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 differen
t 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 algorithms 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.
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 a
djacent 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.
We use high--quality, multi-band observations of Swift GRB120404A, from gamma-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al. (2012) to test the standard synchrotron shock model. The evolution of the radio and
optical afterglow, with its prominent optical rebrightening at t_rest 260-2600 s, is remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re-energization of the shock. We thus constrain the geometry of the jet with half-opening and viewing angles of 23 and 21 deg respectively and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters epsilon_B=2.4e-4 and epsilon_e=9.3e-2 and a circumburst density of n=240 cm^-3. The ability to self-consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X-ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasising the need for a more realistic treatment of cooling in future developments of theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high quality multi-band data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.
We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 37 s after the onset of the GRB. This o
ptical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,z = 0.8 mag), and the well-sampled X-ray-to-near-infrared spectral energy distribution is interesting in requiring an LMC2 extinction profile, in contrast to the majority of GRBs. Comparison of the gamma-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt gamma-rays; instead, we suggest that the optical flash could originate from the inhomogeneity of the relativistic flow. In this scenario, a large velocity irregularity in the flow produces the prompt gamma-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat gamma-ray spectra, roughly F propto nu^-0.1, are observed in many GRBs. If the flat spectrum extends down to the optical band in GRB 080603A, the optical flare could be explained as the low-energy tail of the gamma-ray emission. If this is indeed the case, it provides an important clue to understanding the nature of the emission process in the prompt phase of GRBs and highlights the importance of deep (R> 20 mag), rapid follow-up observations capable of detecting faint, prompt optical emission.
The emission process responsible for the so-called prompt emission of gamma-ray bursts is still unknown. A number of empirical models fitting the typical spectrum still lack a satisfactory interpretation. A few GRB spectral catalogues derived from pa
st and present experiments are known in the literature and allow to tackle the issue of spectral properties of gamma-ray bursts on a statistical ground. We extracted and studied the time-integrated photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an independent statistical characterisation of GRB spectra. The spectra were fit with three models: a simple power-law, a cut-off power law or a Band function. The typical photon spectrum of a bright GRB consists of a low-energy index around 1.0 and a peak energy of the nuFnu spectrum E_p~240 keV in agreement with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ~35% of GRBs can be fit with a power-law with a photon index around 2, indicative of peak energies either close to or outside the GRBM energy boundaries. We confirm the correlation between E_p and fluence, with a logarithmic dispersion of 0.13 around the power-law with index 0.21+-0.06. The low-energy and peak energy distributions are not yet explained in the current literature. The capability of measuring time-resolved spectra over a broadband energy range, ensuring precise measurements of parameters such as E_p, will be crucial for future experiments (abridged).
X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with the peak energy of the time-integrated spectrum, Ep, below 30 keV, whereas classical GRBs have Ep of a few hundreds keV. Apart from Ep and the lower luminosity, the properties of XRFs a
re typical of the classical GRBs. Yet, the nature of XRFs and the differences from that of GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. We examine in detail the case of XRF 080330 discovered by Swift at the redshift of 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich and broadband (from NIR to UV) photometric data set we collected across this phase makes it an ideal candidate to test the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. We present prompt gamma-ray, early and late IR/visible/UV and X-ray observations of the XRF 080330. We derive a SED from NIR to X-ray bands across the plateau phase with a power-law index of 0.79 +- 0.01 and negligible rest-frame dust extinction. The multi-wavelength evolution of the afterglow is achromatic from ~10^2 s out to ~8x10^4 s. We describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model and show that a single-component jet viewed off-axis explains the observations (abriged).
Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we tested the correlation between the intrinsic peak energy of the time-integrated spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the duration of the most
intense parts of the GRB computed as T_0.45 (Firmani correlation). For 41 out of 88 GRBs we could estimate all of the three required properties. Apart from 980425, which appears to be a definite outlier and notoriously peculiar in many respects, we used 40 GRBs to fit the correlation with the maximum likelihood method discussed by DAgostini, suitable to account for the extrinsic scatter in addition to the intrinsic uncertainties affecting every single GRB. We confirm the correlation. However, unlike the results by Firmani et al., we found that the correlation does have a logarithmic scatter comparable with that of the E_p,i-E_iso (Amati) correlation. We also find that the slope of the product L_p,iso T_0.45 is equal to ~0.5, which is consistent with the hypothesis that the E_p,i-L_p,iso-T_0.45 correlation is equivalent to the E_p,i-E_iso correlation (slope ~0.5). We conclude that, based on presently available data, there is no clear evidence that the E_p,i-L_p,iso-T_0.45 correlation is different (both in terms of slope and dispersion) from the E_p,i-E_iso correlation.
We present prompt gamma-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well as for the early and la
te multi-band afterglow of GRB 070311. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt gamma-ray pulse and an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock (abridged).
