1,813 research outputs found
The University of Michigan Centimeter-Band All Stokes Blazar Monitoring Program: Single-Dish Polarimetry as a Probe of Parsec-Scale Magnetic Fields
The University of Michigan 26-m paraboloid was dedicated to obtaining linear
polarization and total flux density observations of blazars from the mid-1960s
until June 2012 providing an unprecedented record tracking centimeter-band
variability over decades at 14.5, 8.0, and 4.8 GHz for both targeted objects
and members of flux-limited samples. In the mid-1970s through the mid-1980s,
and during the last decade of the program, observations were additionally
obtained of circular polarization for a small sample of radio-bright (S>5Jy),
active sources. Key program results include evidence supporting class-dependent
differences in the magnetic field geometry of BL Lac and QSO jets,
identification of linear polarization changes temporally associated with flux
outbursts supporting a shock-in-jet scenario, and determination of the spectral
evolution of the Stokes V amplitude and polarity for testing proposed models.
Recent radiative transfer modeling during large flares supports a jet scenario
with a kinetically-dominated, relativistic flow at parsec scales with embedded
turbulent magnetic fields and dynamically-weak ordered components which may be
helical; the circular polarization observations are consistent with
linear-to-circular mode conversion within this turbulent jet environment.Comment: 8 pages, 4 figures, Proceedings of the conference "Polarised Emission
from Astrophysical Jets", June 12-16, 2017, Ierapetra, Greece, eds. E.
Angelakis, M. Boettcher, and J.-L. Gome
Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from Radiative Transfer Modeling
As part of a program to investigate jet flow conditions during GeV gamma-ray
flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band
total flux density and linear polarization monitoring observations to constrain
radiative transfer models incorporating propagating shocks orientated at an
arbitrary angle to the flow direction. We describe the characteristics of the
model, illustrate how the data are used to constrain the models, and present
results for three program sources with diverse characteristics: PKS 0420-01, OJ
287, and 1156+295. The modeling of the observed spectral behavior yields
information on the sense, strength and orientation of the shocks producing the
radio-band flaring; on the energy distribution of the radiating particles; and
on the observer's viewing angle with respect to the jet independent of VLBI
data. We present evidence that, while a random component dominates the jet
magnetic field, a distinguishing feature of those radio events with an
associated gamma-ray flare is the presence of a weak but non-negligible ordered
magnetic field component along the jet axis.Comment: 6 pages, 4 figures. To appear in the proceedings of "The Innermost
Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spai
The Cross-Wavelet Transform and Analysis of Quasiperiodic Behavior in the Pearson-Readhead VLBI Survey Sources
We introduce an algorithm for applying a cross-wavelet transform to analysis
of quasiperiodic variations in a time-series, and introduce significance tests
for the technique. We apply a continuous wavelet transform and the
cross-wavelet algorithm to the Pearson-Readhead VLBI survey sources using data
obtained from the University of Michigan 26-m parabloid at observing
frequencies of 14.5, 8.0, and 4.8 GHz. Thirty of the sixty-two sources were
chosen to have sufficient data for analysis, having at least 100 data points
for a given time-series. Of these thirty sources, a little more than half
exhibited evidence for quasiperiodic behavior in at least one observing
frequency, with a mean characteristic period of 2.4 yr and standard deviation
of 1.3 yr. We find that out of the thirty sources, there were about four time
scales for every ten time series, and about half of those sources showing
quasiperiodic behavior repeated the behavior in at least one other observing
frequency.Comment: Revised version, accepted by ApJ. 17 pages, 13 figures, color figures
included as gifs, seperate from the text. The addition of statistical
significance tests has resulted in modifying the technique and results, but
the broad conclusion remain the same. A high resolution version may be found
at http://www.astro.lsa.umich.edu/obs/radiotel/prcwdata.htm
Radio Band Observations of Blazar Variability
The properties of blazar variability in the radio band are studied using the
unique combination of temporal resolution from single dish monitoring and
spatial resolution from VLBA imaging; such measurements, now available in all
four Stokes parameters, together with theoretical simulations, identify the
origin of radio band variability and probe the characteristics of the radio jet
where the broadband blazar emission originates. Outbursts in total flux density
and linear polarization in the optical-to-radio bands are attributed to shocks
propagating within the jet spine, in part based on limited modeling invoking
transverse shocks; new radiative transfer simulations allowing for shocks at
arbitrary angle to the flow direction confirm this picture by reproducing the
observed centimeter-band variations observed more generally, and are of current
interest since these shocks may play a role in the gamma-ray flaring detected
by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars
identify the spectral variability properties of circular polarization for the
first time and demonstrate that polarity flips are relatively common.
