Light Curves Of Gamma-ray Bursts Research Articles

Rates and Beaming Angles of Gamma-Ray Bursts Associated with Compact Binary Coalescences

Some, if not all, binary neutron star (BNS) coalescences, and a fraction of neutron star–black hole (NSBH) mergers, are thought to produce sufficient mass ejection to power gamma-ray bursts (GRBs). However, this fraction, as well as the distribution of beaming angles of BNS-associated GRBs, is poorly constrained from observation. Recent work applied machine learning tools to analyze GRB light curves observed by Fermi/Gamma-Ray Burst Monitor (GBM) and Swift/Burst Alert Telescope (BAT). GRBs were segregated into multiple distinct clusters, with the tantalizing possibility that one of them (BNS cluster) could be associated with BNSs and another (NSBH cluster) with NSBHs. As a proof of principle, assuming that all GRBs detected by Fermi/GBM and Swift/BAT associated with BNSs (NSBHs) lie in the BNS (NSBH) cluster, we estimate their rates (Gpc−3 yr−1). We compare these rates with corresponding BNS and NSBH rates estimated by the LIGO–Virgo–KAGRA (LVK) collaboration from the first three observing runs (O1, O2, O3). We find that the BNS rates are consistent with LVK’s rate estimates, assuming a uniform distribution of beaming fractions (f b ∈ [0.01, 0.1]). Conversely, using the LVK’s BNS rate estimates, assuming all BNS mergers produce GRBs, we are able to constrain the beaming angle distribution to θ j ∈ [0.°8, 33.°5] at 90% confidence. We similarly place limits on the fraction of GRB-bright NSBHs as f B ∈ [1.3%, 63%] (f B ∈ [0.4%, 15%]) with Fermi/GBM (Swift/BAT) data.

Generalized linear model study of the T90–T50 relation of gamma-ray bursts

ABSTRACT Gamma-ray bursts (GRBs) can be studied with their linearly dependent parameters alongside the standard $T_{90}$ distribution. The generalized linear model (GLM) identifies the number of linear dependences in a two-parameter space. Classically, GRBs are classified into two classes by the presence of bimodality in the histogram of $T_{90}$. However, additional classes and different features of GRBs are fascinating topics to explore. In this work, we investigate GRB features in the $T_{90} {-}T_{50}$ plane using the GLM for three major catalogues: Swift, the Fermi Gamma-ray Burst Monitor (GBM), and the Burst And Transient Source Experiment (BATSE). This study shows five linear features for the Fermi GBM catalogue and four linear features for the BATSE catalogue, directing us towards the possibility of non-Gaussianity in the light curves of GRBs.

Diversity in Fermi/GBM Gamma-Ray Bursts: New Insights from Machine Learning

Classification of gamma-ray bursts (GRBs) has been a long-standing puzzle in high-energy astrophysics. Recent observations challenge the traditional short versus long viewpoint, where long GRBs are thought to originate from the collapse of massive stars and short GRBs from compact binary mergers. Machine learning (ML) algorithms have been instrumental in addressing this problem, revealing five distinct GRB groups within the Swift Burst Alert Telescope (BAT) light-curve data, two of which are associated with kilonovae (KNe). In this work, we extend our analysis to the Fermi Gamma-ray Burst Monitor catalog and identify five clusters using unsupervised ML techniques, consistent with the Swift/BAT results. These five clusters are well separated in the fluence-duration plane, hinting at a potential link between fluence, duration, and complexities (or structures) in the light curves of GRBs. Further, we confirm two distinct classes of KN-associated GRBs. The presence of GRB 170817A in one of the two KN-associated clusters lends evidence to the hypothesis that this class of GRBs could potentially be produced by binary neutron star mergers. The second KN-associated GRB cluster could potentially originate from neutron star–black hole mergers. Future multimessenger observations of compact binaries in gravitational waves and electromagnetic waves can be paramount in understanding these clusters better.

