Gamma-ray Burst Pulses Research Articles

A mega-electron volt emission line in the spectrum of a gamma-ray burst.

A long gamma-ray burst (GRB) is observed when the collapse of a massive star produces an ultrarelativistic outflow pointed toward Earth. Gamma-ray spectra of long GRBs are smooth, typically modeled by joint power-law segments describing a continuum, with no detected spectral lines. We report a significant (>6σ) narrow emission feature at ~10 mega-electron volts (MeV) in the spectrum of the bright GRB 221009A. Over 80 seconds, it evolves in energy (~12 to ~6 MeV) and in luminosity (~1.1 to <0.43 × 1050 erg second-1) but has a constant width of ~1 MeV. We interpret this feature as a blueshifted spectral line produced by the annihilation of electron-positron pairs, potentially in the same location responsible for emitting the brightest GRB pulses.

The polarization-angle flip in GRB prompt emission

Context. In recent years, some polarization measurements of gamma-ray bursts (GRBs) have been reported, and the polarization-angle (PA) rotation in the prompt emission phase has been found in several bursts. The physical mechanism of the PA evolution is still unclear. In this work, we studied the origin of the PA rotation in a toroidal magnetic field. Aims. We aim to provide an explanation for the PA rotation in GRBs and find the physical conditions that lead to the rotation by 90 degrees in the toroidal magnetic-field (MF) model. Moreover, we present some observable polarization properties in the MF model that can be tested in the future. Methods. We calculated the instantaneous polarization degree (PD) from a top-hat jet with different normalized viewing angles (q = θv/θj), jet opening angles (θj), and jet Lorentz factors (Γ) in three wavebands. When the PD changes between positive and negative values, it means that the PA flips by 90 degrees. On these grounds, we can summarize the range of parameters required for these PA flips. Considering these parameter conditions, we can further estimate the observed rate of the GRBs exhibiting such PA rotations. Results. We find that the PA rotation in the toroidal MF is primarily related to three critical factors: the viewing angle, the jet opening angle, and the jet Lorentz factor. Additionally, the PA can experience flips of 90 degrees twice. The conditions for the flips are q ≳ 0.5 (except for q ≃ 1) and yj = (Γθj)2 ≳ 4. However, the two flips in the PA might not be concurrently observable due to the constraint of flux. Taking these conditions into account and assuming a random orientation between the jet axis and the line of sight (LOS), we obtain a theoretical upper limit (without any constraints) for the observed rate of GRBs in the X-ray or γ-ray band displaying the flips in PA as Rch ≲ 80%. We further constrain the observed rate as Rch ∼ 16% according to the maximal post-flip polarized flux level, where the observed rate of single and double flips each account for ∼8%. It should be noted that the observed rates are different in various wavebands. The observed rate of the second PA flip in the optical bands should be higher than that in the X-ray or γ-ray band since the flux in the optical band declines much slower than that in the X-ray or γ-ray band. Moreover, when the LOS is close to the jet edge (q → 1), it is the easiest case in which to observe the 90-degree PA flip due to the relatively high post-flip polarized flux level. The first and second PA flips in a GRB pulse are most likely to occur at the observed times of tobs ∼ [2 − 3]tpeak and ∼[3 − 4]tpeak, respectively, where tpeak is the peak time of the pulse. It is also noted that the PA flip would not happen before the peak time.

Gamma-Ray Burst Pulses and Lateral Jet Motion

We propose that gamma-ray burst (GRB) pulses are produced when highly relativistic jets sweep across an observer’s line of sight. We hypothesize that axisymmetric jet profiles, coupled with special relativistic effects, produce the time-reversed properties of GRB pulses. Curvature resulting from rapid jet expansion is responsible for much of the observed pulse asymmetry and hard-to-soft evolution. The relative obliqueness with which the jet crosses the line of sight explains the known GRB pulse morphological types. We explore two scenarios: one in which a rigid/semirigid jet moves laterally and another in which a ballistic jet sprays material from a laterally moving nozzle. The ballistic jet model is favored based upon its consistency with standard emission mechanisms.