All-Stokes data are consistent with the production of circular polarization by
linear-to-circular mode conversion in a region that is at least partially
self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes
VLBA observations of 3C 279 support this physical picture and are best
explained by emission from an electron-proton plasma.Comment: 6 pages, 5 figures, uses, jaa.sty. Invited talk presented at the
conference Multifrequency Variability of Blazars, Guangzhou, China, September
22-24, 2010. To appear in J. Astrophys. Ast
Quasinormal mode approach to modelling light-emission and propagation in nanoplasmonics
We describe a powerful and intuitive technique for modeling light-matter
interactions in classical and quantum nanoplasmonics. Our approach uses a
quasinormal mode expansion of the Green function within a metal nanoresonator
of arbitrary shape, together with a Dyson equation, to derive an expression for
the spontaneous decay rate and far field propagator from dipole oscillators
outside resonators. For a single quasinormal mode, at field positions outside
the quasi-static coupling regime, we give a closed form solution for the
Purcell factor and generalized effective mode volume. We augment this with an
analytic expression for the divergent LDOS very near the metal surface, which
allows us to derive a simple and highly accurate expression for the electric
field outside the metal resonator at distances from a few nanometers to
infinity. This intuitive formalism provides an enormous simplification over
full numerical calculations and fixes several pending problems in quasinormal
mode theory
A sensing platform for physiological and contextual feedback to tennis athletes
In this paper we describe our work on creating a multi-modal sensing platform for providing feedback to tennis coaches and players. The platform includes a fixed installation around a tennis court consisting of a video camera network and a localisation system as well as wearable sensing technology deployed to individual athletes. We describe the various components of this platform and explain how we can capture synchronised multi-modal sensor data streams for games or training sessions. We then describe the content-based retrieval system we are building to facilitate the development of novel coaching tools. We provide some examples of the queries that the system can support, where these queries are chosen to be suitably expressive so as to reflect a coach's complex information needs regarding tennis-related performance factors
Oblique Shocks As The Origin Of Radio To Gamma-ray Variability In AGN
The `shock in jet' model for cm-waveband blazar variability is revisited,
allowing for arbitrary shock orientation with respect to the jet flow
direction, and both random and ordered magnetic field. It is shown that oblique
shocks can explain events with swings in polarization position angle much less
than the 90 deg. associated with transverse structures, while retaining the
general characteristics of outbursts, including spectral behavior and level of
peak percentage polarization. Models dominated by a force-free, minimum energy
magnetic field configuration (essentially helical) display a shallow rise in
percentage polarization and frequency dependent swing in polarization position
angle not in agreement with the results of single-dish monitoring observations,
implying that the field is predominantly random in the quiescent state.
Outbursts well-explained by the `shock in jet' model are present during
gamma-ray flaring in several sources, supporting the idea that shock events are
responsible for activity from the radio to gamma-ray bands.Comment: 19 pages, 8 figures, accepted for publication in Ap
Environmental Drivers of Holocene Forest Development in the Middle Atlas, Morocco
In semi-arid regions subject to rising temperatures and drought, palaeoecological insights
into past vegetation dynamics under a range of boundary conditions are needed to
develop our understanding of environmental responses to climatic changes. Here, we
present a new high-resolution record of vegetation history and fire activity spanning the
last 12,000 years from Lake Sidi Ali in the southern Middle Atlas Mountains, Morocco.
The record is underpinned by a robust AMS radiocarbon and 210Pb/137Cs chronology
and multi-proxy approach allowing direct comparison of vegetation, hydroclimate, and
catchment tracers. The record reveals the persistence of steppic landscapes until
10,340 cal yr BP, prevailing sclerophyll woodland with evergreen Quercus until 6,300
cal yr BP, predominance of montane conifers (Cedrus and Cupressaceae) until 1,300
cal yr BP with matorralization and increased fire activity from 4,320 cal yr BP, and
major reduction of forest cover after 1,300 cal yr BP. Detailed comparisons between
the pollen record of Lake Sidi Ali (2,080m a.s.l.) and previously published data from
nearby Tigalmamine (1,626m a.s.l.) highlight common patterns of vegetation change in
response to Holocene climatic and anthropogenic drivers, as well as local differences
relating to elevation and bioclimate contrasts between the sites. Variability in evergreen
Quercus and Cedrus at both sites supports a Holocene summer temperature maximum
between 9,000 and 7,000 cal yr BP in contrast with previous large-scale pollen-based
climate reconstructions, and furthermore indicates pervasive millennial temperature
variability. Millennial-scale cooling episodes are inferred from Cedrus expansion around
10,200, 8,200, 6,100, 4,500, 3,000, and 1,700 cal yr BP, and during the Little Ice Age
(400 cal yr BP). A two-part trajectory of Late Holocene forest decline is evident, with
gradual decline from 4,320 cal yr BP linked to synergism between pastoralism, increased
fire and low winter rainfall, and a marked reduction from 1,300 cal yr BP, attributed to
intensification of human activity around the Early Muslim conquest of Morocco. This
trajectory, however, does not mask vegetation responses to millennial climate variability.
The findings reveal the sensitive response ofMiddle Atlas forests to rapid climate changes
and underscore the exposure of the montane forest ecosystems to future warming
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