1,721,152 research outputs found
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
Testing the gamma-ray burst variability/peak luminosity correlation using the pseudo-redshifts of a large sample of BATSE gamma-ray bursts
No full textWe test the correlation found by Reichart et al. between time variability and peak luminosity of gamma-ray bursts (GRBs). Recently, Guidorzi et al. found that this still holds for a sample of 32 GRBs with spectroscopic redshift, although with a larger scatter than that originally found by Reichart et al. However, Guidorzi et al. also found that a power law does not provide a good description of that. We report on the same test performed on a sample of 551 burst and transient source experiment (BATSE) GRBs with a significant measure of variability assuming the pseudo-redshifts derived by Band et al. (1186 GRBs) through the anticorrelation between spectral lag and peak luminosity. We still find a correlation between variability as defined by Reichart et al. and peak luminosity with higher significance. However, this subsample of BATSE GRBs shows a higher scatter around the best-fitting power law than that found by Reichart et al. in the variability/peak luminosity space. This is in agreement with the result found by Guidorzi et al. on a sample of 32 GRBs with measured redshift. These results confirm that a power law does not provide a satisfactory description for all the GRBs, in contrast with the original findings by Reichart et al
Power-density spectrum of non-stationary short-lived light curves
No full textThe power density spectrum of a light curve is often calculated as the
average of a number of spectra derived on individual time intervals
the light curve is divided into.
This procedure implicitly assumes that each time interval is a
different sample function of the same stochastic ergodic process.
While this assumption can be applied to many astrophysical sources,
there remains a class of transient, highly nonstationary and
short-lived events, such as gamma-ray bursts, for which this
approach is often inadequate.
The power spectrum statistics of a constant signal affected by
statistical (Poisson) noise is known to be a chi2(2) in the
Leahy normalisation. However, this is no more the case when
a nonstationary signal is also present.
As a consequence, the uncertainties on the power spectrum
cannot be calculated based on the chi2(2) properties, as
assumed by tools such as XRONOS powspec.
We generalise the result in the case of a nonstationary signal
affected by uncorrelated white noise and show that the new
distribution is a non-central chi2(2,lambda),
whose non-central value lambda is the power spectrum of
the deterministic function describing the nonstationary signal.
Finally, we test these results in the case of synthetic curves
of gamma-ray bursts.
We end up with a new formula for calculating the power
spectrum uncertainties. This is crucial in the case of
nonstationary short-lived processes affected by uncorrelated
statistical noise, for which ensemble averaging does not
make any physical sense
Esercizi di fisica 1. Con Contenuto digitale (fornito elettronicamente)
No full textRisolvere problemi di fisica è una forma di abilità che richiede studio e costante applicazione, per compenetrare l’aspetto intuitivo con il formalismo.
Questo è il cosiddetto “problem solving”: studio del bagaglio di conoscenze (principi, leggi, teoremi, tecniche di calcolo), che sono l’oggetto del programma del corso, e sviluppo della capacità di impostare un problema, identificare gli strumenti necessari e guidare l’aspetto intuitivo entro il rigore e il formalismo che la fisica richiede.
Attraverso una costante verifica della propria capacità di saper risolvere problemi di varia difficoltà lo studente potrà verificare la solidità delle proprie cono scenze.
Per questo nel libro sono stati pubblicati oltre 200 problemi in ordine di difficoltà crescente, tutti forniti di soluzione ampiamente discussa, che riguardano i seguenti argomenti: calcolo vettoriale, cinematica e dinamica del punto materiale, dinamica dei sistemi di punti materiali e corpi rigidi, gravitazione newtoniana, urti, oscillazioni, onde, fluidodinamica, termodinamica.
