Transient Emission From Dissipative Fronts in Magnetized, Relativistic Outflows. II. Synchrotron Flares

The time dependent synchrotron emission from relativistic jets, and the relation between the synchrotron and ERC emission is considered within the framework of the radiative front model. The timescale and profile of the optically thin emission are shown to be determined, in this model, by the shock formation radius, the thickness of expelled fluid slab and the variation of the front's parameters due to its transverse expansion. For a range of reasonable conditions, a variety of flare shapes can be produced, varying from roughly symmetric with exponential rises and decays, as often seen in blazars, to highly asymmetric with a fast rise and a much slower, power law decay, as seen in GRB afterglows. The onset, duration, and fluence of low-frequency (below the initial turnover frequency) and hard gamma-ray (above the initial gamma-spheric energy) outbursts are limited by opacity effects; the emission at these energies is quite generally delayed and, in the case of sufficiently short length outbursts, severely attenuated. The observational consequences are discussed. One distinctive prediction of this model is that in a single, powerful source, the upper cutoff of the gamma-ray spectrum should be correlated with the timescale of the outburst and with the amplitude of variations at long wavelengths (typically radio to millimeter).Comment: AAS LaTex, 14 pgs, accepted to A

The Generation of Fullerenes

We describe an efficient new algorithm for the generation of fullerenes. Our implementation of this algorithm is more than 3.5 times faster than the previously fastest generator for fullerenes -- fullgen -- and the first program since fullgen to be useful for more than 100 vertices. We also note a programming error in fullgen that caused problems for 136 or more vertices. We tabulate the numbers of fullerenes and IPR fullerenes up to 400 vertices. We also check up to 316 vertices a conjecture of Barnette that cubic planar graphs with maximum face size 6 are hamiltonian and verify that the smallest counterexample to the spiral conjecture has 380 vertices.Comment: 21 pages; added a not

Thermal X-ray emission from shocked ejecta in Type Ia Supernova Remnants. Prospects for explosion mechanism identification

The explosion mechanism behind Type Ia supernovae is a matter of continuing debate. The diverse attempts to identify or at least constrain the physical processes involved in the explosion have been only partially successful so far. In this paper we propose to use the thermal X-ray emission from young supernova remnants originated in Type Ia events to extract relevant information concerning the explosions themselves. We have produced a grid of thermonuclear supernova models representative of the paradigms currently under debate: pure deflagrations, delayed detonations, pulsating delayed detonations and sub-Chandrasekhar explosions, using their density and chemical composition profiles to simulate the interaction with the surrounding ambient medium and the ensuing plasma heating, non-equilibrium ionization and thermal X-ray emission of the ejecta. Key observational parameters such as electron temperatures, emission measures and ionization time scales are presented and discussed. We find that not only is it possible to identify the explosion mechanism from the spectra of young Type Ia Supernova Remnants, it is in fact necessary to take the detailed ejecta structure into account if such spectra are to be modeled in a self-consistent way. Neither element line flux ratios nor element emission measures are good estimates of the true ratios of ejected masses, with differences of as much as two or three orders of magnitude for a given model. Comparison with observations of the Tycho SNR suggests a delayed detonation as the most probable explosion mechanism. Line strengths, line ratios, and the centroid of the Fe Kalpha line are reasonably well reproduced by a model of this kind.Comment: 11 pages, 8 figures (5 of them color), accepted for publication by the Ap

Scattered Light in a Flood-plain Lake of Central Amazônia()

The light intensity below the surface of Lago Castanho was recorded at 10 minute intervals and compared to the total light intensity above the water surface. Scattered light of the lake water was rather constant and it was found to be sufficiently high to support the growth of photoautotrophic organisms in the near-surface layer. Below the floating plant communities scattered light was extremely low in intensity.Foram medidas, cada 10 minutos, a luz total incidindo sobre a superfície do Lago do Castanho e a luz difusa abaixo da superfície do lago. Verificou-se, nas camadas superficiais da água do lago, intensidade luminosa suficiente para assegurar o desenvolvimento de organismos foto-autotróficos, com variações mínimas durante o dia. Embaixo das massas de capim flutuante, a intensidade da luz difusa era extremamente baixa, máximo: 20 Lux

