Search for isotropic γ radiation in the cosmological window between 65 and 200 TeV

Electromagnetic energy injected into the universe above a few hundred TeV is expected to pile up as γ radiation in a relatively narrow energy interval below 100 TeV due to its interaction with the 2.7^°K background radiation. We present an upper limit (90% C.L.) on the ratio of primary γ to charged cosmic rays in the energy interval 65–160 TeV (80–200 TeV) of 10.3 • 10^−3 (7.8 • 10^−3). Data from the HEGRA cosmic-ray detector complex consisting of a wide angle Čerenkov array (AIROBICC) measuring the lateral distribution of air Čerenkov light and a scintillator array, were used with a novel method to discriminate γ-ray and hadron induced air showers. If the presently unmeasured universal far infrared background radiation is not too intense, the result rules out a topological-defect origin of ultrahigh energy cosmic rays for masses of the X particle released by the defects equal to or larger than about 10^16 GeV.

Search for very high energy gamma radiation from the radio bright region DR4 of the SNR G78.2+2.1

Data from the HEGRA air shower array are used to set an upper limit on the emission of gamma-radiation above 25 (18) TeV from the direction of the radio bright region DR4 within the SNR G78.2 + 2.1 of 2.5 (7.1). 10^-13 cm^-2 sec^-1. The shock front of SNR G78.2 + 2.1 probably recently overtook the molecular cloud Gong 8 which then acts as a target for the cosmic rays produced within the SNR, thus leading to the expectation of enhanced gamma-radiation. Using a model of Drury, Aharonian and Völk which assumes that SNRs are the sources of galactic cosmic rays via first order Fermi acceleration, we calculated a theoretical prediction for the gamma-ray flux from the DR4 region and compared it with our experimental flux limit. Our 'best estimate' value for the predicted flux lies a factor of about 18 above the upper limit for gamma-ray energies above 25 TeV. Possible reasons for this discrepancy are discussed.

Psychometric functions for detection and discrimination with and without flankers.

Recent studies have reported that flanking stimuli broaden the psychometric function and lower detection thresholds. In the present study, we measured psychometric functions for detection and discrimination with and without flankers to investigate whether these effects occur throughout the contrast continuum. Our results confirm that lower detection thresholds with flankers are accompanied by broader psychometric functions. Psychometric functions for discrimination reveal that discrimination thresholds with and without flankers are similar across standard levels, and that the broadening of psychometric functions with flankers disappears as standard contrast increases, to the point that psychometric functions at high standard levels are virtually identical with or without flankers. Threshold-versus-contrast (TvC) curves with flankers only differ from TvC curves without flankers in occasional shallower dippers and lower branches on the left of the dipper, but they run virtually superimposed at high standard levels. We discuss differences between our results and other results in the literature, and how they are likely attributed to the differential vulnerability of alternative psychophysical procedures to the effects of presentation order. We show that different models of flanker facilitation can fit the data equally well, which stresses that succeeding at fitting a model does not validate it in any sense.

Reminder and 2AFC tasks provide similar estimates of the difference limen: a reanalysis of data from Lapid, Ulrich, and Rammsayer (2008) and a discussion of Ulrich and Vorberg (2009)

Lapid, Ulrich, and Rammsayer (2008) reported that estimates of the difference limen (DL) from a two-alternative forced choice (2AFC) task are higher than those obtained from a reminder task. This article reanalyzes their data in order to correct an error in their estimates of the DL from 2AFC data. We also extend the psychometric functions fitted to data from both tasks to incorporate an extra parameter that has been shown to allow obtaining accurate estimates of the DL that are unaffected by lapses. Contrary to Lapid et al.'s conclusion, our reanalysis shows that DLs estimated with the 2AFC task are only minimally (and not always significantly) larger than those estimated with the reminder task. We also show that their data are contaminated by response bias, and that the small remaining difference between DLs estimated with 2AFC and reminder tasks can be reasonably attributed to the differential effects that response bias has in either task as they were defined in Lapid et al.'s experiments. Finally, we discuss a novel approach presented by Ulrich and Vorberg (2009) for fitting psychometric functions to 2AFC discrimination data.

