On the Internal Absorption of Galaxy Clusters

A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas, possibly resulting from cooling flows. Since this amount of cold gas is not confirmed by observations at other wavelengths, we examined whether this excess absorption is present in the ROSAT PSPC observations of 20 bright galaxy clusters. For 3/4 of the clusters, successful spectral fits were obtained with absorption due only to the Galaxy, and therefore no extra absorption is needed within the clusters, in disagreement with the results from the Einstein SSS data for some of the same clusters. For 1/4 of the clusters, none of our spectral fits was acceptable, suggesting a more complicated cluster medium than the two-temperature and cooling flow models considered here. However, even for these clusters, substantial excess absorption is not indicated.Comment: accepted by the Astrophysical Journa

Compatibility of various approaches to heavy-quark fragmentation

We find that the definition of the heavy-quark fragmentation function given by Jaffe and Randall differs by a factor of the longitudinal-momentum fraction z from the standard Collins-Soper definition. Once this factor is taken into account, the explicit calculation of Braaten et al. is found to be in agreement with the general analysis of Jaffe and Randall. We also examine the model of Peterson et al. for heavy-quark fragmentation and find that the quoted values of the width and of the value of $z$ at the maximum are in error. The corrected values are in agreement with the analysis of Jaffe and Randall.Comment: 4 pages late

Two Clusters with Radio-quiet Cooling Cores

Radio lobes inflated by active galactic nuclei at the centers of clusters are a promising candidate for halting condensation in clusters with short central cooling times because they are common in such clusters. In order to test the AGN-heating hypothesis, we obtained Chandra observations of two clusters with short central cooling times yet no evidence for AGN activity: Abell 1650 and Abell 2244. The cores of these clusters indeed appear systematically different from cores with more prominent radio emission. They do not have significant central temperature gradients, and their central entropy levels are markedly higher than in clusters with stronger radio emission, corresponding to central cooling times ~ 1 Gigayear. Also, there is no evidence for fossil X-ray cavities produced by an earlier episode of AGN heating. We suggest that either (1) the central gas has not yet cooled to the point at which feedback is necessary to prevent it from condensing, possibly because it is conductively stabilized, or (2) the gas experienced a major heating event $\gtrsim 1$ Gyr in the past and has not required feedback since then. The fact that these clusters with no evident feedback have higher central entropy and therefore longer central cooling times than clusters with obvious AGN feedback strongly suggests that AGNs supply the feedback necessary to suppress condensation in clusters with short central cooling times.Comment: ApJ Letter, in pres

Can an Electric Field Induce an Antiferroelectric Phase Out of a Ferroelectric Phase?

It has been widely accepted that electric fields favor the ferroelectric phase with parallel electric dipoles over the antiferroelectric phase. With detailed measurements in polycrystalline ceramics of Pb(0.99)Nb(0.02)[(Zr(0.57)Sn(0.43))(1-y)Ti(y)](0.98)O(3), we demonstrate in this Letter that electric fields can induce an antiferroelectric phase out of a ferroelectric phase, i.e., trigger an apparently unlikely ferroelectric-to-antiferroelectric phase transition. We suggest that it is caused by the volume contraction from the converse piezoelectric effect at the coercive field with a reversed polarity.open211

Concentration phase diagram of Ba(x)Sr(1-x)TiO3 solid solutions

Method of derivation of phenomenological thermodynamic potential of solid solutions is proposed in which the interaction of the order parameters of constituents is introduced through the account of elastic strain due to misfit of the lattice parameters of the end-members. The validity of the method is demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of ferroelectric solid solution. Its phase diagram is determined using experimental data for the coefficients in the phenomenological potentials of SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for small Ba concentration, there are a tricritical point and two multiphase points one of which is associated with up to 6 possible phases.Comment: 8 pages, 3 figure

Gravitational waves from galaxy encounters

We discuss the emission of gravitational radiation produced in encounters of dark matter galactic halos. To this aim we perform a number of numerical simulations of typical galaxy mergers, computing the associated gravitational radiation waveforms as well as the energy released in the processes. Our simulations yield dimensionless gravitational wave amplitudes of the order of $10^{-13}$ and gravitational wave frequencies of the order of $10^{-16}$ Hz, when the galaxies are located at a distance of 10 Mpc. These values are of the same order as those arising in the gravitational radiation originated by strong variations of the gravitational field in the early Universe, and therefore, such gravitational waves cannot be directly observed by ground-based detectors. We discuss the feasibility of an indirect detection by means of the B-mode polarization of the Cosmic Microwave Background (CMB) induced by such waves. Our results show that the gravitational waves from encounters of dark matter galactic halos leave much too small an imprint on the CMB polarization to be actually observed with ongoing and future missions.Comment: 9 pages with revtex style, 3 ps figures; to be published in Physical Review

