392 research outputs found
Linking radio and gamma ray emission in Ap Librae
Ap Lib is one of the rare Low Synchrotron Peaked blazars detected so far at
TeV energies. This type of source is not properly modelled by standard one-zone
leptonic Synchrotron-self-Compton (SSC) emission scenarios. The aim of this
paper is to study the relevance of additional components which should naturally
occur in a SSC scenario for a better understanding of the emission mechanisms,
especially at very high energies (VHE). Methods. We use simultaneous data from
a multi-wavelength campaign of Planck, Swift-UVOT and Swift-XRT telescopes
carried out in February 2010, as well as quasi-simultaneous data of WISE, Fermi
and H.E.S.S. taken in 2010. The multi-lambda emission of Ap Lib is modelled by
a blob-in-jet SSC scenario including the contribution of the base of the VLBI
extended jet, the radiative blob-jet interaction, the accretion disk and its
associated external photon field. We show that signatures of a strong
parsec-scale jet and of an accretion disk emission are present in the SED. We
can link the observationnal VLBI jet features from MOJAVE to parameters
expected for a VHE emitting blob accelerated near the jet base. The VHE
emission appears to be dominated by the inverse-Compton effect of the blob
relativistic electrons interacting with the jet synchrotron radiation. In such
scenario Ap Lib appears as an intermediate source between BL Lac objects and
Flat Spectrum Radio Quasars. Ap Lib could be a bright representative of a
specific class of blazars, in which the parsec-scale jet luminosity is no more
negligible compared to the blob and contributes to the high energy emission via
inverse Compton processes.Comment: 12 pages, 7 figures, Accepted for publication in A&
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
Mass diffusion measurements in liquid crystals by a novel optical method
Binary mass diffusion measurements have been performed in fluid media doped with photochromic dye molecules. A periodic concentration of tagged molecules is created by illuminating the sample with a fringe intensity pattern. The photo induced change of refractive index and/or absorption of the dye molecules creates an optical grating which is observed through Bragg diffraction of an auxiliary laser beam. When the flash excitation is switched off. this grating will relax since photoexcited molecules will diffuse into nonphotoexcited zones and vice versa. The main advantages of this method are that: (1) the duration of the experiment is strongly reduced compared to classical tracer methods since diffusion lengths. defined by the fringe spacing. are small (1-100 fLm); (2) the possibility of studying anisotropic diffusion is readily available; and (3) small sample volumes are required. Results are reported on the diffusion of methyl red in a homogeneously aligned sample of MBBA. In the nematic phase. the diffusion is faster along the local optical axis than perpendicular, the anisotropic ratio DU / Dl being 1.6±O.l at 22 "C. in good agreement with previous tracer data. The temperature dependence yields an activation energy of 5.8±O.7 kcallmole for DII and 6.0±O.8 kcallmole for D 1 • In the isotropic phase. the activation energy is found to be \O± 1.5 kcallmole
SST-GATE: A dual mirror telescope for the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) will be the world's first open
observatory for very high energy gamma-rays. Around a hundred telescopes of
different sizes will be used to detect the Cherenkov light that results from
gamma-ray induced air showers in the atmosphere. Amongst them, a large number
of Small Size Telescopes (SST), with a diameter of about 4 m, will assure an
unprecedented coverage of the high energy end of the electromagnetic spectrum
(above ~1TeV to beyond 100 TeV) and will open up a new window on the
non-thermal sky. Several concepts for the SST design are currently being
investigated with the aim of combining a large field of view (~9 degrees) with
a good resolution of the shower images, as well as minimizing costs. These
include a Davies-Cotton configuration with a Geiger-mode avalanche photodiode
(GAPD) based camera, as pioneered by FACT, and a novel and as yet untested
design based on the Schwarzschild-Couder configuration, which uses a secondary
mirror to reduce the plate-scale and to allow for a wide field of view with a
light-weight camera, e.g. using GAPDs or multi-anode photomultipliers. One
objective of the GATE (Gamma-ray Telescope Elements) programme is to build one
of the first Schwarzschild-Couder prototypes and to evaluate its performance.