A Comprehensive Analysis of Insight-HXMT Gamma-Ray Burst Data. I. Power Density Spectrum

The power density spectrum (PDS) is a powerful tool to study light curves of gamma-ray bursts (GRBs). We show the average PDS and individual PDS analysis with GRB data from the Hard X-ray Modulation Telescope (also named Insight-HXMT). The values of the power-law index of the average PDS ( αP¯ ) for long GRBs (LGRBs) vary from 1.58 to 1.29 (for 100–245, 245–600, and 600–2000 keV). The Insight-HXMT data allow us to extend the energy of the LGRBs up to 2000 keV, and a relation between αP¯ and energy E is obtained: αP¯∝E−0.09 (8–2000 keV). We first systematically investigate the average PDS and individual PDSs for short GRBs (SGRBs), and obtain αP¯∝E−0.07 (8–1000 keV), where the values of αP¯ vary from 1.86 to 1.34. The distribution of the power-law index of an individual PDS (α) of an SGRB is consistent with that of an LGRB, and the α value for the group with a dominant timescale (the bent power law) is higher than that for the group with no dominant timescale (the single power law). Both LGRBs and SGRBs show similar α and αP¯ , which indicates that they may be the result of similar stochastic processes. Typical values of the dominant timescale τ for LGRBs and SGRBs are 1.58 s and 0.02 s, respectively. It seems that τ varies in proportion to the duration of GRBs T 90, with a relation τ∝T900.86 . The GRB light curve may result from superposing a number of pulses with different timescales. No periodic or quasi-periodic signal above the 3σ significance threshold is found in our sample.

Long gamma-ray burst light curves as the result of a common stochastic pulse--avalanche process

The complexity and variety exhibited by the light curves of long gamma-ray bursts (GRBs) enclose a wealth of information that has not yet been fully deciphered. Despite the tremendous advance in the knowledge of the energetics, structure, and composition of the relativistic jet that results from the core collapse of the progenitor star, the nature of the inner engine, how it powers the relativistic outflow, and the dissipation mechanisms remain open issues. A promising way to gain insights is describing GRB light curves as the result of a common stochastic process. In the Burst And Transient Source Experiment (BATSE) era, a stochastic pulse avalanche model was proposed and tested through the comparison of ensemble-average properties of simulated and real light curves. Here our aim was to revive and further test this model. We applied it to two independent datasets, BATSE and Swift /BAT, through a machine learning approach: the model parameters are optimised using a genetic algorithm. The average properties were successfully reproduced. Notwithstanding the different populations and passbands of both datasets, the corresponding optimal parameters are interestingly similar. In particular, for both sets the dynamics appear to be close to a critical state, which is key to reproducing the observed variety of time profiles. Our results propel the avalanche character in a critical regime as a key trait of the energy release in GRB engines, which underpins some kind of instability.

Spectral Lags of 90 Swift Gamma-Ray Bursts and the Constraint on the Lorentz Invariance Violation

The arrival times of photons with different energy could be different, even when they are emitted from the same source simultaneously. Such a spectral lag is a common property among high-energy astrophysics phenomena like gamma-ray bursts (GRBs). The potential violation of the Lorentz invariance caused by quantum fluctuations in background spacetime metrics could cause the spectral lag. In this paper, we try to make a constraint on Lorentz invariance violation (LIV) with multiple energy bands light curves of GRBs. With a sample of 90 Swift GRBs with redshifts, we calculate their spectral offsets from four fixed energy bands within their source frame. With help from the cross-correlation function and kernel smooth procedure, we extract improved spectral shifts. By using these more general spectral lags, we obtain an upper limit on LIV, equivalent to a lower limit on the quantum-gravitational energy scale of E QG ≥ 2.2 × 1014GeV.