I problemi sono suddivisi in 12 capitoli (secondo i classici argomenti di un corso di Fisica 1 per i CdS di Fisica, Astrofisica, Chimica e Ingegneria) raccolti per argomenti, seguendo l’ordine con il quale gli stessi vengono affrontati durante il corso di lezioni. In un capitolo finale ne sono raccolti alcuni a carattere riepilogativo, più complessi e trasversali agli argomenti
The two large flares from SGR1900+14 observed with the BeppoSAX Gamma-Ray Burst Monitor: new results
No full textWe present the results of a thorough timing and spectral analysis of the BeppoSAX Gamma-Ray Burst Monitor data of the two large flares from SGR1900+14: the giant one of August 27, 1998 and the intermediate one of April 18, 2001. We compare the two flares, showing interesting common spectral and temporal properties, despite their apparent different profiles and fluences. New findings concerning the presence of timing noise and the time-averaged energy spectra are discussed and interpreted in the light of the magnetar model
Extremely energetic
No full textThe origin, reliability, and dispersion of the Ep,i – Eiso and other spectral energy correlations is a highly debated topic in GRB astrophysics. GRB 080916C, with its enormous radiated energy ( ~ 1055 erg in the 1 keV-10 GeV cosmological rest-frame energy band) and its intense GeV emission measured by Fermi, provides a unique opportunity to investigate this issue. In our analysis, we also study another extremely energetic event, GRB 090323, more recently detected and localized by Fermi/LAT, whose radiated energy is comparable to that of GRB 080916C
in the 1 keV-10 MeV energy range. Based on Konus/WIND and Fermi spectral measurements,
we find that both events are fully consistent with the Ep,i – Eiso correlation (updated to include 95 GRBs with the data available as of April 2009), thus further confirming and extending it,
and providing evidence against a possible flattening or increased dispersion at very high energies.
This also suggests that the physics behind the emission of peculiarly bright and hard GRBs is the same as for medium-bright and soft-weak long events (XRFs), which all follow the correlation.
In addition, we find that the normalization of the correlation obtained by considering these two GRBs and the other long ones for which was measured to high accuracy by the Fermi/GBM are fully consistent with those obtained by other instruments (e.g., BeppoSAX, Swift, Konus/WIND), thus indicating that the correlation is not affected significantly by “data truncation” because of detector thresholds and limited energy bands. A Fermi/GBM accurate estimate of the peak energy of a
very bright and hard short GRB with a measured redshift, GRB 090510, provides robust evidence that short GRBs do not follow the Ep,i – Eiso correlation and that the Ep,i – Eiso plane can be used to discriminate between, and understand, the two classes of events. Prompted by the extension of the spectrum of GRB 080916C to several GeV (in the cosmological rest-frame) without any excess or cut-off, we also investigated whether the evaluation of in the commonly adopted 1 keV-10 MeV energy band may bias the Ep,i – Eiso correlation and/or contribute to its scatter.
By computing from 1 keV to 10 GeV, the slope of the correlation becomes slightly flatter, while its dispersion does not change significantly. Finally, we find that GRB 080916C is also consistent with most of the other spectral energy correlations derived from it, with the
possible exception of the Ep,i – Eiso – tb correlation
Autocorrelation analysis of GRBM--Beppo-SAX burst data
No full textAn autocorrelation function (ACF) analysis was performed on 17 gamma-ray bursts with known redshift, using data from the GRBM on board Beppo-SAX. When corrected from the cosmic time dilation effect, the ACFs show a bimodal distribution at about half-maximum, in agreement with a previous study based on BATSE and Konus burst data. Although the results show more dispersion, the separation between the two classes is highly significant
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
Broadband turbulent spectra in gamma-ray burst light curves
No full textBroadband power density spectra offer a window to understanding turbulent behavior in the emission mechanism and, at the highest frequencies, in the putative inner engines powering long gamma-ray bursts (GRBs).We describe a chirp search method alongside Fourier analysis for signal detection in the Poisson noise-dominated, 2 kHz sampled, BeppoSAX light curves. An efficient numerical implementation is described in O(Nn log n) operations, where N is the number of chirp templates and n is the length of the light-curve time series, suited for embarrassingly parallel processing. For the detection of individual chirps over a 1 s duration, the method is one order of magnitude more sensitive in signal-to-noise ratio than Fourier analysis. The Fourier–chirp spectra of GRB 010408 and GRB 970816 show a continuation of the spectral slope with up to 1 kHz of turbulence identified in low-frequency Fourier analysis. The same continuation is observed in an average spectrum of 42 bright, long GRBs. An outlook on a similar analysis of upcoming gravitational wave data is included
Evidence for Luminosity Evolution of Long Gamma-ray Bursts in Swift Data
No full textWe compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE experiment in two different scenarios: i) the GRB luminosity evolves with redshift and ii) GRBs form preferentially in low-metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at z>2.5 and z>3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 10^53 erg s^-1) detected by Swift in its first three years of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that pure luminosity evolution models can account for the number of sure identifications. In the case of a pure density evolution scenario, models with Z_th>0.3 Zsun are ruled out with high confidence. For lower metallicity thresholds, the model results are still statistically consistent with available lower limits. However, many factors can increase the discrepancy between model results and data, indicating that some luminosity evolution in the GRB LF may be needed also for such low values of Z_th. Finally, using these new constraints, we derive robust upper limits on the bright-end of the GRB LF, showing that this cannot be steeper than ~2.6
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