428 research outputs found
Gamma-ray burst engines may have no memory
No full textContext. A sizeable fraction of gamma-ray burst (GRB) time profiles consist of a temporal sequence of pulses. The nature of this stochastic process carries information on how GRB inner engines work. The so-called interpulse time defines the interval between adjacent pulses, excluding the long quiescence periods during which the signal drops to the background level. It was found by many authors in the past that interpulse times are lognormally distributed, at variance with the exponential case that is expected for a memoryless process.
Aims. We investigated whether the simple hypothesis of a temporally uncorrelated sequence of pulses is really to be rejected, as a lognormal distribution necessarily implies.
Methods. We selected and analysed a number of multi-peaked CGRO/BATSE GRBs and simulated similar time profiles, with the crucial difference that we assumed exponentially distributed interpulse times, as is expected for a memoryless stationary Poisson process. We then identified peaks in both data sets using a novel peak search algorithm, which is more efficient than others used in the past.
Results. We independently confirmed that the observed interpulse time distribution is approximately lognormal. However, we found the same results on the simulated profiles, in spite of the intrinsic exponential distribution. Although intrinsic lognormality cannot be ruled out, this shows that intrinsic interpulse time distribution in real data could still be exponential, while the observed lognormal could be ascribed to the low efficiency of peak search algorithms at short values combined with the limitations of a bin-integrated profile.
Conclusions. Our result suggests that GRB engines may emit pulses after the fashion of nuclear radioactive decay, that is, as a memoryless process
The slope of the gamma-ray burst variability/peak luminosity correlation
No full textFirst using a sample of 32 gamma-ray bursts (GRBs) with known redshift and then a sample of 551 BATSE GRBs with derived pseudo-redshift, the time variability/peak luminosity correlation (V versus L), originally found by Reichart et al. using a sample of 18 GRBs, was tested. For both samples the correlation is still found is but less relevant because of a much higher spread of the data. Assuming a straight line in the logL-logV plane (logL = m logV + b), both Guidorzi et al. and Guidorzi found that the line slope for both samples is much lower than that derived by Reichart et al.: m = 1.3+0.8-0.4 and 0.85 +/- 0.02, to be compared with m = 3.3+1.1-0.9 by Reichart et al. Reichart & Nysewander discuss our results and attribute the different slope to the fact that we do not take into account the variance of the sample in the fit (also called the slop), and demonstrate that, using the method presented by Reichart, the expanded data set in the logL-logV plane is still well described by a line with slope m = 3.4+0.9-0.6. Here we compare the results of two methods accounting for the slope of the sample: the method implemented by Reichart and that by D'Agostini. We demonstrate that the first method provides an inconsistent estimate of the parameter when the sample variance is comparable to the interval of values covered by the GRB variability. We also demonstrate that, using the second method, the slope of the logL-logV correlation is still consistent with that derived by us earlier and inconsistent with that derived by Reichart & Nysewander. Finally we discuss the implications on the interpretations proposed for the V-L correlation and show that our results are in agreement with the peak energy/variability correlation found by Lloyd-Ronning & Ramirez-Ruiz and the peak energy/peak luminosity correlation found by Yonetoku et al. and Ghirlanda et al
Optimizing the exponential sine sweep (ESS) signal for in situ measurements on noise barriers
No full textThe measurement of sound reflection and airborne sound insulation of noise barriers in based on the acquisition of impulse responses by means of digital devices. The most widely used methods employ MLS (Maximum Length Sequence) or ESS (Exponential Sine Sweep) as test signals. The theory behind MLS generation and use is well developed and does not involve computational problems. That is why in CEN/TS 1793-5:2003 an MLS signal is recommended. During the European project QUIESST also an ESS signal was applied. The ESS signal has some advantages over MLS, such as a better signal to noise ratio (SNR) and a robust non-linearity rejection. Anyway, the generation of an ESS signal and the subsequent analysis of impulse responses involve some problems whose solutions are not yet common practice. These solutions are discussed here and practical application examples are presented
Necessary conditions and non-existence results for autonomous nonconvex variational problems
No full textAbstractClassical one-dimensional, autonomous Lagrange problems are considered. In absence of any smoothness, convexity or coercivity condition on the energy density, we prove a DuBois-Reymond type necessary condition, expressed as a differential inclusion involving the subdifferential of convex analysis. As a consequence, a non-existence result is obtained
Metodo per il calcolo di una posizione e l’eventuale mappatura di una variabile spazio-correlata tramite impiego di segnali acustici e corrispondente apparato per l’implementazione del metodo
