566 research outputs found
Radio Properties of the -ray Emitting CSO Candidate 2234+282
Most of the gamma-ray emitting active galactic nuclei (AGN) are blazars,
although there is still a small fraction of non-blazar AGN in the Fermi/LAT
catalog. Among these misaligned gamma-ray-emitting AGN, a few can be classified
as Compact Symmetric Objects (CSOs). In contrast to blazars in which gamma-ray
emission is generally thought to originate from highly beamed relativistic
jets, the source of gamma-ray emission in unbeamed CSOs remains an open
question. The rarity of the gamma-ray emitting CSOs is a mystery as well. Here
we present the radio properties of the gamma-ray CSO candidate 2234+282.Comment: 4 pages, accepted for publication in Astronomische Nachrichte
Precursors of short gamma-ray bursts
We carried out a systematic search of precursors on the sample of short GRBs
observed by Swift. We found that ~8-10% of short GRBs display such early
episode of emission. One burst (GRB 090510) shows two precursor events, the
former ~13 s and the latter ~0.5 s before the GRB. We did not find any
substantial difference between the precursor and the main GRB emission, and
between short GRBs with and without precursors. We discuss possible mechanisms
to reproduce the observed precursor emission within the scenario of compact
object mergers. The implications of our results on quantum gravity constraints
are also discussed.Comment: 8 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Studies of active galactic nuclei with CTA
In this paper, we review the prospects for studies of active galactic nuclei
(AGN) using the envisioned future Cherenkov Telescope Array (CTA). This review
focuses on jetted AGN, which constitute the vast majority of AGN detected at
gamma-ray energies. Future progress will be driven by the planned lower energy
threshold for very high energy (VHE) gamma-ray detections to ~10 GeV and
improved flux sensitivity compared to current-generation Cherenkov Telescope
facilities. We argue that CTA will enable substantial progress on gamma-ray
population studies by deepening existing surveys both through increased flux
sensitivity and by improving the chances of detecting a larger number of
low-frequency peaked blazars because of the lower energy threshold. More
detailed studies of the VHE gamma-ray spectral shape and variability might
furthermore yield insight into unsolved questions concerning jet formation and
composition, the acceleration of particles within relativistic jets, and the
microphysics of the radiation mechanisms leading to the observable high-energy
emission. The broad energy range covered by CTA includes energies where
gamma-rays are unaffected from absorption while propagating in the
extragalactic background light (EBL), and extends to an energy regime where VHE
spectra are strongly distorted. This will help to reduce systematic effects in
the spectra from different instruments, leading to a more reliable EBL
determination, and hence will make it possible to constrain blazar models up to
the highest energies with less ambiguity.Comment: invited review article, 15 pages, 9 figures, Astroparticle Physics,
Special Issue on Physics with the Cherenkov Telescope Array, in pres
Constraints on Cold Magnetized Shocks in Gamma-Ray Bursts
We consider a model in which the ultra-relativistic jet in a gamma-ray burst
(GRB) is cold and magnetically accelerated. We assume that the energy flux in
the outflowing material is partially thermalized via internal shocks or a
reverse shock, and we estimate the maximum amount of radiation that could be
produced in such magnetized shocks. We compare this estimate with the available
observational data on prompt gamma-ray emission in GRBs. We find that, even
with highly optimistic assumptions, the magnetized jet model is radiatively too
inefficient to be consistent with observations. One way out is to assume that
much of the magnetic energy in the post-shock, or even pre-shock, jet material
is converted to particle thermal energy by some unspecified process, and then
radiated. This can increase the radiative efficiency sufficiently to fit
observations. Alternatively, jet acceleration may be driven by thermal pressure
rather than magnetic fields. In this case, which corresponds to the traditional
fireball model, sufficient prompt GRB emission could be produced either from
shocks at a large radius or from the jet photosphere closer to the center.Comment: MNRAS, in press. 9 pages, 4 figures, uses mn2e.cl
The Dark Knight Falters
Potential line emission at 111 and 129 GeV from 16 unassociated Fermi-LAT
point sources has been reported recently by Su & Finkbeiner (2012c). Together
with similar features seen by Fermi in a region near the Galactic Centre, the
evidence has been interpreted as the spectral signature of dark matter
annihilation or internal bremsstrahlung. Through a combination of supervised
machine-learning algorithms and archival multiwavelength observations we find
that 14 out of the 16 unassociated sources showing the line emission in the Su
& Finkbeiner sample are most likely active galactic nuclei (AGN). Based on this
new evidence, one must widen the range of possible solutions for the 100-140
GeV excess to include a very distinct astrophysical explanation. While we
cannot rule out a dark matter origin for the line emission in the Galactic
Centre, we posit that if the detection in the Su & Finkbeiner sample is indeed
real it might be related to accretion, bubble, or jet activity in nearby (z <
0.2) AGN. Alternatively, given the right conditions, the similarity could be
due to a chance occurrence caused by extragalactic background light (EBL)
