Duration Distribution Of Gamma-ray Bursts Research Articles

Can the Cosmological Dilation Explain the Skewness in the Gamma-Ray Burst Duration Distribution?

Abstract In order to explain the origin of skewness in the gamma-ray burst (GRB) duration distribution, a statistical model, based on the cosmological time dilation and taking into account the GRB formation rate and instrumental effects, is examined. It is concluded that the considered effects are able to account for only a small fraction of the observed skewness. Therefore, its origin needs to be searched for in the processes governing the progenitors of GRBs​.

A note on ‘Analysis of gamma-ray burst duration distribution using mixtures of skewed distributions’

A note on ‘Analysis of gamma-ray burst duration distribution using mixtures of skewed distributions’

A common central engine for long gamma-ray bursts and Type Ib/c supernovae

Long-duration, spectrally-soft Gamma-Ray Bursts (GRBs) are associated with Type Ic Core Collapse (CC) Supernovae (SNe), and thus arise from the death of massive stars. In the collapsar model, the jet launched by the central engine must bore its way out of the progenitor star before it can produce a GRB. Most of these jets do not break out, and are instead "choked" inside the star, as the central-engine activity time, $t_{\rm e}$, is not long enough. Modelling the long-soft GRB duration distribution assuming a power-law distribution for their central-engine activity times, $\propto t_{\rm e}^{-\alpha}$ for $t_{\rm e}>t_{\rm b}$, we find a steep distribution ($\alpha\sim4$) and a typical GRB jet breakout time of $t_{\rm b}\sim 60\text{ s}$ in the star's frame. The latter suggests the presence of a low-density, extended envelope surrounding the progenitor star, similar to that previously inferred for low-luminosity GRBs. Extrapolating the range of validity of this power law below what is directly observable, to $t_{\rm e}<t_{\rm b}$, by only a factor of $\sim$4-5 produces enough events to account for all Type Ib/c SNe. Such extrapolation is necessary to avoid fine-tuning the distribution of central engine activity times with the breakout time, which are presumably unrelated. We speculate that central engines launching relativistic jets may operate in all Type Ib/c SNe. In this case, the existence of a common central engine would imply that (i) the jet may significantly contribute to the energy of the SN; (ii) various observational signatures, like the asphericity of the explosion, could be directly related to jet's interaction with the star.

Analysis of gamma-ray burst duration distribution using mixtures of skewed distributions

Two classes of GRBs have been confidently identified thus far and are prescribed to different physical scenarios -- NS-NS or NS-BH mergers, and collapse of massive stars, for short and long GRBs, respectively. A third, intermediate in duration class, was suggested to be present in previous catalogs, such as BATSE and $Swift$, based on statistical tests regarding a mixture of two or three log-normal distributions of $T_{90}$. However, this might possibly not be an adequate model. This paper investigates whether the distributions of $\log T_{90}$ from BATSE, $Swift$, and $Fermi$ are described better by a mixture of skewed distributions rather than standard Gaussians. Mixtures of standard normal, skew-normal, sinh-arcsinh and alpha-skew-normal distributions are fitted using a maximum likelihood method. The preferred model is chosen based on the Akaike information criterion. It is found that mixtures of two skew-normal or two sinh-arcsinh distributions are more likely to describe the observed duration distribution of $Fermi$ than a mixture of three standard Gaussians, and that mixtures of two sinh-arcsinh or two skew-normal distributions are models competing with the conventional three-Gaussian in the case of BATSE and $Swift$. Based on statistical reasoning, existence of a third (intermediate) class of GRBs in $Fermi$ data is rejected, and it is shown that other phenomenological models may describe the observed $Fermi$, BATSE, and $Swift$ duration distributions at least as well as a mixture of standard normal distributions.

Duration distributions for gamma-ray bursts registered in various experiments since VENERA11/KONUS up to Fermi/GBM.

Gamma-ray bursts duration distributions properties for events registered by experiments CGRO/BATSE, VENERA11/KONUS, VENERA12/KONUS, Swift/BAT, GRANAT/PHEBUS, Suzaku/WAM, RHESSI and Fermi/GBM are considered. GRBs observed since 1967 and now several thousands of events were listed in more than 30 catalogues. Gamma-ray bursts duration distribution was the first analysed using data of BATSE instrument onboard the CGRO. The GRBs duration distribution analysis had shown the existence of two bursts classes: long and short separated by t90 = 2 s. But results of similar distributions for bursts observed by other detectors have shown shifting of boundary between short and long events from value of 2 s. For example, Swift/BAT GRBs subset analysis gives the value of ∼1 s for this separator point. Moreover, t90 has dependence from instrument registered this burst - it is function of detector sensitivity threshold and operation energy band. For instance, the duration of GRB060418 burst t90 is ∼52 s according to Swift/BAT data and only 36 s according to RHESSI data. Therefore, the type of GGB (whether it short or long) should be defined only taking into account distinctive features of instrument detected this event. Also attributes of third intermediate GRBs subgroup appearance in events subsets for various detectors are discussed. Firstly this subgroup was found some years ago in BATSE GRB duration and duration-hardness distributions.

