns the inner disk radius
. We discuss the global energetics in
the system, arguing that while the accretion proceeds at the Eddington rate,
with the accretion-related bolometric luminosity erg s , the jet total kinetic energy
erg s, inferred from the dynamical
modelling of the giant radio lobes in the source, constitutes only a small
fraction of the available accretion power.Comment: 9 pages and 6 figures, ApJ accepte
High Energy gamma-rays From FR I Jets
Thanks to Hubble and Chandra telescopes, some of the large scale jets in
extragalactic radio sources are now being observed at optical and X-ray
frequencies. For the FR I objects the synchrotron nature of this emission is
surely established, although a lot of uncertainties - connected for example
with the particle acceleration processes involved - remain. In this paper we
study production of high energy gamma-rays in FR I kiloparsec-scale jets by
inverse-Compton emission of the synchrotron-emitting electrons. We consider
different origin of seed photons contributing to the inverse-Compton
scattering, including nuclear jet radiation as well as ambient, stellar and
circumstellar emission of the host galaxies. We discuss how future detections
or non-detections of the evaluated gamma-ray fluxes can provide constraints on
the unknown large scale jet parameters, i.e. the magnetic field intensity and
the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find
measurable fluxes of TeV photons resulting from synchrotron self-Compton
process and from comptonisation of the galactic photon fields, respectively. In
the case of Centaurus A, we also find a relatively strong emission component
due to comptonisation of the nuclear blazar photons, which could be easily
observed by GLAST at energy ~10 GeV, providing important test for the
unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for
publication in Astrophysical Journa
UVSat: a concept of an ultraviolet/optical photometric satellite
Time-series photometry from space in the ultraviolet can be presently done
with only a few platforms, none of which is able to provide wide-field
long-term high-cadence photometry. We present a concept of UVSat, a twin space
telescope which will be capable to perform this kind of photometry, filling an
observational niche. The satellite will host two telescopes, one for
observations in the ultraviolet, the other for observations in the optical
band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of
the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck
Particle Diffusion and Acceleration by Shock Wave in Magnetized Filamentary Turbulence
We expand the off-resonant scattering theory for particle diffusion in
magnetized current filaments that can be typically compared to astrophysical
jets, including active galactic nucleus jets. In a high plasma beta region
where the directional bulk flow is a free-energy source for establishing
turbulent magnetic fields via current filamentation instabilities, a novel
version of quasi-linear theory to describe the diffusion of test particles is
proposed. The theory relies on the proviso that the injected energetic
particles are not trapped in the small-scale structure of magnetic fields
wrapping around and permeating a filament but deflected by the filaments, to
open a new regime of the energy hierarchy mediated by a transition compared to
the particle injection. The diffusion coefficient derived from a quasi-linear
type equation is applied to estimating the timescale for the stochastic
acceleration of particles by the shock wave propagating through the jet. The
generic scalings of the achievable highest energy of an accelerated ion and
electron, as well as of the characteristic time for conceivable energy
restrictions, are systematically presented. We also discuss a feasible method
of verifying the theoretical predictions. The strong, anisotropic turbulence
reflecting cosmic filaments might be the key to the problem of the acceleration
mechanism of the highest energy cosmic rays exceeding 100 EeV (10^{20} eV),
detected in recent air shower experiments.Comment: 39 pages, 2 figures, accepted for publication in Ap
Radiation from the Relativistic Jet: a Role of the Shear Boundary Layer
Recent radio and optical large scale jets' observations suggest a
two-component jet morphology, consisting of a fast central spine surrounded
with a boundary layer with a velocity shear. We study radiation of electrons
accelerated at such boundary layers as an option for standard approaches
involving internal shocks in jets. The acceleration process in the boundary
layer yields in a natural way a two component electron distribution: a
power-law continuum with a bump at the energy, where energy gains equal
radiation losses, followed by a cut-off. For such distributions we derive the
observed spectra of synchrotron and inverse-Compton radiation, including
comptonization of synchrotron and CMB photons. Under simple assumptions of
energy equipartition between the relativistic particles and the magnetic field,
the relativistic jet velocity at large scales and a turbulent character of the
shear layer, the considered radiation can substantially contribute to the jet
radiative output. In the considered conditions the synchrotron emission is
characterized by a spectral index of the radio-to-optical continuum being
approximately constant along the jet. A characteristic feature of the obtained
broad-band synchrotron spectrum is an excess at X-ray frequencies, similar to
the one observed in some objects by Chandra. As compared to the uniform jet
models, the velocity shear across the radiating boundary region leads to
decrease and frequency dependence of the observed jet-counterjet radio
brightness asymmetry. We conclude that a careful investigation of the
observational data looking for the derived effects can allow to evaluate the
role of the boundary layer acceleration processes and/or impose constraints for
the physical parameters of such layers in large scale jets.Comment: 30 pages, 4 figures included. Modified version, accepted for
publication in Astrophysical Journa
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