On the Detection and Characterization of Quasiperiodic Oscillations in Astronomical Time Series: Gamma-Ray Burst X-Ray Light Curves as a Test Case

The study of temporal properties of variable sources can elucidate their physical processes. In this context, we present a critical study comparing three approaches to periodic or quasiperiodic behavior: Gaussian process, power spectrum, and wavelet analysis, using celerite, Lomb–Scargle periodograms, and weighted wavelet Z-transforms, respectively. We use 15 Swift X-ray Telescope light curves of short gamma-ray bursts (sGRBs) as examples. A comprehensive analysis of two sGRB X-ray light curves is performed. The results reveal the importance of artifacts, largely in the form of false quasiperiodic oscillation signals, possibly introduced by preprocessing (such as detrending) or other aspects of the analysis. The exploration described in this paper can be helpful for future studies of variability in gamma-ray bursts, active galactic nuclei, and other astronomical sources.

One Fits All: A Unified Synchrotron Model Explains GRBs with FRED-shape Pulses

The analysis of gamma-ray burst (GRB) spectra often relies on empirical models lacking a distinct physical explanation. Previous attempts to couple physical models with observed data focus on individual burst studies, fitting models to segmented spectra with independent physical parameters. However, these approaches typically neglect to explain the time evolution of observed spectra. In this study, we propose a novel approach by incorporating the synchrotron radiation model to provide a self-consistent explanation for a selection of single-pulse GRBs. Our study comprehensively tests the synchrotron model under a unified physical condition, such as a single injection event of electrons. By tracing the evolution of cooling electrons in a decaying magnetic field, our model predicts time-dependent observed spectra that align well with the data. Using a single set of physical parameters, our model successfully fits all time-resolved spectra within each burst. Our model suggests that the rising phase of the GRB light curve results from the increasing number of radiating electrons, while the declining phase is attributed to the curvature effect, electron cooling, and the decaying magnetic field. Our model provides a straightforward interpretation of the peak energy’s evolution, linked to the decline of the magnetic field and electron cooling due to the expansion of the GRB emission region. Our findings strongly support the notion that spectral and temporal evolution in GRB pulses originates from the expansion of the GRB emission region, with an initial radius of approximately 1015 cm, and synchrotron radiation as the underlying emission mechanism.

What absorbs the early TeV photons of GRB 221009A?

ABSTRACT The tera-electronvolt (TeV) light curve of gamma-ray burst 221009A exhibits an unprecedentedly rapid rise at its start. This phenomenon could be due to the strong absorption of photons and electrons within the emitting region. As the external shock expands outward and the radius increases, the volume of matter also grows, resulting in a gradual reduction of the optical depth for TeV photons. We investigate several potential explanations for the early TeV light curves. We calculate the optical depth for TeV photons, considering both annihilation with lower energy photons in the external shock and their scattering by electrons generated through the cascading of the TeV emission. Even under the favourable assumptions, we have determined that the optical depths for these processes are orders of magnitude too small to explain the observed light curve. Additional sources of absorbers, including electrons in the ejecta or external shock, also do not result in sufficient optical depths. Hence, the cause behind the early peculiar TeV light curve remains unclear.

GRB 220408B: A Three-episode Burst from a Precessing Jet

Jet precession has previously been proposed to explain the apparently repeating features in the light curves of a few gamma-ray bursts (GRBs). In this paper, we further apply the precession model to a bright GRB 220408B by examining both its temporal and spectral consistency with the predictions of the model. As one of the recently confirmed GRBs observed by our GRID CubeSat mission, GRB 220408B is noteworthy as it exhibits three apparently similar emission episodes. Furthermore, the similarities are reinforced by their strong temporal correlations and similar features in terms of spectral evolution and spectral lags. Our analysis demonstrates that these features can be well explained by the modulated emission of a Fast-Rise-Exponential-Decay (FRED) shape light curve intrinsically produced by a precessing jet with a precession period of s, a nutation period of s and viewed off-axis. This study provides a straightforward explanation for the complex yet similar multi-episode GRB light curves.

