2,999 research outputs found
An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?
High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or
1ES 0347-121, recently focused interest both for blazar and jet physics and for
the implication on the extragalactic background light and intergalactic
magnetic field estimate. However, the number of these extreme highly peaked BL
Lac objects (EHBL) is still rather small. Aiming at increase their number, we
selected a group of EHBL candidates starting from the BL Lac sample of Plotkin
et al. (2011), considering those undetected (or only barely detected) by the
Large Area Telescope onboard Fermi and characterized by a high X-ray vs. radio
flux ratio. We assembled the multi-wavelength spectral energy distribution of
the resulting 9 sources, profiting of publicly available archival observations
performed by the Swift, Galex and Fermi satellites, confirming their nature.
Through a simple one-zone synchrotron self-Compton model we estimate the
expected VHE flux, finding that in the majority of cases it is within the reach
of present generation of Cherenkov arrays or of the forthcoming Cherenkov
Telescope Array (CTA).Comment: 12 pages, 5 figures, accepted for publication in MNRA
Extreme BL Lacs: probes for cosmology and UHECR candidates
High-energy observations of extreme BL Lac objects, such as 1ES0229+200 or
1ES 0347-121, recently focused interest both for blazar and jet physics and for
the implication on the extragalactic background light and intergalactic
magnetic field estimate. Moreover, their enigmatic properties have been
interpreted in a scenario in which their primary high- energy output is through
a beam of high-energy hadrons. However, despite their possible important role
in all these topics, the number of these extreme highly peaked BL Lac objects
(EHBL) is still rather small. Aiming at increase their number, we selected a
group of EHBL candidates considering those undetected (or only barely detected)
by the LAT onboard Fermi and characterized by a high X-ray versus radio flux
ratio. We assembled the multi-wavelength spectral energy distribution of the
resulting 9 sources, using available archival data of Swift, GALEX, and Fermi
satellites, confirming their nature. Through a simple one-zone synchrotron
self-Compton model we estimate the expected very high energy flux, finding that
in the majority of cases it is within the reach of present generation of
Cherenkov arrays or of the forthcoming CTA.Comment: 6 pages, to appear in the Proceedings of the Conference "High-Energy
Phenomena and Relativistic Outflows V", held in La Plata, 5-8 October 201
Sonic Booms in Atmospheric Turbulence (SonicBAT): The Influence of Turbulence on Shaped Sonic Booms
The objectives of the Sonic Booms in Atmospheric Turbulence (SonicBAT) Program were to develop and validate, via research flight experiments under a range of realistic atmospheric conditions, one numeric turbulence model research code and one classic turbulence model research code using traditional N-wave booms in the presence of atmospheric turbulence, and to apply these models to assess the effects of turbulence on the levels of shaped sonic booms predicted from low boom aircraft designs. The SonicBAT program has successfully investigated sonic boom turbulence effects through the execution of flight experiments at two NASA centers, Armstrong Flight Research Center (AFRC) and Kennedy Space Center (KSC), collecting a comprehensive set of acoustic and atmospheric turbulence data that were used to validate the numeric and classic turbulence models developed. The validated codes were incorporated into the PCBoom sonic boom prediction software and used to estimate the effect of turbulence on the levels of shaped sonic booms associated with several low boom aircraft designs. The SonicBAT program was a four year effort that consisted of turbulence model development and refinement throughout the entire period as well as extensive flight test planning that culminated with the two research flight tests being conducted in the second and third years of the program. The SonicBAT team, led by Wyle, includes partners from the Pennsylvania State University, Lockheed Martin, Gulfstream Aerospace, Boeing, Eagle Aeronautics, Technical & Business Systems, and the Laboratory of Fluid Mechanics and Acoustics (France). A number of collaborators, including the Japan Aerospace Exploration Agency, also participated by supporting the experiments with human and equipment resources at their own expense. Three NASA centers, AFRC, Langley Research Center (LaRC), and KSC were essential to the planning and conduct of the experiments. The experiments involved precision flight of either an F-18A or F-18B executing steady, level passes at supersonic airspeeds in a turbulent atmosphere to create sonic boom signatures that had been distorted by turbulence. The flights spanned a range of atmospheric turbulence conditions at NASA Armstrong and Kennedy in order to provide a variety of conditions for code validations. The SonicBAT experiments at both sites were designed to capture simultaneous F-18A or F-18B onboard flight instrumentation data, high fidelity ground based and airborne acoustic data, surface and upper air meteorological data, and additional meteorological data from ultrasonic anemometers and SODARs to determine the local atmospheric turbulence and boundary layer height