The construction of the SST-GATE prototype on the campus of the Paris
Observatory in Meudon is under way. We report on the current status of the
project and provide details of the opto-mechanical design of the prototype, the
development of its control software, and simulations of its expected
performance.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
Post-Tanner spreading of nematic droplets
The quasistationary spreading of a circular liquid drop on a solid substrate
typically obeys the so-called Tanner law, with the instantaneous base radius
R(t) growing with time as R ~ t^{1/10} -- an effect of the dominant role of
capillary forces for a small-sized droplet. However, for droplets of nematic
liquid crystals, a faster spreading law sets in at long times, so that R ~
t^alpha with alpha significantly larger than the Tanner exponent 1/10. In the
framework of the thin film model (or lubrication approximation), we describe
this "acceleration" as a transition to a qualitatively different spreading
regime driven by a strong substrate-liquid interaction specific to nematics
(antagonistic anchoring at the interfaces). The numerical solution of the thin
film equation agrees well with the available experimental data for nematics,
even though the non-Newtonian rheology has yet to be taken into account. Thus
we complement the theory of spreading with a post-Tanner stage, noting that the
spreading process can be expected to cross over from the usual
capillarity-dominated stage to a regime where the whole reservoir becomes a
diffusive film in the sense of Derjaguin.Comment: 15 pages, 4 figures, accepted in JPCM special issu
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Flow-induced dynamic surface tension effects at nanoscale
The aim of this study is to investigate flow-induced dynamic surface tension effects, similar to the well-known Marangoni phenomena, but solely generated by the nanoscale topography of the substrates. The flow-induced surface tension effects are examined on the basis of a sharp interface theory. It is demonstrated how nanoscale objects placed at the boundary of the flow domain result in the generation of substantial surface forces acting on the bulk flow
Post-Tanner stages of droplet spreading: the energy balance approach revisited
The spreading of a circular liquid drop on a solid substrate can be described
by the time evolution of its base radius R(t). In complete wetting the
quasistationary regime (far away from initial and final transients) typically
obeys the so-called Tanner law, with R t^alpha_T, alpha_T=1/10. Late-time
spreading may differ significantly from the Tanner law: in some cases the drop
does not thin down to a molecular film and instead reaches an equilibrium
pancake-like shape; in other situations, as revealed by recent experiments with
spontaneously spreading nematic crystals, the growth of the base radius
accelerates after the Tanner stage. Here we demonstrate that these two
seemingly conflicting trends can be reconciled within a suitably revisited
energy balance approach, by taking into account the line tension contribution
to the driving force of spreading: a positive line tension is responsible for
the formation of pancake-like structures, whereas a negative line tension tends
to lengthen the contact line and induces an accelerated spreading (a transition
to a faster power law for R(t) than in the Tanner stage).Comment: 12 pages, 1 figur
Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR
We present a study of -ray emission from the core-collapse supernova
remnant Cas~A in the energy range from 0.1GeV to 10TeV. We used 65 hours of
VERITAS data to cover 200 GeV - 10 TeV, and 10.8 years of \textit{Fermi}-LAT
data to cover 0.1-500 GeV. The spectral analysis of \textit{Fermi}-LAT data
shows a significant spectral curvature around GeV that is
consistent with the expected spectrum from pion decay. Above this energy, the
joint spectrum from \textit{Fermi}-LAT and VERITAS deviates significantly from
a simple power-law, and is best described by a power-law with spectral index of
with a cut-off energy of TeV. These
results, along with radio, X-ray and -ray data, are interpreted in the
context of leptonic and hadronic models. Assuming a one-zone model, we exclude
a purely leptonic scenario and conclude that proton acceleration up to at least
6 TeV is required to explain the observed -ray spectrum. From modeling
of the entire multi-wavelength spectrum, a minimum magnetic field inside the
remnant of is deduced.Comment: 33 pages, 9 Figures, 6 Table
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
Very-high-energy observations of the binaries V 404 Cyg and 4U 0115+634 during giant X-ray outbursts
Transient X-ray binaries produce major outbursts in which the X-ray flux can
increase over the quiescent level by factors as large as . The low-mass
X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such
major outbursts in June and October 2015, respectively. We present here
observations at energies above hundreds of GeV with the VERITAS observatory
taken during some of the brightest X-ray activity ever observed from these
systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of
observations of the microquasar V 404 Cyg from 2015, June 20-21. The upper flux
limits derived from these observations on the gamma-ray flux above 200 GeV of F
cm s correspond to a tiny fraction (about
) of the Eddington luminosity of the system, in stark contrast to that
seen in the X-ray band. No gamma rays have been detected during observations of
4U 0115+634 in the period of major X-ray activity in October 2015. The flux
upper limit derived from our observations is F cm
s for gamma rays above 300 GeV, setting an upper limit on the ratio of
gamma-ray to X-ray luminosity of less than 4%.Comment: Accepted for publication in the Astrophysical Journa
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