Evidence for a luminosity–decay correlation in GRB GeV light curves

ABSTRACT Correlations between intrinsic properties of gamma-ray burst (GRB) light curves provide clues to the nature of the central engine, the jet, and a possible means to standardize GRBs for cosmological use. Here, we report on the discovery of a correlation between the intrinsic early-time luminosity, LG,10 s, measured at rest frame 10 s, and the average decay rate measured from rest frame 10 s onward, $\alpha _{\mathrm{G,avg\gt 10\, s}}$, in a sample of 13 Fermi Large Area Telescope long GRB light curves. We note that our selection criteria, in particular the requirement for a redshift to construct luminosity light curves, naturally limits our sample to energetic GRBs. A Spearman’s rank correlation gives a coefficient of –0.74, corresponding to a confidence level of 99.6 per cent, indicating that brighter afterglows decay faster than less luminous ones. Assuming a linear relation with log(LG,10s), we find $\alpha _{\mathrm{G,avg\gt 10\, s}}$$= -0.31_{-0.09}^{+0.12}\log ($LG,10s$) + 14.43_{-5.97}^{+4.55}$. The slope of −0.31 is consistent at 1σ with previously identified correlations in the optical/ultraviolet and X-ray light curves. We speculate that differences in the rate at which energy is released by the central engine or differences in observer viewing angle may be responsible for the correlation.

The Temporal Symmetrical and Translational Structure in Gamma-Ray Burst Light Curves

Tremendous information is hidden in the light curve of a gamma-ray burst (GRB). Based on Compton Gamma Ray Observatory/Burst And Transient Source Experiment (BATSE) data, Hakkila found a majority of GRBs can be characterized by a smooth, single-peaked component superposed with a temporally symmetrical residual structure, i.e., a mirror feature for the fast-varying component. In this study, we conduct a similar analysis on the same data, as well as on Fermi/Gamma-Ray Burst Monitor data. We obtained a similar conclusion, which is that most GRBs have this symmetrical fast-varying component. Furthermore, we chose an alternative model to characterize the smooth component and used a three-parameter model to identify the residual, i.e., the fast component. By choosing 226 BATSE GRBs based on a few criteria, we checked the time-symmetrical feature and time-translational feature for the fast components and found the ratio is roughly 1:1. We propose that both features could come from the structure of the ejected shells. In the future, the Square Kilometre Array might be able to observe the early radio emission from the collision of the shells.

The signature of refreshed shocks in the afterglow of GRB 030329

ABSTRACT GRB 030329 displays one clear and, possibly, multiple less intense fast-rising (Δt/t ∼ 0.3) jumps in its optical afterglow light curve. The decay rate of the optical light curve remains the same before and after the flux jumps. This may be the signature of energy injection into the shocked material at the front of the jet. In this study, we model the Gamma-ray Burst (GRB) ejecta as a series of shells. We follow the dynamical evolution of the ejecta as it interacts with itself (i.e. internal shocks) and with the circumburst medium (i.e. external forward and reverse shocks), and calculate the emission from each shock event assuming synchrotron emission. We confirm the viability of the proposed model in which the jumps in the optical afterglow light curve of GRB 030329 are produced via refreshed shocks. The refreshed shocks may be the signatures of collisions between earlier ejected material with an average Lorentz factor $\bar{\Gamma }\gtrsim 100$ and later ejected material with $\bar{\Gamma } \sim 10$ once the early material has decelerated due to interaction with the circumburst medium. We show that even if the late material is ejected with a spread of Lorentz factors, internal shocks naturally produce a narrow distribution of Lorentz factors (ΔΓ/Γ ≲ 0.1), which is a necessary condition to produce the observed quick rise times of the jumps. These results imply a phase of internal shocks at some point in the dynamical evolution of the ejecta, which requires a low magnetization in the outflow.

