MEPSA: a flexible peak search algorithm designed for uniformly spaced time series

We 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 by MEPSA can be tailored and optimised to the kind of data to be analysed without modifying the code. The C code is made publicly available.Comment: 9 pages, 7 figures, accepted by Astronomy and Computin

Gamma-ray burst engines may have no memory

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. We investigated whether the simple hypothesis of a temporally uncorrelated sequence of pulses is really to be rejected, as a lognormal distribution necessarily implies. 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. 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. Our result suggests that GRB engines may emit pulses after the fashion of nuclear radioactive decay, that is, as a memoryless process.Comment: 4 pages, 3 figures, accepted in A&

Gamma Ray Bursts in the Era of Rapid Followup

We present a status report on the study of gamma-ray bursts (GRB) in the era of rapid follow-up 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.Comment: 20 pages, 12 figures; Review article accepted for publication in Advances in Astronomy, special issue 'Robotic Astronomy (Hindawi Publishing Corporation)

A search for Galactic transients disguised as gamma-ray bursts

A significant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a diffusion process. Possible evidence has recently been reported by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs discovered with Swift. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered different subsamples. We tested the degree of isotropy through the dipole and the quadrupole moment distributions, both with reference to the Galaxy and in a coordinate-system-independent way, as well as with the two-point angular autocovariance function. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. We found marginal (~3 sigma) evidence for an excess of FREDs with unknown redshift towards the Galactic plane compared with what is expected for an isotropic distribution corrected for the non-uniform sky exposure. However, when we account for the observational bias against optical follow-up observations of low-Galactic latitude GRBs, the evidence for anisotropy decreases to ~2 sigma. In addition, we found no statistical evidence for different spectral or temporal properties from the bulk of cosmological GRBs. We found marginal evidence for the presence of a disguised Galactic population among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +- 11)%, with an upper limit of 34% (90% confidence).Comment: 6 pages, 4 figures, accepted by A&

A search for pulsations in short gamma-ray bursts to constrain their progenitors

We searched for periodic and quasiperiodic signal in the prompt emission of a sample of 44 bright short gamma-ray bursts detected with Fermi/GBM, Swift/BAT, and CGRO/BATSE. The aim was to look for the observational signature of quasiperiodic jet precession which is expected from black hole-neutron star mergers, but not from double neutron star systems. Thus, this kind of search holds the key to identify the progenitor systems of short GRBs and, in the wait for gravitational wave detection, represents the only direct way to constrain the progenitors. We tailored our search to the nature of the expected signal by properly stretching the observed light curves by an increasing factor with time, after calibrating the technique on synthetic curves. In none of the GRBs of our sample we found evidence for periodic or quasiperiodic signals. In particular, for the 7 unambiguously short GRBs with best S/N we obtained significant upper limits to the amplitude of the possible oscillations. This result suggests that BH-NS systems do not dominate the population of short GRB progenitors as described by the kinematic model of Stone, Loeb, & Berger (2013).Comment: 7 pages, 5 figures, accepted to ApJ, added reference

Average power density spectrum of long GRBs detected with BeppoSAX/GRBM and with Fermi/GBM

From past experiments the average power density spectrum (PDS) of GRBs with unknown redshift was found to be modelled from 0.01 to 1 Hz with a power-law, f^(-alpha), with alpha broadly consistent with 5/3. Recent analyses of the Swift/BAT catalogue showed analogous results in the 15-150 keV band. We carried out the same analysis on the bright GRBs detected by BeppoSAX/GRBM and Fermi/GBM. The BeppoSAX/GRBM data, in the energy range 40-700 keV and with 7.8 and 0.5-ms time resolutions, allowed us to explore for the first time the average PDS at very high frequencies (up to 1 kHz) and reveal a break around 1-2 Hz, previously found in CGRO/BATSE data. The Fermi/GBM data, in the energy band 8-1000 keV, allowed us to explore for the first time the average PDS within a broad energy range. Our results confirm and extend the energy dependence of the PDS slope, according to which harder photons have shallower PDS.Comment: 13 pages, 9 figures, accepted to MNRA

A common stochastic process rules gamma-ray burst prompt emission and X-ray flares

Prompt gamma-ray and early X-ray afterglow emission in gamma-ray bursts (GRBs) are characterized by a bursty behavior and are often interspersed with long quiescent times. There is compelling evidence that X-ray flares are linked to prompt gamma-rays. However, the physical mechanism that leads to the complex temporal distribution of gamma-ray pulses and X-ray flares is not understood. Here we show that the waiting time distribution (WTD) of pulses and flares exhibits a power-law tail extending over 4 decades with index ~2 and can be the manifestation of a common time-dependent Poisson process. This result is robust and is obtained on different catalogs. Surprisingly, GRBs with many (>=8) gamma-ray pulses are very unlikely to be accompanied by X-ray flares after the end of the prompt emission (3.1 sigma Gaussian confidence). These results are consistent with a simple interpretation: an hyperaccreting disk breaks up into one or a few groups of fragments, each of which is independently accreted with the same probability per unit time. Prompt gamma-rays and late X-ray flares are nothing but different fragments being accreted at the beginning and at the end, respectively, following the very same stochastic process and likely the same mechanism.Comment: 11 pages, 7 figures, accepted by Ap

SN/GRB connection: a statistical approach with BATSE and Asiago Catalogues

Recent observations suggest that some types of GRB are physically connected with SNe of type Ib/c. However, it has been pointed out by several authors that some GRBs could be associated also with other types of core-collapse SNe (type IIdw/IIn). On the basis of a comphrensive statistical study, which has made use of the BATSE and Asiago catalogues, we have found that: i) the temporal and spacial distribution of SNe-Ib/c is marginally correlated with that of the BATSE GRBs; ii) we do not confirm the existence of an association between GRBs and SNe-IIdw/IIn.Comment: Proceeding of the 4th workshop on Gamma Ray Bursts in the Afterglow Era, Rome, 2004; 4 page

A New Frequency-Luminosity Relation for Long GRBs?

We have studied power density spectra (PDS) of 206 long Gamma-Ray Bursts (GRBs). We fitted the PDS with a simple power-law and extracted the exponent of the power-law (alpha) and the noise-crossing threshold frequency (f_th). We find that the distribution of the extracted alpha peaks around -1.4 and that of f_th around 1 Hz. In addition, based on a sub-set of 58 bursts with known redshifts, we show that the redshift-corrected threshold frequency is positively correlated with the isotropic peak luminosity. The correlation coefficient is 0.57 +/- 0.03.Comment: 9 pages, 17 figures, 1 table; Accepted for publication in MNRA