6,126 research outputs found
Properties of optically selected BL Lac candidates from the SDSS
\textbf{Context.} Deep optical surveys open the avenue for find large numbers
of BL Lac objects that are hard to identify because they lack the unique
properties classifying them as such. While radio or X-ray surveys typically
reveal dozens of sources, recent compilations based on optical criteria alone
have increased the number of BL Lac candidates considerably. However, these
compilations are subject to biases and may contain a substantial number of
contaminating sources. \textbf{Aims.} In this paper we extend our analysis of
182 optically selected BL Lac object candidates from the SDSS with respect to
an earlier study. The main goal is to determine the number of bona fide BL Lac
objects in this sample. \textbf{Methods.} We examine their variability
characteristics, determine their broad-band radio-UV SEDs, and search for the
presence of a host galaxy. In addition we present new optical spectra for 27
targets with improved S/N with respect to the SDSS spectra. \textbf{Results.}
At least 59% of our targets have shown variability between SDSS DR2 and our
observations by more than 0.1-0.27 mag de- pending on the telescope used. A
host galaxy was detected in 36% of our targets. The host galaxy type and
luminosities are consistent with earlier studies of BL Lac host galaxies.
Simple fits to broad-band SEDS for 104 targets of our sample derived
synchrotron peak frequencies between with a peak at
. Our new optical spectra do not reveal any new
redshift for any of our objects. Thus the sample contains a large number of
bona fide BL Lac objects and seems to contain a substantial fraction of
intermediate-frequency peaked BL Lacs.Comment: Accepted for publication in A\&
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Excitonic - vibronic coupled dimers: A dynamic approach
The dynamical properties of exciton transfer coupled to polarization
vibrations in a two site system are investigated in detail. A fixed point
analysis of the full system of Bloch - oscillator equations representing the
coupled excitonic - vibronic flow is performed. For overcritical polarization a
bifurcation converting the stable bonding ground state to a hyperbolic unstable
state which is basic to the dynamical properties of the model is obtained. The
phase space of the system is generally of a mixed type: Above bifurcation chaos
develops starting from the region of the hyperbolic state and spreading with
increasing energy over the Bloch sphere leaving only islands of regular
dynamics. The behaviour of the polarization oscillator accordingly changes from
regular to chaotic.Comment: uuencoded compressed Postscript file containing text and figures. In
case of questions, please, write to [email protected]
Modified Scaling Relation for the Random-Field Ising Model
We investigate the low-temperature critical behavior of the three dimensional
random-field Ising ferromagnet. By a scaling analysis we find that in the limit
of temperature the usual scaling relations have to be modified as far
as the exponent of the specific heat is concerned. At zero
temperature, the Rushbrooke equation is modified to , an equation which we expect to be valid also for other systems with
similar critical behavior. We test the scaling theory numerically for the three
dimensional random field Ising system with Gaussian probability distribution of
the random fields by a combination of calculations of exact ground states with
an integer optimization algorithm and Monte Carlo methods. By a finite size
scaling analysis we calculate the critical exponents , , and .Comment: 4 pages, Latex, Postscript Figures include
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