Distributions of Energy, Luminosity, Duration, and Waiting Times of Gamma-Ray Burst Pulses with Known Redshift Detected by Fermi/GBM

Discovered more than 50 years ago, gamma-ray burst (GRB) prompt emission remains the most puzzling aspect of GRB physics. Its complex and irregular nature should reveal how newborn GRB engines release their energy. In this respect, the possibility that GRB engines could operate as self-organized critical (SOC) systems has been put forward. Here, we present the energy, luminosity, waiting time, and duration distributions of individual pulses of GRBs with known redshift detected by the Fermi Gamma-ray Burst Monitor. This is the first study of this kind in which selection effects are accounted for. The compatibility of our results with the framework of SOC theory is discussed. We found evidence for an intrinsic break in the power-law models that describe the energy and the luminosity distributions.

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.

Temporal and Spectral Evolution of Gamma-Ray Burst Broad Pulses: Identification of High-latitude Emission in the Prompt Emission

We perform a detailed analysis of broad pulses in bright gamma-ray bursts (GRBs) to understand the evolution of GRB broad pulses. Using the temporal and spectral properties, we test the high-latitude emission (HLE) scenario in the decaying phase of broad pulses. The HLE originates from the curvature effect of a relativistic spherical jet, where higher-latitude photons are delayed and softer than the observer’s line-of-sight emission. The signature of HLE has not yet been identified undisputedly during the prompt emission of GRBs. The HLE theory predicts a specific relation, ∝ , between the peak energy E p in ν F ν spectra and the spectral flux F ν measured at E p , . We search for evidence of this relation in 2157 GRBs detected by the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope from 2008 to 2017. After imposing unbiased selection criteria in order to minimize contamination in a signal by background and overlaps of pulses, we build a sample of 32 broad pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly consistent with the HLE relation. For the 18 broad pulses, the exponent δ in the relation of ∝ E p δ is distributed as a Gaussian function with a median and width of 1.99 and 0.34, respectively. This result provides a constraint on the emission radius of GRBs with the HLE signature.

Evidence for the Self-organized Criticality Phenomenon in the Prompt Phase of Short Gamma-Ray Bursts

The prompt phase of gamma-ray bursts (GRBs) contains essential information regarding their physical nature and central engine, which are as yet unknown. In this paper, we investigate the self-organized criticality phenomenon in GRB prompt phases as done in X-ray flares of GRBs. We obtain the differential and cumulative distributions of 243 short GRB pulses, such as peak flux, FWHM, rise time, decay time, and peak time in the fourth BATSE Time-Tagged Event Catalog with the Markov Chain Monte Carlo technique. It is found that these distributions can be well described by power-law models. In particular, comparisons are made with 182 short GRB pulses in the third Swift GRB Catalog from 2004 December to 2019 July. The results are essentially consistent with the BATSE ones. We notice that there is no obvious power-law index evolution across different energy bands for either BATSE or Swift short GRBs. The joint analysis suggests that the GRB prompt phase can be explained by a fractal-diffusive self-organized criticality system with the spatial dimension S = 3 and the classical diffusion β = 1. Our findings show that GRB prompt phases and X-ray flares possess the very same magnetically dominated stochastic process and mechanism.

Jets in a Gamma-Ray Burst during Its Prompt Emission: Evolution of the Lorentz Factor

Knowledge about the Lorentz factor and its evolution of relativistic jets in gamma-ray bursts (GRBs) is crucial to understand their physics. An exact value of the bulk Lorentz factor can be estimated based on a high-energy spectral cutoff, which may appear in the GRBs’ prompt emission owing to the absorption of photon–photon pair production. In this work, we focus on an investigation of the bulk Lorentz factor evolution of jets in an individual burst. Based on Fermi observations, we search for bursts with multiple GRB pulses characterized by a high-energy spectral cutoff, and nine GRBs are obtained. Together with the estimation of the pulse duration and radiation spectrum, the Lorentz factor of jets corresponding to different pulses in an individual GRB is estimated. It is shown that the Lorentz factor of jets in an individual GRB fluctuates within a certain range and without a general trend in these nine GRBs. In addition, the Lorentz factors of the jets in GRBs 130821A, 160509A, and 160625B seem to increase with time. We also study the relations among L iso, E p,z, and Γ for the pulses in our sample, which are found to be consistent with those found in previous works.