A Stochastic Approach to Reconstruct Gamma-Ray-burst Light Curves

Gamma-ray bursts (GRBs), as they are observed at high redshift (z = 9.4), are vital to cosmological studies and investigating Population III stars. To tackle these studies, we need correlations among relevant GRB variables with the requirement of small uncertainties on their variables. Thus, we must have good coverage of GRB light curves (LCs). However, gaps in the LC hinder the precise determination of GRB properties and are often unavoidable. Therefore, extensive categorization of GRB LCs remains a hurdle. We address LC gaps using a stochastic reconstruction, wherein we fit two preexisting models (the Willingale model; W07; and a broken power law; BPL) to the observed LC, then use the distribution of flux residuals from the original data to generate data to fill in the temporal gaps. We also demonstrate a model-independent LC reconstruction via Gaussian processes. At 10% noise, the uncertainty of the end time of the plateau, its correspondent flux, and the temporal decay index after the plateau decreases by 33.3%, 35.03%, and 43.32% on average for the W07, and by 33.3%, 30.78%, 43.9% for the BPL, respectively. The uncertainty of the slope of the plateau decreases by 14.76% in the BPL. After using the Gaussian process technique, we see similar trends of a decrease in uncertainty for all model parameters for both the W07 and BPL models. These improvements are essential for the application of GRBs as standard candles in cosmology, for the investigation of theoretical models, and for inferring the redshift of GRBs with future machine-learning analyses.

A Concept of Assessment of LIV Tests with THESEUS Using the Gamma-Ray Bursts Detected by Fermi/GBM

According to Einstein’s special relativity theory, the speed of light in a vacuum is constant for all observers. However, quantum gravity effects could introduce its dispersion depending on the energy of photons. The investigation of the spectral lags between the gamma-ray burst (GRB) light curves recorded in distinct energy ranges could shed light on this phenomenon: the lags could reflect the variation of the speed of light if it is linearlydependent on the photon energy and a function of the GRB redshift. We propose a methodology to start investigating the dispersion law of light propagation in a vacuum using GRB light curves. This technique is intended to be fully exploited using the GRB data collected with THESEUS.

Afterglow Light Curves from Off-Axis GRB Jets in Stratified Circumburst Medium

We study the gamma-ray burst (GRB) afterglow light curves produced by an off-axis jet in a stratified circumburst medium and summarize the temporal indices of the coasting phase, the deceleration phase, the Newtonian phase, and the deep Newtonian phase for various viewing angles and power-law indices of medium density. Generally, the afterglow light curves of off-axis GRBs in the homogeneous interstellar medium have a steep rise arising due to jet deceleration. In the stratified medium, the flux rises is more shallow but peaks earlier for the same viewing angle due to faster deceleration of the jet running into the denser stratified medium, compared with the case of the interstellar medium (ISM). Observations of off-axis bursts will possibly increase over the coming years due to the arrival of the multi-messenger era and the forthcoming surveys in multiple bands. The temporal indices of off-axis afterglows derived in the paper will provide a reference for comparison with the observations and can diagnose the circumburst environment. The numerical code calculating the afterglow light curve also can be used to fit the multi-wavelength light curves.

The Synchrotron Low-energy Spectrum Arising from the Cooling of Electrons in Gamma-Ray Bursts

This work is a continuation of a previous effort (Panaitescu) to study the cooling of relativistic electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses, with application to the spectra and light curves of the synchrotron gamma-ray burst (GRB) produced by such cooling electrons. Here, we derive the low-energy slope β LE of a GRB pulse-integrated spectrum and quantify the implications of the measured distribution of β LE. Radiative processes that produce soft integrated spectra can accommodate the harder slopes measured by CGRO/BATSE and Fermi/GBM only if the magnetic field lifetime t B is shorter than the time during which the typical GRB electrons cool to radiate below 1–10 keV, which is less than (at most) 10 radiative cooling timescales t rad of the typical GRB electron. In this case, there is a one-to-one correspondence between t B and β LE. To account for low-energy slopes β LE > −3/4, the adiabatic electron-cooling requires a similar restriction on t B . In this case, the diversity of slopes arises mostly from how the electron-injection rate varies with time (temporal power-law injection rates yield power-law low-energy GRB spectra) and not from the magnetic field timescale.

