4,509 research outputs found
The Classification of Extragalactic X-ray Jets
The overall classification of X-ray jets has clung to that prevalent in the
radio: FRI vs. FRII (including quasars). Indeed, the common perception is that
X-ray emission from FRI's is synchrotron emission whereas that from FRII's may
be IC/CMB and/or synchrotron. Now that we have a sizable collection of sources
with detected X-ray emission from jets and hotspots, it seems that a more
unbiased study of these objects could yield additional insights on jets and
their X-ray emission. The current contribution is a first step in the process
of analyzing all of the relevant parameters for each detected component for the
sources collected in the XJET website. This initial effort involves measuring
the ratio of X-ray to radio fluxes and evaluating correlations with other jet
parameters. For single zone synchrotron X-ray emission, we anticipate that
larger values of fx/fr should correlate inversely with the average magnetic
field strength (if the acceleration process is limited by loss time equals
acceleration time). Beamed IC/CMB X-rays should produce larger values of fx/fr
for smaller values of the angle between the jet direction and the line of sight
but will also be affected by the low frequency radio spectral index.Comment: 4 pages; to appear in the conference proceedings: "X-Ray Astronomy
2009: Present Status, Multiwavelength Approach and Future Perspectives";
Bologna, Italy, September 2009, Editors: A. Comastri, M. Cappi, L. Angelini,
2010 AIP (in press
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Spectral Hardening of Large Solar Flares
RHESSI observations are used to quantitatively study the hard X-ray evolution
in 5 large solar flares selected for spectral hardening in the course of the
event. The X-ray bremsstrahlung emission from non-thermal electrons is
characterized by two spectroscopically distinct phases: impulsive and gradual.
The impulsive phase usually consists of several emission spikes following a
soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as
spectral hardening while the flux slowly decreases. Both the soft-hard-soft
(impulsive) phase and the hardening (gradual) phase are well described by
piecewise linear dependence of the photon spectral index on the logarithm of
the hard X-ray flux. The different linear parts of this relation correspond to
different rise and decay phases of emission spikes. The temporal evolution of
the spectra is compared with the configuration and motion of the hard X-ray
sources in RHESSI images. These observations reveal that the two stages of
electron acceleration causing these two different behaviors are closely related
in space and time. The transition between the impulsive and gradual phase is
found to be smooth and progressive rather than abrupt. This suggests that they
arise because of a slow change in a common accelerator rather than being caused
by two independent and distinct acceleration processes. We propose that the
hardening during the decay phase is caused by continuing particle acceleration
with longer trapping in the accelerator before escape.Comment: accepted by Ap
The Emerging Federal Class Actions Brand
Class actions raise a fundamental question about our judicial system. Is the purpose first and foremost to achieve a societally-desired level of law enforcement and deterrence, or is the primary goal to foster citizen participation in the resolution of private disputes? This Article provides the first extended analysis of this question in light of five recent Supreme Court decisions regarding class actions, the evolution of legislative initiatives in the area, and the docket activity in sixteen of the largest recent federal securities class actions. A single conclusion follows: we are witnessing the emergence of a new national âbrandâ of class action litigation grounded in the protection and growth of individual participatory rights
The Emerging Federal Class Actions Brand
Class actions raise a fundamental question about our judicial system. Is the purpose first and foremost to achieve a societally-desired level of law enforcement and deterrence, or is the primary goal to foster citizen participation in the resolution of private disputes? This Article provides the first extended analysis of this question in light of five recent Supreme Court decisions regarding class actions, the evolution of legislative initiatives in the area, and the docket activity in sixteen of the largest recent federal securities class actions. A single conclusion follows: we are witnessing the emergence of a new national âbrandâ of class action litigation grounded in the protection and growth of individual participatory rights
CLaSPS: a new methodology for Knowledge extraction from complex astronomical dataset
In this paper we present the Clustering-Labels-Score Patterns Spotter
(CLaSPS), a new methodology for the determination of correlations among
astronomical observables in complex datasets, based on the application of
distinct unsupervised clustering techniques. The novelty in CLaSPS is the
criterion used for the selection of the optimal clusterings, based on a
quantitative measure of the degree of correlation between the cluster
memberships and the distribution of a set of observables, the labels, not
employed for the clustering. In this paper we discuss the applications of
CLaSPS to two simple astronomical datasets, both composed of extragalactic
sources with photometric observations at different wavelengths from large area
surveys. The first dataset, CSC+, is composed of optical quasars
spectroscopically selected in the SDSS data, observed in the X-rays by Chandra
and with multi-wavelength observations in the near-infrared, optical and
ultraviolet spectral intervals. One of the results of the application of CLaSPS
to the CSC+ is the re-identification of a well-known correlation between the
alphaOX parameter and the near ultraviolet color, in a subset of CSC+ sources
with relatively small values of the near-ultraviolet colors. The other dataset
consists of a sample of blazars for which photometric observations in the
optical, mid and near infrared are available, complemented for a subset of the
sources, by Fermi gamma-ray data. The main results of the application of CLaSPS
to such datasets have been the discovery of a strong correlation between the
multi-wavelength color distribution of blazars and their optical spectral
classification in BL Lacs and Flat Spectrum Radio Quasars and a peculiar
pattern followed by blazars in the WISE mid-infrared colors space. This pattern
and its physical interpretation have been discussed in details in other papers
by one of the authors.Comment: 18 pages, 9 figures, accepted for publication in Ap
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
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