A Memetic Analysis of a Phrase by Beethoven: Calvinian Perspectives on Similarity and Lexicon-Abstraction
This article discusses some general issues arising from the study of similarity in music, both human-conducted and computer-aided, and then progresses to a consideration of similarity relationships between patterns in a phrase by Beethoven, from the first movement of the Piano Sonata in A flat major op. 110 (1821), and various potential memetic precursors. This analysis is followed by a consideration of how the kinds of similarity identified in the Beethoven phrase might be understood in psychological/conceptual and then neurobiological terms, the latter by means of William Calvinâs Hexagonal Cloning Theory. This theory offers a mechanism for the operation of David Copeâs concept of the lexicon, conceived here as a museme allele-class. I conclude by attempting to correlate and map the various spaces within which memetic replication occurs
A jet-dominated model for a broad-band spectral energy distribution of the nearby low-luminosity active galactic nucleus in M94
We have compiled a new multiwavelength spectral energy distribution (SED) for
the closest obscured low-ionization emission-line region active galactic
nucleus (AGN), NGC 4736, also known as M94. The SED comprises mainly
high-resolution (mostly sub-arcsecond, or, at the distance to M94, <23 pc from
the nucleus) observations from the literature, archival data, as well as
previously unpublished sub-millimetre data from the Plateau de Bure
Interferometer (PdBI) and the Combined Array for Research in Millimeter-wave
Astronomy, in conjunction with new electronic MultiElement Radio
Interferometric Network (e-MERLIN) L-band (1.5 GHz) observations. Thanks to the
e-MERLIN resolution and sensitivity, we resolve for the first time a double
structure composed of two radio sources separated by ~1 arcsec, previously
observed only at higher frequency. We explore this data set, which further
includes non-simultaneous data from the Very Large Array, the Gemini telescope,
the Hubble Space Telescope and the Chandra X-ray observatory, in terms of an
outflow-dominated model. We compare our results with previous trends found for
other AGN using the same model (NGC 4051, M81*, M87 and Sgr A*), as well as
hard- and quiescent-state X-ray binaries. We find that the nuclear broad-band
spectrum of M94 is consistent with a relativistic outflow of low inclination.
The findings in this work add to the growing body of evidence that the physics
of weakly accreting black holes scales with mass in a rather straightforward
fashion.Comment: 18 pages, 7 figure
Faraday Conversion in Turbulent Blazar Jets
Low () levels of circular polarization (CP) detected at radio
frequencies in the relativistic jets of some blazars can provide insight into
the underlying nature of the jet plasma. CP can be produced through linear
birefringence, in which initially linearly polarized emission produced in one
region of the jet is altered by Faraday rotation as it propagates through other
regions of the jet with varying magnetic field orientation. Marscher has begun
a study of jets with such magnetic geometries using the Turbulent Extreme
Multi-Zone (TEMZ) model, in which turbulent plasma crossing a standing shock in
the jet is represented by a collection of thousands of individual plasma cells,
each with distinct magnetic field orientations. Here we develop a radiative
transfer scheme that allows the numerical TEMZ code to produce simulated images
of the time-dependent linearly and circularly polarized intensity at different
radio frequencies. In this initial study, we produce synthetic polarized
emission maps that highlight the linear and circular polarization expected
within the model.Comment: 17 pages, 13 figures, accepted for publication in Ap
Pulsar Timing Constraints on the Fermi Massive Black-Hole Binary Blazar Population
Blazars are a sub-population of quasars whose jets are nearly aligned with
the line-of-sight, which tend to exhibit multi-wavelength variability on a
variety of timescales. Quasi-periodic variability on year-like timescales has
been detected in a number of bright sources, and has been connected to the
orbital motion of a putative massive black hole binary. If this were indeed the
case, those blazar binaries would contribute to the nanohertz
gravitational-wave stochastic background. We test the binary hypothesis for the
blazar population observed by the \textit{Fermi} Gamma-Ray Space Telescope,
which consists of BL Lacertae objects and flat-spectrum radio quasars. Using
mock populations informed by the luminosity functions for BL Lacertae objects
and flat-spectrum radio quasars with redshifts , we calculate the
expected gravitational wave background and compare it to recent pulsar timing
array upper limits. The two are consistent only if a fraction of blazars hosts a binary with orbital periods years. We
therefore conclude that binarity cannot significantly explain year-like
quasi-periodicity in blazars.Comment: 5 pages, 4 figures, accepted by MNRAS Letter
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