Comprehension of Jet Physics from the Analysis of the Gamma‐Ray Burst Afterglow Parameter Distributions

AbstractThe discovery of a short gamma‐ray burst (GRB), GRB170817A, in association with a gravitational wave (GW) and a bright kilonova started a new era in the high‐energy astrophysics. The observation of GRB170817A and more recently, GRB200826A and GRB211211A, a short and a long burst, respectively, with a possible kilonova, reinforce the concern about new ways of classification. For this reason, a new machine learning technique is applied to Swift‐BAT data, searching for morphological similarities in the light curves. The resulting map is characterized by two distinct groups, although still correlated with standard T90 duration. Since a jet viewed off‐axis could explain the emission from GRB170817A, the modeling of this kind of sources is of great importance. A public code called JetFit, based on the “boosted fireball” model, is applied to fit Swift‐XRT afterglow light curves of short and long GRBs, with known red‐shift, from 2005 to 2021. JetFit does not model the flaring activity. For this purpose, a new technique to remove the time flaring phases, is developed. This analysis provides a comprehensive study of the prompt and of the afterglow phase, trough the study of the best‐fit parameters.

On the Jet Structures of GRB 050820A and GRB 070125

We present the broadband numerical modeling of afterglows for two remarkably bright long gamma-ray bursts (GRBs), GRB 050820A and GRB 070125, with a wide range of observations from the radio band to the X-ray band. In our work, we fit light curves and constrain physical parameters using a standard forward shock model from the afterglowpy Python package, considering different jet structures and the jet lateral expansion. For GRB 050820A, the constrained jet is close to a top-hat jet with an extremely small half opening angle of about 0.015 rad, and the circumburst matter density is as small as 10−7 cm−3, which suggests that this peculiar long GRB might originate from metal-poor stars with low mass-loss rates. To explain the late time optical light curves of GRB 070125, the effects of the lateral expansion and the participation factor of electrons that are accelerated by the shock have to be taken into account. The constrained results for GRB 070125 show that the jet is also close to a top-hat jet with a half opening angle of about 0.1 rad, the viewing angle is about 0.05 rad, the circumburst density is about 10 cm−3, and the participation factor is about 0.1. The jet energy of the two bursts is required to be ∼1051–1052 erg, which can be produced by a millisecond magnetar or a hyper-accreting black hole.

The spectral analysis and study of GRB 120709A, a burst with three distinct emission episodes

The reason for quiescent intervals and isolated emission episodes in gamma-ray burst (GRB) light curves is still not well understood. Probing these questions further requires the study of the spectral characteristics and properties of the outflow of such GRBs. GRB 120709A is a burst with three distinct emission episodes, separated by quiescent periods lasting several seconds. The episodes are comparable with each other in terms of peak flux and duration. The GRB was observed by the Fermi LAT and GBM instruments, covering over six orders of magnitude of energy. In this work, we carry out the time-integrated and time-resolved spectral analysis of GRB 120709A. We use these results to study the temporal variation of its parameters. In particular, we find that the peak energy of the spectrum of each episode traces the flux of those episodes. We also study and parameterise the correlation between the isotropic luminosity and intrinsic peak energy. Finally, we determine the parameters of the outflow based on gamma-gamma opacity calculations. We find that if we require the MeV emission to occur above the photosphere, then the ratio of the GeV and MeV emission radii can not be greater than ∼1.2. This implies an internal origin of the GeV emission. This also allows us to estimate the average value of the Lorentz factor to be in the ∼100–200 range. We also evaluate the photospheric and MeV emission radii and find them to be in the (4.5–5.3) ×1012 cm and (2.1–5.2) ×1014 cm ranges, respectively. We note the limitations of this exercise due to the relative faintness of the GRB which prevents us from doing a very fine time-resolved analysis or fitting the spectra with multi-component models. We also discuss GRB 120709A in the context of models for distinct emission episodes and quiescent intervals.