A comparison of fixed-step-size and Bayesian staircases for sensory threshold estimation

Fixed-step-size (FSS) and Bayesian staircases are widely used methods to estimate sensory thresholds in 2AFC tasks, although a direct comparison of both types of procedure under identical conditions has not previously been reported. A simulation study and an empirical test were conducted to compare the performance of optimized Bayesian staircases with that of four optimized variants of FSS staircase differing as to up-down rule. The ultimate goal was to determine whether FSS or Bayesian staircases are the best choice in experimental psychophysics. The comparison considered the properties of the estimates (i.e. bias and standard errors) in relation to their cost (i.e. the number of trials to completion). The simulation study showed that mean estimates of Bayesian and FSS staircases are dependable when sufficient trials are given and that, in both cases, the standard deviation (SD) of the estimates decreases with number of trials, although the SD of Bayesian estimates is always lower than that of FSS estimates (and thus, Bayesian staircases are more efficient). The empirical test did not support these conclusions, as (1) neither procedure rendered estimates converging on some value, (2) standard deviations did not follow the expected pattern of decrease with number of trials, and (3) both procedures appeared to be equally efficient. Potential factors explaining the discrepancies between simulation and empirical results are commented upon and, all things considered, a sensible recommendation is for psychophysicists to run no fewer than 18 and no more than 30 reversals of an FSS staircase implementing the 1-up/3-down rule.

Stopping rules in Bayesian adaptive threshold estimation

Threshold estimation with sequential procedures is justifiable on the surmise that the index used in the so-called dynamic stopping rule has diagnostic value for identifying when an accurate estimate has been obtained. The performance of five types of Bayesian sequential procedure was compared here to that of an analogous fixed-length procedure. Indices for use in sequential procedures were: (1) the width of the Bayesian probability interval, (2) the posterior standard deviation, (3) the absolute change, (4) the average change, and (5) the number of sign fluctuations. A simulation study was carried out to evaluate which index renders estimates with less bias and smaller standard error at lower cost (i.e. lower average number of trials to completion), in both yes–no and two-alternative forced-choice (2AFC) tasks. We also considered the effect of the form and parameters of the psychometric function and its similarity with themodel function assumed in the procedure. Our results show that sequential procedures do not outperform fixed-length procedures in yes–no tasks. However, in 2AFC tasks, sequential procedures not based on sign fluctuations all yield minimally better estimates than fixed-length procedures, although most of the improvement occurs with short runs that render undependable estimates and the differences vanish when the procedures run for a number of trials (around 70) that ensures dependability. Thus, none of the indices considered here (some of which are widespread) has the diagnostic value that would justify its use. In addition, difficulties of implementation make sequential procedures unfit as alternatives to fixed-length procedures.

CETA (Tratado de Libre Comercio Canadá - Unión Europea) y Contratación Pública

Artículo que analiza el impacto que va a tener el Tratado de Libre Comercio entre Canadá y la Unión Europea en la contratación pública

Visibilidad pública y tratamiento informativo del movimiento de cooperación al desarrollo (1992-2002)

En España los años noventa ven aparecer el primer gran movimiento social vinculado a la problemática de la solidaridad y el desarrollo internacional, impulsado por tres grandes campañas: la inicial (1994) está centrada principalmente en la reclamación simbólica de la cesión de 0,7 % del PIB para cooperación internacional con el objetivo de dotar de un mayor control sobre aquellos instrumentos de cooperación vinculados al comercio exterior; y la tercera (2000) impulsa una consulta social sobre la deuda externa que se desarrolla de manera paralela a las elecciones generales ese año. Esta creciente politización de los objetivo s del movimiento recibe un tratamiento informativo muy diferenciado en estos tres momentos, poniendo de manifiesto la relación entre la estructura de oportunidad mediática y las alianzas político mediática, entre, en este caso, prensa escrita y partidos (en su posición relativa de gobierno y oposición). El análisis de esta relación entre partidos y medios da cuenta de las profundas transformaciones que está viviendo la esfera pública en las coordenadas de la sociedad de la información y como pesa sobre el espacio público en el que trabaja la sociedad civil en las sociedades democráticas.