Properties of the Prompt Optical Counterpart Arising from the Cooling of Electrons in Gamma-Ray Bursts

This work extends a contemporaneous effort to study the properties of the lower-energy counterpart synchrotron emission produced by the cooling of relativistic gamma-ray burst (GRB) electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses. We derive the major characteristics (pulse duration, lag time after burst, and brightness relative to the burst) of the prompt optical counterpart (POC) occurring during or after the GRB. Depending on the magnetic field lifetime, duration of electron injection, and electron transit time Δt o from hard X-ray (GRB) to optical-emitting energies, a POC may appear during the GRB pulse (of duration δ t γ ) or after (delayed OC). The signature of counterparts arising from the cooling of GRB electrons is that POC pulses (Δt o < δ t γ ) last as long as the corresponding GRB pulse (δ t o ≃ δ t γ ), while delayed OC pulses (Δt o > δ t γ ) last as long as the transit time (δ t o ≃ Δt o ). If OC variability can be measured, then another signature for this OC mechanism is that the GRB variability is passed on to POCs but not to delayed OCs. Within the GRB electron cooling model for counterparts, POCs should be on average dimmer than delayed ones (consistent with the data), and harder GRB low-energy slopes β LE should be associated more often with the dimmer POCs. The latter sets an observational bias against detecting POCs from (the cooling of electrons in) GRBs with a hard slope β LE, making it more likely that the detected POCs of such bursts arise from another mechanism.

The hardness–intensity correlation of photospheric emission from a structured jet for gamma-ray bursts

ABSTRACT For many gamma-ray bursts (GRBs), hardness–intensity correlation (HIC) can be described by a power-law function, Ep ∝ Fκ, where Ep is the peak energy of νFν spectrum, and F is the instantaneous energy flux. In this paper, HIC of the non-dissipative photospheric emission from a structured jet is studied in different regimes. An intermediate photosphere, which contains both of unsaturated and saturated emissions is introduced, and we find positive κ &amp;lt; 1/4 in this case. The same conclusion could be generalized to the photospheric emission from a hybrid jet without magnetic dissipations, or that with sub-photospheric magnetic dissipations and fully thermalized. This may imply that the contribution peaking at ∼1/2 in the distribution of observed κ are mainly from the prompt emission of GRBs with synchrotron origin. Besides, emissions of the intermediate photosphere could give a smaller low-energy photon index α than that in the unsaturated regime, and naturally reproduce anticorrelation in α − Ep in a GRB pulse.

A Comprehensive Study of Multiflare GRB Spectral Lag

Abstract We select 48 multiflare gamma-ray bursts (GRBs) (including 137 flares) from the Swift/XRT database and estimate the spectral lag with the discrete correlation function. It is found that 89.8% of the flares have positive lags and only 9.5% of the flares show negative lags when fluctuations are taken into account. The median lag of the multiflares (2.75 s) is much greater than that of GRB pulses (0.18 s), which can be explained by the fact that we confirm that multiflare GRBs and multipulse GRBs have similar positive lag–duration correlations. We investigate the origin of the lags by checking the E peak evolution with the two brightest bursts and find the leading models cannot explain all of the multiflare lags and there may be other physical mechanisms. All of the results above reveal that X-ray flares have the same properties as GRB pulses, which further supports the observation that X-ray flares and GRB prompt-emission pulses have the same physical origin.

How Temporal Symmetry Defines Morphology in BATSE Gamma-Ray Burst Pulse Light Curves

Abstract We present compelling evidence that most gamma-ray burst (GRB) pulse light curves can be characterized by a smooth single-peaked component coupled with a more complex emission structure that is temporally symmetric around the time of the pulse peak. The model successfully fits 86% of Burst and Transient Source Experiment GRB pulses bright enough for structural properties to be measured. Surprisingly, a GRB pulse’s light-curve morphology can be accurately predicted by the pulse asymmetry and the stretching/compression needed to align the structural components preceding the temporal mirror with the time-reversed components following it. Such a prediction is only possible because GRB pulses exhibit temporal symmetry. Time-asymmetric pulses include fast rise exponential decays, rollercoaster pulses, and asymmetric u-pulses, while time-symmetric pulses include u-pulses and crowns. Each morphological type is characterized by specific asymmetries, stretching parameters, durations, and alignments between the smooth and structured components, and a delineation in the asymmetry/stretching distribution suggests that symmetric pulses and asymmetric pulses may belong to separate populations. Furthermore, pulses belonging to the short GRB class exhibit similar morphologies to the long GRB class, but appear to simply occur on shorter timescales.

