325 research outputs found
Stochastic model of optical variability of BL Lacertae
We use optical photometric and polarimetric data of BL Lacertae that cover a
period of 22 years to study the variability of the source. The long-term
observations are employed for establishing parameters of a stochastic model
consisting of the radiation from a steady polarized source and a number of
variable components with different polarization parameters, proposed by
Hagen-Thorn et al. earlier. We infer parameters of the model from the
observations using numerical simulations based on a Monte Carlo method, with
values of each model parameter selected from a Gaussian distribution. We
determine the best set of model parameters by comparing model distributions to
the observational ones using the chi-square criterion. We show that the
observed photometric and polarimetric variability can be explained within a
model with a steady source of high polarization, ~40%, and with direction of
polarization parallel to the parsec scale jet, along with 10+-5 sources of
variable polarization.Comment: 4 pages, 10 figures, published by Astronomy and Astrophysics; v2:
typos correcte
The Outburst of the Blazar AO 0235+164 in 2006 December: Shock-in-Jet Interpretation
We present the results of polarimetric ( band) and multicolor photometric
() observations of the blazar AO 0235+16 during an outburst in 2006
December. The data reveal a short timescale of variability (several hours),
which increases from optical to near-IR wavelengths; even shorter variations
are detected in polarization. The flux density correlates with the degree of
polarization, and at maximum degree of polarization the electric vector tends
to align with the parsec-scale jet direction. We find that a variable component
with a steady power-law spectral energy distribution and very high optical
polarization (30-50%) is responsible for the variability. We interpret these
properties of the blazar withina model of a transverse shock propagating down
the jet. In this case a small change in the viewing angle of the jet, by
, and a decrease in the shocked plasma compression by a factor of
1.5 are sufficient to account for the variability.Comment: 22 pages, 8 figures, accepted for Ap
Color Variability of the Blazar AO 0235+16
Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The
light curves were compiled at the Turin Observatory from literature data and
the results of observations obtained in the framework of the WEBT program
(http://www.to.astro/blazars/webt/). The color variability of the blazar was
studied in eight time intervals with a sufficient number of multicolor optical
observations; JHK data are available for only one of these. The spectral energy
distribution (SED) of the variable component remained constant within each
interval, but varied strongly from one interval to another. After correction
for dust absorption, the SED can be represented by a power law in all cases,
providing evidence for a synchrotron nature of the variable component. We show
that the variability at both optical and IR wavelengths is associated with the
same variable source.Comment: 11 pages, 9 figures, 4 tables, accepted for publication in Astronomy
Report
Perturbative Renormalization in Quantum Mechanics
Some quantum mechanical potentials, singular at short distances, lead to
ultraviolet divergences when used in perturbation theory. Exactly as in quantum
field theories, but much simpler, regularization and renormalization lead to
finite physical results, which compare correctly to the exact ones. The Dirac
delta potential, because of its relevance to triviality, and the Aharonov-Bohm
potential, because ot its relevance to anyons, are used as examples here.Comment: LATEX, 13 pages, UB-ECM-PF 19-9
Macronutrients in tree stems and foliage: a comparative study of six temperate forest species planted at the same sites
Common European tree species ( oak, ash, beech, birch, lime and spruce) planted in adjacent stands on six sites were compared in terms of macronutrient concentrations in foliar and stem wood ( including bark) biomass. The nutrient concentrations in both biomass compartments were much more dependent on species than on site although soil conditions differed between the sites. Differences between species regarding stem wood nutrient concentrations only partly corresponded to the differences in species foliage. The concentrations in spruce were considerably lower than in deciduous species, except P in foliage, and Ca in both stem wood and foliar biomass. Differences were also observed between the deciduous species both regarding foliar and stem wood nutrient concentrations. The differences should be considered when modelling nutrient circulation in forest stands and when evaluating the long-term sustainability of forest management
The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A
We present the results of stellar photometry of polar-ring galaxies NGC 2685
and NGC 4650A, using the archival data obtained with the Hubble Space
Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were
resolved into ~800 and ~430 stellar objects in the B, V and Ic bands,
considerable part of which are blue supergiants located in the young stellar
complexes. The stellar features in the CM-diagrams are best represented by
isochrones with metallicity Z = 0.008. The process of star formation in the
polar rings of both galaxies was continuous and the age of the youngest
detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Comment: 21 pages, 9 figures, AJ 2004 February, accepte
Month-Timescale Optical Variability in the M87 Jet
A previously inconspicuous knot in the M87 jet has undergone a dramatic
outburst and now exceeds the nucleus in optical and X-ray luminosity.
Monitoring of M87 with the Hubble Space Telescope and Chandra X-ray Observatory
during 2002-2003, has found month-timescale optical variability in both the
nucleus and HST-1, a knot in the jet 0.82'' from the nucleus. We discuss the
behavior of the variability timescales as well as spectral energy distribution
of both components. In the nucleus, we see nearly energy-independent
variability behavior. Knot HST-1, however, displays weak energy dependence in
both X-ray and optical bands, but with nearly comparable rise/decay timescales
at 220 nm and 0.5 keV. The flaring region of HST-1 appears stationary over
eight months of monitoring. We consider various emission models to explain the
variability of both components. The flares we see are similar to those seen in
blazars, albeit on longer timescales, and so could, if viewed at smaller
angles, explain the extreme variability properties of those objects.Comment: 4 pages, 3 figures, ApJ Lett., in pres
The nature of optical and near-infrared variability of BL Lacertae
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the analysis of this huge dataset has been the study of
colour variability. The massive amount of optical and near-infrared data
collected during the campaigns enables us to perform a deep analysis of
multiband data, with the aim of understanding the flux variability mechanisms.
We use a new approach for the analysis of these data, focusing on the source
spectral evolution. We show that the overall behaviour of the BL Lacertae light
and colour curves can be explained in terms of changing viewing angle of a
moving, discrete emitting region, which causes variable Doppler boosting of the
corresponding radiation. A fractal helical structure is suggested to be at the
origin of the different time scales of variability.Comment: 6 pages, 8 figures, accepted by A&
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