On the structure of the scalar mesons f0(975)f_0(975) and a0(980)a_0(980)

We investigate the structure of the scalar mesons $f_0(975)$ and $a_0(980)$ within realistic meson-exchange models of the $\pi\pi$ and $\pi\eta$ interactions. Starting from a modified version of the J\"ulich model for $\pi\pi$ scattering we perform an analysis of the pole structure of the resulting scattering amplitude and find, in contrast to existing models, a somewhat large mass for the $f_0(975)$ ($m_{f_0}=1015$ MeV, $\Gamma_{f_0}=30$ MeV). It is shown that our model provides a description of $J/\psi\rightarrow\phi\pi\pi/\phi KK$ data comparable in quality with those of alternative models. Furthermore, the formalism developed for the $\pi\pi$ system is consistently extended to the $\pi\eta$ interaction leading to a description of the $a_0(980)$ as a dynamically generated threshold effect (which is therefore neither a conventional $q\overline{q}$ state nor a $K\overline{K}$ bound state). Exploring the corresponding pole position the $a_0(980)$ is found to be rather broad ($m_{a_0}=991$ MeV, $\Gamma_{a_0}=202$ MeV). The experimentally observed smaller width results from the influence of the nearby $K\overline{K}$ threshold on this pole.Comment: 25 pages, 15 Postscript figure

Octahedral tilting, monoclinic phase and the phase diagram of PZT

Anelastic and dielectric spectroscopy measurements on PZT close to the morphotropic (MPB) and antiferroelectric boundaries provide new insight in some controversial aspects of its phase diagram. No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, in agreement with recent structural studies supporting a coexistence of the two phases over a broad composition range x < 0.5, with the fraction of M increasing toward the MPB. It is also discussed why the observed maximum of elastic compliance appears to be due to a rotational instability of the polarisation and therefore cannot be explained by extrinsic softening from finely twinned R phase alone, but indicates the presence also of M phase, not necessarily homogeneous. A new diffuse transition is found within the ferroelectric phase near x ~ 0.1, at a temperature T_IT higher than the well established boundary T_T to the phase with tilted octahedra. It is proposed that around T_IT the octahedra start rotating in a disordered manner and finally become ordered below T_T. In this interpretation, the onset temperature for octahedral tilting monotonically increases up to the antiferroelectric transition of PbZrO3, and the depression of T_T(x) below x = 0.18 would be a consequence of the partial relieve of the mismatch between the cation radii with the initial stage of tilting below T_IT.Comment: submitted to J. Phys.: Condens. Matte

Coaxial Jets and Sheaths in Wide-Angle-Tail Radio Galaxies

We add 20, 6 and 3.6 cm wavelength VLA observations of two WATs, 1231+674 and 1433+553, to existing VLA data at 6 and 20 cm, in order to study the variations of spectral index as a function of position. We apply the spectral tomography process that we introduced in our analysis of 3C67, 3C190 and 3C449. Both spectral tomography and polarization maps indicate that there are two distinct extended components in each source. As in the case of 3C449, we find that each source has a flat spectrum jet surrounded by a steeper spectrum sheath. The steep components tend to be more highly polarized than the flat components. We discuss a number of possibilities for the dynamics of the jet/sheath systems, and the evolution of their relativistic electron populations. While the exact nature of these two coaxial components is still uncertain, their existence requires new models of jets in FR I sources and may also have implications for the dichotomy between FR Is and FR IIs.Comment: 29 text pages plus 13 figures. Scheduled for publication in May 10, 1999 Ap

How to measure patent thickets – a novel approach

The existing literature identifies patent thickets indirectly. In this paper we propose a novel measure based on patent citations which allows us to measure the density of patent thickets directly. We discuss the algorithm which generates the measure and present descriptive results validating it. Moreover, we identify technology areas which are particularly impacted by patent thickets