Light-curve Test of GRB 090717 as a Gravitationally Lensed Echo

Abstract Light curve comparison is a simple but statistically powerful test for the presence of gravitational lensing in gamma-ray bursts (GRBs). Since gravitational lensing does not change relative timing internal to images, all source images created by gravitational lensing should show the same light curve to within a scale factor in amplitude. Applied to the recent claim that the two pulses in GRB 090717 are lensed images of the same pulse, it was found that the light curves for these pulses actually differ at about the 5.0σ confidence level (99.999956%) in a χ 2 comparison test. Under the given assumptions, GRB 090717 therefore does not present a compelling example of gravitational lensing.

Hardness Test of GRB 950830 as a Gravitationally Lensed Echo

Abstract Cumulative hardness comparisons are a simple but statistically powerful test for the presence of gravitational lensing in gamma-ray bursts (GRBs). Since gravitational lensing does not change photon energies, all source images should have the same spectra—and hence hardness. Applied to the recent claim that the two pulses in GRB 950830 are lensed images of the same pulse, the measured flux ratio between the two main pulses should be the same at all energies. After summing up all the counts in both of GRB 950830's two pulses in all four BATSE energy bands, it was found that in energy channel 3, the second pulse appears somewhat weak. In comparison with the other BATSE energy channels, the difference was statistically significant at above 90%. This model-independent test indicates that the case for GRB 950830 involving a gravitational lens may be intriguing—but should not be considered proven.

Adiabatic and Radiative Cooling of Relativistic Electrons Applied to Synchrotron Spectra and Light Curves of Gamma-Ray Burst Pulses

Abstract We investigate the adiabatic and radiative (synchrotron and inverse-Compton) cooling of relativistic electrons whose injected or initial distribution with energy is a power law. Analytical and numerical results are presented for the cooling-tail and the cooled-injected distribution that develop below and above the typical energy of injected electrons, for the evolution of the peak energy E p of the synchrotron emission spectrum. The pulse shape resulting from an episode of electron injection is also analyzed. The synchrotron emission calculated numerically is compared with the spectrum and shape of Gamma-ray burst (GRB) pulses. Both adiabatic and radiative cooling processes lead to a softening of the pulse spectrum, and both types of cooling processes lead to pulses peaking earlier and lasting shorter at higher energy, quantitatively consistent with observations. For adiabatic-dominated electron cooling, a power-law injection rate R i suffices to explain the observed power-law GRB low-energy spectra. Synchrotron-dominated cooling leads to power-law cooling-tails that yield the synchrotron standard slope α = −3/2 provided that R i ∼ B 2, which is exactly the expectation if the magnetic field is a constant fraction of the post-shock energy density. Increasing (decreasing) R i and decreasing (increasing) B(t) lead to harder (softer, respectively) slopes α than the standard value and to nonpower-law (curved) cooling-tails. Inverse-Compton cooling yields four values for the slope α but, as for synchrotron, other R i or B histories yield a wider range of slopes and curved low-energy spectra. Feedback between the power-law segments that develop below and above the typical injected electron leads to a synchrotron spectrum with many breaks.

Time-reversed Gamma-Ray Burst Light-curve Characteristics as Transitions between Subluminal and Superluminal Motion

Abstract We introduce a simple model to explain the time-reversed and stretched residuals in gamma-ray burst (GRB) pulse light curves. In this model an impactor wave in an expanding GRB jet accelerates from subluminal to superluminal velocities, or decelerates from superluminal to subluminal velocities. The impactor wave interacts with the surrounding medium to produce Cerenkov and/or other collisional radiation when traveling faster than the speed of light in this medium, and other mechanisms (such as thermalized Compton or synchrotron shock radiation) when traveling slower than the speed of light. These transitions create both a time-forward and a time-reversed set of light-curve features through the process of relativistic image doubling. The model can account for a variety of unexplained yet observed GRB pulse behaviors, including the amount of stretching observed in time-reversed GRB pulse residuals and the relationship between stretching factor and pulse asymmetry. The model is applicable to all GRB classes since similar pulse behaviors are observed in long/intermediate GRBs, short GRBs, and X-ray flares. The free model parameters are the impactor’s Lorentz factor when moving subluminally, its Lorentz factor when moving superluminally, and the speed of light in the impacted medium.