Light Environment in Tropical Rain Forest of Central Amazonia

Light intensity above the groundstoreys (120 cm) and inbetween the ground strata of a Riverine forest, a Carrasco forest and a terra firme Rain forest was recorded through two day periods of time in the rain season (April/May) and the dry season (August) of 1969. Measurements were undertaken between 6 a.m. and 18 p.m. in minute by minute intervals (51.840 readings). The relative frequency of light intensity was computed for seven intensity classes and three periods of time per day. The spectral composition of light was determined as relative frequency of light intensities for five filter ranges of wavelengths, seven intensity classes and three periods of time per day. The riverine forest (best light conditions for the undertorey plant communities) and the terra firme Rain forest (worst conditions) developed extremely oposite positions with respect to forest light climate, while the Carrasco forest showed up with a somewhat intermediate character, but with a strong shift to terra firme Rain forest conditions. The spectral composition of light received by the ground strata of all three forest stands was as follows1) light intensities peaked in the RG 630 filter range of wavelenths (5.920 ​​​Ǻ - 7.500 ​​​Ǻ), 2) a secondary intensity peak covered the VG 9 filter (4.420 ​​​Ǻ - 6.440 ​​​Ǻ) and 3) a less important secondary peak matched the BG 12 filter band (3.500 ​​​Ǻ - 5.150 ​​​Ǻ). Inbetween the understorey communities, spectral light intensities were pretty low and showed another considerable red-shift.O Autor apresenta um estudo comparativo do fator "luz" em três tipos de floresta na região central da Amazônia: floresta ribeirinha (floresta da baixa terra firme), carrasco e mata primária. São discutidas as dificuldades e a complexidade do problema enfatizando-se a importância das correlações entre luz e estrutura da floresta. É caracterizado o equipamento utilizado e descrito o procedimento seguido para a determinação da intensidade luminosa total, utilizando-se diferentes filtros em 51.840 determinações durante a estação chuvosa e a estação sêca, procurando verificar a intensidade luminosa total e espectral em cada stratum e como são aquelas intensidades distribuidas no tempo. A frequência relativa da intensidade luminosa foi computada em sete classes de intensidade e três períodos de tempo por dia. A composição espectral da luz foi determinada como frequência relativa de intensidades por meio de filtros para cinco faixas de comprimento de ondas. Na floresta ribeirinha foram encontradas as melhores condições de luz e na mata primária foram registrados os valores luminosos mais baixos. O carrasco ocupa uma posição intermediária, com nítida tendência às condições encontradas na mata primária. Quanto à composição espectral da luz recebida pelo stratum ao nível do solo nos três tipos de floresta, foi encontrado um pico de intensidade na faixa 5.950 ​​​Ǻ - 7.500 ​​​Ǻ, um pico secundário entre 4.420 e 6.440 ​​​Ǻ e um terceiro pico, menos importante, entre 3.500 a 5.150 ​​​

XMM-Newton Observations of High Redshift Quasars

We report on our XMM observations of the high redshift quasars BR 2237--0607 (z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found in the XMM public archive. Contrary to former reports, we do not find high redshift radio-loud quasars to be more absorbed than their radio-quiet counterparts. We find that the optical to X-ray spectral index alpha-ox is correlated with the luminosity density at 2500 A, but does not show a correlation with redshift. The mean 2-10 keV power-law slope of the 9 high redshift radio-quiet quasars in our sample for which a spectral analysis can be performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and significantly different from alpha-x=0.78 found for low redshift Broad-Line Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars look remarkably similar, we find a first indication of a difference in their X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios L/L_Edd. These observations give credence to the hypothesis of Mathur (2000) that NLS1s are low luminosity cousins of high redshift quasars, both likely to be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006

Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below T$_{SP}$=14 K a singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to 227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in Physica B (1996

X-ray Observations of Gravitationally Lensed Quasars; Evidence for a Hidden Quasar Population

The large magnification factors of gravitationally lensed (GL) systems allow us to investigate the properties of quasars with X-ray luminosities that are substantially lower than those of unlensed ones and also provide an independent means of estimating the contribution of faint quasars to the hard X-ray component of the cosmic X-ray background. Our spectral analysis indicate a flattening of the spectral index in the hard band (4-20keV restframe) for 2 radio-loud quasars in the GL quasar sample for which the data have moderate signal-to-noise ratio. We have identified a large fraction of Broad Absorption Line (BAL) quasars amongst the GL quasar population. We find that approximately 35% of radio-quiet GL quasars contain BAL features which is significantly larger than the 10% fraction of BAL quasars presently found in optically selected flux limited quasar samples. We present a simple model that estimates the effects of attenuation and lens magnification on the luminosity function of quasars and that explains the observed fraction of GL BAL quasars. These observations suggest that a large fraction of BAL quasars are missed from flux limited optical surveys. Modeling of several X-ray observations of the GL BAL quasar PG1115+080 suggests that the observed large X-ray variability may be caused in part by a variable intrinsic absorber consistent with previously observed variability of the BAL troughs in the UV band. The observed large X-ray flux variations in PG1115+080 offer the prospect of considerably reducing errors in determining the time delay with future X-ray monitoring of this system and hence constraining the Hubble constant H$_{0}$.Comment: 19 pages, 10 figures, 9 Tables, accepted for publication in Ap

Generalized Symmetries of Impulsive Gravitational Waves

We generalize previous \cite{AiBa2} work on the classification of ($C^\infty$) symmetries of plane-fronted waves with an impulsive profile. Due to the specific form of the profile it is possible to extend the group of normal-form-preserving diffeomorphisms to include non-smooth transformations. This extension entails a richer structure of the symmetry algebra generated by the (non-smooth) Killing vectors.Comment: 18 pages, latex2e, no figure

Non-singular radiation cosmological models

In this paper we analyse the possibility of constructing singularity-free inhomogeneous cosmological models with a pure radiation field as matter content. It is shown that the conditions for regularity are very easy to implement and therefore there is a huge number of such spacetimes.Comment: 13 pages, LaTex, ws-mpla, to appear in Modern Physics Letters