No full textMetodo per il calcolo di una posizione e l’eventuale mappatura di una variabile spazio-correlata tramite impiego di segnali acustici comprendente le seguenti fasi: predisporre in corrispondenza di uno spazio tridimensionale di misura almeno tre emettitori acustici di posizione (A, B, C, D) con posizione relativa nota e almeno un ricevitore acustico (R) posto ad una distanza (D1, D2, D3, D4) non nota da ciascuno degli emettitori di posizione (A, B, C, D); far emettere a ciascun emettitore di posizione (A, B, C, D) un segnale acustico di posizione (X1, X2, X3, X4); acquisire, per mezzo del ricevitore acustico (R), un segnale acustico ricevuto (J) comprendente contributi legati ai singoli segnali acustici di posizione (X1, X2, X3, X4); ricavare un segnale identificativo di posizione (H1, H2, H3, H4) per ogni segnale acustico di posizione (X1, X2, X3, X4); ricavare, dai segnali identificativi di posizione (H1, H2, H3, H4), il tempo di volo (T1, T2, T3, T4); calcolare le distanze (D1, D2, D3, D4) a partire dai tempi di volo (T1, T2, T3, T4); calcolare la posizione (P) del ricevitore acustico (R) nello spazio tridimensionale di misura a partire da dette distanze (D1, D2, D3, D4)
Existence of Minimizers of Free Autonomous Variational Problems Via Solvability of Constrained Ones
No full textWe consider the following autonomous variational problem:
minimize {\int_a^b f(v(x), v′(x))dx: v ∈ W^{1,1}(a, b), v(a) = α, v(b) = β} where the Lagrangian f is assumed to be continuous, but not necessarily coercive, nor convex. We show that the existence of the minimum is linked to the solvability of certain constrained variational problems. This allows us to derive existence theorems covering a wide class of nonconvex noncoercive problems
Gamma-Ray Bursts in the Era of Rapid Followup
No full textWe present a status report on the study of gamma-ray bursts (GRBs) in the era
of rapid followup using the world's largest robotic optical telescopes—the 2 m Liverpool
and Faulkes telescopes. Within the context of key unsolved issues in GRB
physics, we describe (1) our innovative software that allows real-time automatic
analysis and interpretation of GRB light curves, (2) the novel instrumentation that
allows unique types of observations (in particular, early time polarisation measurements),
and (3) the key science questions and discoveries to which robotic observations
are ideally suited, concluding with a summary of current understanding of GRB
physics provided by combining rapid optical observations with simultaneous observations
at other wavelengths
Clonidine Inhibition of Norepinephrine Release from Normal and Morphine‐Tolerant Guinea Pig Cortical Slices
No full textEndogenous norepinephrine (NE) release in cerebral cortex slices taken from normal and morphine-tolerant guinea pigs was measured by HPLC. In normal slices, a linear relationship was found between electrically evoked NE release and the log of the frequency of stimulation in the range of 1-20 Hz. The efficiency of the alpha 2-mediated autofeedback was tested by adding the alpha 2-agonist clonidine and the alpha 2 agonist idazoxan. NE release was dose-dependently reduced by clonidine (1 nmol/L-1 mumol/L) and increased by idazoxan (10-100 nmol/L). The inhibition by clonidine was significantly greater at 1 Hz than at 3 Hz, whereas the absolute increase in NE release induced by idazoxan was greater at 3 Hz than at 1 Hz. Morphine at 1 mumol/L (a concentration per se ineffective) shifted to the left the clonidine concentrations able to inhibit NE release at 3 and 1 Hz (1-10 nmol/L), but at both frequencies, the opiate reduced the maximal inhibition induced by clonidine at 1 mumol/L. In slices taken from morphine-tolerant guinea pigs (in the presence of morphine at 1 mumol/L), clonidine (1 nmol/L-1 mumol/L) was ineffective at the stimulation rate of 3 Hz, but it was more active than in normal slices at 1 Hz. Such a response pattern suggests a reduced availability of alpha 2 receptors and an increase in their sensitivity to clonidine. However, chronic morphine treatment did not influence the physiological autoinhibition because the increase in NE release elicited by idazoxan (10-100 nmol/L) at 1 and 3 Hz was the same in normal and in "morphine-tolerant" slices.(ABSTRACT TRUNCATED AT 250 WORDS)Abstract: Endogenous norepinephrine (NE) release in cerebral cortex slices taken from normal and morphine‐tolerant guinea pigs was measured by HPLC. In normal slices, a linear relationship was found between electrically evoked NE release and the log of the frequency of stimulation in the range of 1–20 Hz. The efficiency of the α2‐mediated autofeedback was tested by adding the α2 agonist clonidine and the α2 antagonist idazoxan. NE release was dose‐dependently reduced by clonidine (1 nmol/L–1 μmol/L) and increased by idazoxan (10–100 nmol/L). The inhibition by clonidine was significantly greater at 1 Hz than at 3 Hz, whereas the absolute increase in NE release induced by idazoxan was greater at 3 Hz than at 1 Hz. Morphine at 1 μmol/L (a concentration per se ineffective) shifted to the left the clonidine concentrations able to inhibit NE release at 3 and 1 Hz (1–10 nmol/L), but at both frequencies, the opiate reduced the maximal inhibition induced by clonidine at 1 μmol/L. In slices take..
MEPSA: a flexible peak search algorithm designed for uniformly spaced time series
No full textWe present a novel algorithm aimed at identifying peaks within a uniformly sampled time series affected by uncorrelated Gaussian
noise. The algorithm, called “MEPSA” (multiple excess peak search algorithm), essentially scans the time series at different timescales by comparing a given peak candidate with a variable number of adjacent bins. While this has originally been conceived for the analysis of gamma–ray burst light (GRB) curves, its usage can be readily extended to other astrophysical transient phenomena, whose activity is recorded through different surveys. We tested and validated it through simulated featureless profiles as well as simulated GRB time profiles. We showcase the algorithm’s potential by comparing with the popular algorithm by Li and Fenimore, that is frequently adopted in the literature. Thanks to its high flexibility, the mask of excess patterns used byMEPSA can be tailored and optimised to the kind of data to be analysed without modifying the code. The C code is made publicly available
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