absorption. Or else one must concede that the features are an artefact of
instrumental or calibration issues.Comment: 5 pages, 2 figures, 2 tables, accepted for publication in MNRA
Dark Matter Constraints from Observations of 25 Milky Way Satellite Galaxies with the Fermi Large Area Telescope
The dwarf spheroidal satellite galaxies of the Milky Way are some of the most dark-matter-dominated objects known. Due to their proximity, high dark matter content, and lack of astrophysical backgrounds, dwarf spheroidal galaxies are widely considered to be among the most promising targets for the indirect detection of dark matter via gamma rays. Here we report on gamma ray observations of 25 Milky Way dwarf spheroidal satellite galaxies based on 4 years of Fermi Large Area Telescope (LAT) data. None of the dwarf galaxies are significantly detected in gamma rays, and we present gamma ray flux upper limits between 500MeV and 500 GeV. We determine the dark matter content of 18 dwarf spheroidal galaxies from stellar kinematic data and combine LAT observations of 15 dwarf galaxies to constrain the dark matter annihilation cross section. We set some of the tightest constraints to date on the annihilation of dark matter particles with masses between 2 GeV and 10TeV into prototypical standard model channels. We find these results to be robust against systematic uncertainties in the LAT instrument performance, diffuse gamma ray background modeling, and assumed dark matter density profile
The Connection Between Thermal and Non-Thermal Emission in Gamma-ray Bursts: General Considerations and GRB090902B as a Case Study
Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB)
"fireball" model. We show here, that inclusion of this component in the
analysis of the GRB prompt emission phase naturally explains some of the prompt
GRB spectra seen by the Fermi satellite over its entire energy band. The
sub-MeV peak is explained as multi-color black body emission, and the high
energy tail, extending up to the GeV band, results from roughly similar
contributions of synchrotron emission, synchrotron self Compton(SSC) and
Comptonization of the thermal photons by energetic electrons originating after
dissipation of the kinetic energy above the photosphere. We show how this
analysis method results in a complete, self consistent picture of the physical
conditions at both emission sites of the thermal and non-thermal radiation. We
study the connection between the thermal and non-thermal parts of the spectrum,
and show how the values of the free model parameters are deduced from the data.
We demonstrate our analysis method on GRB090902B: We deduce a Lorentz factor in
the range 920 <= \eta <= 1070, photospheric radius r_{ph} ~ 7.2 - 8.4 * 10^{11}
cm and dissipation radius r_\gamma >= 3.5 - 4.1 * 10^{15} cm. By comparison to
afterglow data, we deduce that a large fraction, epsilon_d ~85% - 95% of the
kinetic energy is dissipated, and that large fraction, ~equipartition of this
energy is carried by the electrons and the magnetic field. This high value of
epsilon_d questions the "internal shock" scenario as the main energy
dissipation mechanism for this GRB.Comment: 15 pages, 5 figures; minor revisions, typos corrected. Accepted for
publication in MNRA
Open Questions in GRB Physics
Open questions in GRB physics are summarized as of 2011, including
classification, progenitor, central engine, ejecta composition, energy
dissipation and particle acceleration mechanism, radiation mechanism, long term
engine activity, external shock afterglow physics, origin of high energy
emission, and cosmological setting. Prospects of addressing some of these
problems with the upcoming Chinese-French GRB mission, SVOM, are outlined.Comment: 27 pages. To appear in a special issue of Comptes Rendus Physique
"GRB studies in the SVOM era", Eds. F. Daigne, G. Dubu
Quantitative principles of cis-translational control by general mRNA sequence features in eukaryotes.
BackgroundGeneral translational cis-elements are present in the mRNAs of all genes and affect the recruitment, assembly, and progress of preinitiation complexes and the ribosome under many physiological states. These elements include mRNA folding, upstream open reading frames, specific nucleotides flanking the initiating AUG codon, protein coding sequence length, and codon usage. The quantitative contributions of these sequence features and how and why they coordinate to control translation rates are not well understood.ResultsHere, we show that these sequence features specify 42-81% of the variance in translation rates in Saccharomyces cerevisiae, Schizosaccharomyces pombe, Arabidopsis thaliana, Mus musculus, and Homo sapiens. We establish that control by RNA secondary structure is chiefly mediated by highly folded 25-60 nucleotide segments within mRNA 5' regions, that changes in tri-nucleotide frequencies between highly and poorly translated 5' regions are correlated between all species, and that control by distinct biochemical processes is extensively correlated as is regulation by a single process acting in different parts of the same mRNA.ConclusionsOur work shows that general features control a much larger fraction of the variance in translation rates than previously realized. We provide a more detailed and accurate understanding of the aspects of RNA structure that directs translation in diverse eukaryotes. In addition, we note that the strongly correlated regulation between and within cis-control features will cause more even densities of translational complexes along each mRNA and therefore more efficient use of the translation machinery by the cell
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