Smoke and Mirrors: Signal-to-noise Ratio and Time-reversed Structures in Gamma-Ray Burst Pulse Light Curves

Abstract We demonstrate that the “smoke” of limited instrumental sensitivity smears out structure in gamma-ray burst (GRB) pulse light curves, giving each a triple-peaked appearance at moderate signal-to-noise ratio (S/N) and a simple monotonic appearance at low S/N. We minimize this effect by studying six very bright GRB pulses (S/N generally &gt;100), discovering surprisingly that each exhibits complex time-reversible wavelike residual structures. These “mirrored” wavelike structures can have large amplitudes, occur on short timescales, begin/end long before/after the onset of the monotonic pulse component, and have pulse spectra that generally evolve hard to soft, rehardening at the time of each structural peak. Among other insights, these observations help explain the existence of negative pulse spectral lags and allow us to conclude that GRB pulses are less common, more complex, and have longer durations than previously thought. Because structured emission mechanisms that can operate forward and backward in time seem unlikely, we look to kinematic behaviors to explain the time-reversed light-curve structures. We conclude that each GRB pulse involves a single impactor interacting with an independent medium. Either the material is distributed in a bilaterally symmetric fashion, the impactor is structured in a bilaterally symmetric fashion, or the impactor’s motion is reversed such that it returns along its original path of motion. The wavelike structure of the time-reversible component suggests that radiation is being both produced and absorbed/deflected dramatically, repeatedly, and abruptly relative to the emission of the monotonic component.

How pulses in short gamma-ray bursts constrain HMXRB evolution

AbstractWe demonstrate how pulse structures in Short gamma-ray bursts (SGRBs), coupled with observations of GRB/GW 170817A, constrain the geometries of dying HMXRB systems composed of merging neutron stars.

Properties of Short Gamma-ray Burst Pulses from a BATSE TTE GRB Pulse Catalog

Abstract We analyze pulse properties of short gamma-ray bursts (GRBs) from a new catalog containing 434 pulses from 387 BATSE time-tagged event (TTE) GRBs. Short GRB pulses exhibit correlated properties of duration, fluence, hardness, and amplitude, and they evolve hard to soft while undergoing similar triple-peaked light curves similar to those found in long/intermediate bursts. We classify pulse light curves using their temporal complexities, demonstrating that short GRB pulses exhibit a range of complexities from smooth to highly variable. Most of the bright, hard, chaotic emission seen in complex pulses seems to represent a separate highly variable emission component. Unlike long/intermediate bursts, as many as 90% of short GRBs are single-pulsed. However, emission in short multipulsed bursts is coupled such that the first pulse’s duration is a predictor of both the interpulse separation and subsequent pulse durations. These results strongly support the idea that external shocks produce the prompt emission seen in short GRBs. The similarities between the triple-peaked structures and spectral evolution of long, short, and intermediate GRBs then suggests that external shocks are responsible for the prompt emission observed in all GRB classes. In addition to these findings, we identify a new type of gamma-ray transient in which peak amplitudes occur at the end of the burst rather than at earlier times. Some of these “crescendo” bursts are preceded by rapid-fire “staccato” pulses, whereas the remaining are preceded by a variable episode that could be unresolved staccato pulses.

Spectral evolution of GRBs with negative spectral lag using Fermi GBM observations

The positive spectral lag of Gamma Ray Bursts (GRBs) is often explained in terms of hard-to-soft spectral evolution of GRB pulses. While positive lags of GRBs is very common, there are few GRB pulses that exhibits negative spectral lags. In the present work we examine whether negative lags of GRBs also can be interpreted in terms of spectral evolution of GRB pulses or not. Using Fermi-GBM data, we identify two GRBs, GRB 090426C and GRB 150213A, with clean pulses that exhibit negative spectral lag. An indication of soft to hard transition has been noticed for the negative spectral lag events from the spectral evolution study. The implication of the present findings on the models of GRB spectral lags are discussed.