41 research outputs found
Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.Comment: to be published in A&
Parsec-Scale Blazar Monitoring: Flux and Polarization Variability
We present analysis of the flux and polarization variability of parsec scale
radio jets from a dual-frequency, six-epoch, VLBA polarization experiment
monitoring 12 blazars. The observations were made at 15 and 22 GHz at bimonthly
intervals over 1996. Here we analyze the flux, fractional polarization, and
polarization position angle behavior of core regions and jet features,
considering both the linear trends of these quantities with time and more rapid
fluctuations about the linear trends. The dual frequency nature of the
observations allows us to examine spectral evolution, to separate Faraday
effects from changes in magnetic field order, and also to deduce empirical
estimates for the uncertainties in measuring properties of VLBI jet features
(abridged).Comment: 35 pages, 30 figures, accepted by the Astrophysical Journal (Changes
from original version: typos corrected and a clarification in terminology
Disentangling the synchrotron and Inverse Compton variability in the X-ray emission of the intermediate BL Lac object S5 0716+71
The possibility to detect simultaneously in the X-ray band the synchrotron
and Inverse Compton (IC)emission of intermediate BL Lac objects offers the
unique opportunity to study contemporaneously the low- and high-energy tails of
the electron distribution in the jets of these sources. We attempted to
disentangle the X-ray spectral variability properties of both the low- and
high-energy ends of the synchrotron and Inverse Compton emission of the
intermediate BL Lac object S5 0716+71. We carried out spectral, temporal and
cross-correlation analyses of the data from a long XMM-Newton pointing of S5
0716+71 and we compared our findings with previous results from past X-ray
observations. Strong variability was detected during the XMM exposure.Both the
synchrotron and Inverse Compton components were found to vary on time scales of
hours, implying a size of the emitting region of R\la 0.7\delta /(1+z)
light-hours. The synchrotron emission was discovered to become dominant during
episodes of flaring activity, following a harder-when-brighter trend. Tight
correlations were observed between variations in different energy bands. Upper
limits on time lags between the soft and hard X-ray light curves are of the
order of a few hundred seconds.Comment: 14 pages, 9 figures, 5 tables, A&A in press, minor corrections to
match the version to be publishe
CMB Telescopes and Optical Systems
The cosmic microwave background radiation (CMB) is now firmly established as
a fundamental and essential probe of the geometry, constituents, and birth of
the Universe. The CMB is a potent observable because it can be measured with
precision and accuracy. Just as importantly, theoretical models of the Universe
can predict the characteristics of the CMB to high accuracy, and those
predictions can be directly compared to observations. There are multiple
aspects associated with making a precise measurement. In this review, we focus
on optical components for the instrumentation used to measure the CMB
polarization and temperature anisotropy. We begin with an overview of general
considerations for CMB observations and discuss common concepts used in the
community. We next consider a variety of alternatives available for a designer
of a CMB telescope. Our discussion is guided by the ground and balloon-based
instruments that have been implemented over the years. In the same vein, we
compare the arc-minute resolution Atacama Cosmology Telescope (ACT) and the
South Pole Telescope (SPT). CMB interferometers are presented briefly. We
conclude with a comparison of the four CMB satellites, Relikt, COBE, WMAP, and
Planck, to demonstrate a remarkable evolution in design, sensitivity,
resolution, and complexity over the past thirty years.Comment: To appear in: Planets, Stars and Stellar Systems (PSSS), Volume 1:
Telescopes and Instrumentatio
Causal Loop Analysis of coastal geomorphological systems
As geomorphologists embrace ever more sophisticated theoretical frameworks that shift from simple notions of evolution towards single steady equilibria to recognise the possibility of multiple response pathways and outcomes, morphodynamic modellers are facing the problem of how to keep track of an ever-greater number of system feedbacks. Within coastal geomorphology, capturing these feedbacks is critically important, especially as the focus of activity shifts from reductionist models founded on sediment transport fundamentals to more synthesist ones intended to resolve emergent behaviours at decadal to centennial scales. This paper addresses the challenge of mapping the feedback structure of processes controlling geomorphic system behaviour with reference to illustrative applications of Causal Loop Analysis at two study cases: (1) the erosionâaccretion behaviour of graded (mixed) sediment beds, and (2) the local alongshore sediment fluxes of sand-rich shorelines. These case study examples are chosen on account of their central role in the quantitative modelling of geomorphological futures and as they illustrate different types of causation. Causal loop diagrams, a form of directed graph, are used to distil the feedback structure to reveal, in advance of more quantitative modelling, multi-response pathways and multiple outcomes. In the case of graded sediment bed, up to three different outcomes (no response, and two disequilibrium states) can be derived from a simple qualitative stability analysis. For the sand-rich local shoreline behaviour case, two fundamentally different responses of the shoreline (diffusive and anti-diffusive), triggered by small changes of the shoreline cross-shore position, can be inferred purely through analysis of the causal pathways. Explicit depiction of feedback-structure diagrams is beneficial when developing numerical models to explore coastal morphological futures. By explicitly mapping the feedbacks included and neglected within a model, the modeller can readily assess if critical feedback loops are included
2nd Highly Polarized Radio Outburst In Cygnus-x-3
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62557/1/262669a0.pd