On the observed duration distribution of gamma-ray bursts from collapsars

The duration of the prompt emission of long gamma-ray bursts is generally considered to be fairly similar to the duration of the activity of the engine in the center of the progenitor star. Here, we investigate the relation between the duration of the engine activity and that of the observed light curve, using inputs from both numerical simulations and observations. We find that the observed burst duration is a good proxy for the engine duration after the time necessary for the jet to break out the star's surface is subtracted. However, the observed duration is a function of the viewing angle and can be significantly shorter than the duration of the engine activity. We also show that the observed, redshift-corrected burst duration evolves only moderately with redshift for both observations and synthetic light curves. We conclude that the broad distribution of the observed duration of long BATSE gamma-ray bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being due to viewing and redshift effects. Our results do not rule out the existence of engines with very long duration. However, we find that they are constrained to be a small minority of the BATSE detected bursts.

Duration distributions for different softness groups of gamma-ray bursts

Gamma-ray bursts (GRBs) are divided into two classes according to their durations. We investigate if the softness of bursts plays a role in the conventional classification of the objects. We employ the BATSE (Burst and Transient Source Experiment) catalog and analyze the duration distributions of different groups of GRBs associated with distinct softness. Our analysis reveals that the conventional classification of GRBs with the duration of bursts is influenced by the softness of the objects. There exists a bimodality in the duration distribution of GRBs for each group of bursts and the time position of the dip in the bimodality histogram shifts with the softness parameter. Our findings suggest that the conventional classification scheme should be modified by separating the two well-known populations in different softness groups, which would be more reasonable than doing so with a single sample. According to the relation between the dip position and the softness parameter, we get an empirical function that can roughly set apart the short-hard and long-soft bursts: SP = (0.100 ± 0.028)T 90 − (0.85 ± 0.18) , where SP is the softness parameter adopted in this paper.

On the origin of the bimodal duration distribution of gamma-ray bursts and the subjet model

The modified version of a bullet model for gamma-ray bursts (GRBs) is studied. The central engine of the source produces multiple subjets that are contained within a cone. The emission of photons in the source frame of a subjet takes place either in an infinitesimally thin shell, or during its expansion for a finite time. The analysis of the observed profiles of GRBs taken by the Burst And Transient Source Experiment leads us to the conclusion that the latter possibility is much more favoured. We also study the statistical distribution of GRBs in the context of their bimodality of durations, taking into account the capability of the detector of observing the signal above a certain flux limit. The model with shells emitting for a finite time is able to reproduce only one class of bursts, short or long, depending on the adopted physical parameters. Therefore, we suggest that the GRB bimodality is intrinsically connected with two separate classes of sources.

Gamma-ray burst phenomenology explained through the blast wave model

The essential physics of the blast wave model is contained in a formula recently proposed to describe the temporal evolution of the nonthermal synchrotron spectra from gamma-ray bursts (GRBs). This formula accounts for many of the well-established empirical trends in GRB physics, including the basic form of the fast-rise, gentle-decay FRED-type GRB light curves, the characteristic hard-to-soft spectral evolution and intensity-hardness correlations of GRB light curves, and the recently uncovered relations between the peak energies and the photon and energy fluences. On the basis of this formula, we predict the tendency for pulses to align at high energies and to lag at low energies. The overall GRB duration distribution is understood when triggering properties of GRB detectors are considered for fireballs with a range of baryon loading parameters. The existence of new classes of clean and dirty fireballs are implied. Short timescale variability of GRB light curves is attributed to density variations in the circumburster medium (CBM).

On Fueling Gamma-Ray Bursts and Their Afterglows with Pulsars

Gamma-ray bursts (GRBs) and their afterglows seem to result from the dissipation of bulk energy in relativistic outflows, but their engine has not been identified. The engine could be a young pulsar formed from accretion-induced collapse with a dynamo-amplified magnetic field. Elsewhere, we suggest that such strong field (cosmological) pulsars may help to explain the bimodal GRB duration distribution. Here we discuss the possible roles of a pulsar for the afterglow. We derive the expected bolometric luminosity decay and emphasize that the extracted rotational energy could dissipate by shocks and/or by large-amplitude electromagnetic waves (LAEMWs). The simplest LAEMW approach predicts a slower decay in the observed afterglow peak frequency and a faster decay in the flux than the simplest blast wave, although more complicated models of both can produce different dependences. LAEMWs do not require the rapid magnetic field amplification demanded of the blast-wave approach because the emission originates from a nearly fixed radius. Different time evolution of GRB and post-GRB emission is also predicted. Evidence for a pulsar in a GRB would make some GRB engine models, such as neutron star mergers and black holes, unlikely. Thus, the question of a pulsar presence is important even if it could drive a canonical fireball.

An Explanation for the Bimodal Duration Distribution of Gamma-Ray Bursts: Millisecond Pulsars from Accretion-induced Collapse

Cosmological gamma-ray bursts (GRBs) could be driven by dissipation of pure electromagnetic energy (Poynting flux) extracted from rapidly rotating compact objects with strong magnetic fields. One such possibility is a young millisecond pulsar (MSP) formed from the accretion-induced collapse (AIC) of a white dwarf. The combination of an efficient magnetic dynamo, likely operating during the first seconds of the initially hot and turbulent MSP interior, and the subsequent modest beaming of gamma ray-emitting outflows, would easily account for energy constraints. But the remarkable feature of such models is that they may naturally explain the puzzling bimodal distribution in GRB time durations. The two burst classes could correspond to MSPs that form spinning above and below, respectively, a gravitationally unstable limit. In the former case, the spin-down timescale is caused by gravitational radiation emission (<1 s), while the spin-down timescale of the latter is caused by electromagnetic dipole emission (1 s). These two timescales account for the short and long GRB durations, i.e., the observed bimodal GRB duration distribution. A natural prediction is that the short-duration GRBs would be accompanied by strong gravitational radiation emission, which is absent from the longer class. Both would show millisecond variabilities.