Prompt Emission Phase Research Articles

Robust constraints on the physics of the MeV emission line in GRB 221009A from optical depth arguments

ABSTRACT The brightest-of-all-time gamma-ray burst (GRB), GRB 221009A, is the first GRB observed to have emission line (up to 37 MeV) in its prompt emission spectra. It is naturally explained as $e^-/e^+$ annihilation line that was Doppler boosted in the relativistic jet of the GRB. In this work, we repeatedly apply the simple optical depth argument to different physical processes necessary to produce an observable $e^-/e^+$ annihilation line. This approach results in robust constraints on the physics of the line: We conclude that in GRB 221009A, the $e^-/e^+$ pairs were produced at a radius greater than $4.3\times 10^{15}$ cm from the central engine, and annihilated in a region between $1.4\times 10^{16}$ and $4.3\times 10^{16}$ cm. From these constraints, we established a self-consistent picture of $e^-/e^+$ production, cooling, and annihilation. We also derived a criterion for pair production in the GRB prompt emission: $E_{\rm {iso}} \gtrsim 3.3\times 10^{53} E_{\rm {peak},100} (1+z) R^2_{\rm {prod},16}~\text{erg}$. Using this criterion, we find tens of candidate GRBs that could have produced $e^-/e^+$ in prompt emissions to annihilate. GRB 221009A is with the highest likelihood according to this criterion. We also predict the presence of a thermal radiation, with a time-evolving blackbody temperature, sweeping through soft X-ray during the prompt emission phase.

Early photometric and spectroscopic observations of the extraordinarily bright INTEGRAL-detected GRB221009A

initially detected as an X-ray transient by was later revealed to have triggered the satellite about an hour earlier, marking it as a post-peak observation of the event's emission. This GRB distinguished itself as the brightest ever recorded, presenting an unparalleled opportunity to probe the complexities of GRB physics. The unprecedented brightness, however, challenged observation efforts, as it led to the saturation of several high-energy instruments. Our study seeks to investigate the nature of the and elucidate the environmental conditions conducive to these exceptionally powerful bursts. Moreover, we aim to understand the fundamental physics illuminated by the detection of teraelectronvolt (TeV) photons emitted by grb. We conducted detailed analyses of early photometric and spectroscopic observations that span from the trigger through to the initial days following the prompt emission phase in order to characterize afterglow, and we complemented these analyses with a comparative study. Our findings from analyzing data confirm as the most energetic event observed to date. Early optical observations during the prompt phase negate the presence of bright optical emissions with internal or external shock origins. Spectroscopic analyses enabled us to measure distance and line-of-sight properties. The afterglow's temporal and spectral analysis suggests prolonged activity of the central engine and a transition in the circumburst medium's density. Finally, we discuss the implications for fundamental physics of detecting photons as energetic as 18 TeV from grb. Early optical observations have proven invaluable for distinguishing between the potential origins of optical emissions in underscoring their utility in GRB physics studies. However, the rarity of such data underscores the need for dedicated telescopes capable of synchronous multiwavelength observations. Additionally, our analysis suggests that the host galaxies of TeV GRBs share commonalities with those of long and short GRBs. Expanding the sample of TeV GRBs could further solidify these findings.

Polarization Degree of Magnetic Field Structure Changes Caused by Random Magnetic Field in Gamma-Ray Burst

In a Poynting-flux-dominated jet that exhibits an ordered magnetic field, a transition toward turbulence and magnetic disorder follows after magnetic reconnection and energy dissipation during the prompt emission phase. In this process, the configuration of the magnetic field evolves with time, rendering it impossible to entirely categorize the magnetic field as ordered. Therefore, we assumed a crude model that incorporates a random magnetic field and an ordered magnetic field, and takes into account the proportionality of the random magnetic field strength to the ordered magnetic field, in order to compute the polarization degree (PD) curve for an individual pulse. It has been discovered that the random magnetic field has a significant impact on the PD results of the low-energy X-ray. In an ordered magnetic field, the X-ray segment maintains a significant PD compared to those in the hundreds of keV and MeV ranges even after electron injection ceases, making PD easier to detect by polarimetry. However, when the random magnetic field is introduced, the low-energy and high-energy PDs exhibit a similar trend, with the X-ray PD being lower than that of the high-energy segment. Of course, this is related to the rate of disorder in the magnetic field. Additionally, there are two rotations of the polarization angles (PAs) that were not present previously, and the rotation of the PA in the high-energy segment occurs slightly earlier. These results are unrelated to the structure of the ordered magnetic field.

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.

What Powered the Kilonova-like Emission after GRB 230307A in the Framework of a Neutron Star–White Dwarf Merger?

The second brightest gamma-ray burst, GRB 230307A (with a duration T 90 ∼ 40 s), exhibited characteristics indicative of a magnetar engine during the prompt emission phase. Notably, a suspected kilonova was identified in its follow-up optical and infrared observations. Here we propose that the origin of GRB 230307A is a neutron star–white dwarf (NS–WD) merger as this could naturally explain the long duration and the large physical offset from the center of its host galaxy. In the framework of such an NS–WD merger event, the late-time kilonova-like emission is very likely to be powered by the spin-down of the magnetar and the radioactive decay of 56Ni, rather than by the decay of r-process elements as these heavy elements may not easily be synthesized in an NS-WD merger. It is demonstrated that the above scenario can be supported by our fit to the late-time observational data, where a mass of ∼10−3 M ⊙ 56Ni is involved in the ejecta of a mass of ∼0.1 M ⊙. Particularly, the magnetar parameters required by the fit are consistent with those derived from the early X-ray observation.

Constraining the Jet Composition of GRB 221009A with the Prompt TeV Emission Limit

Recent LHAASO observations of the prompt emission phase of the brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio between the TeV and MeV emissions, F TeV/F MeV ≤ 2 × 10−5, during the period 220–230 s after the trigger. This period covers the peak of the main MeV burst and is just before the TeV afterglow emerges. Within the framework of internal shocks, we study the internal γγ absorption in GRB 221009A by generating a set of synthetic bursts in a simulation that reproduces the observed feature of GRB 221009A. We find that the γγ absorption does not lead to an exponential cutoff, but rather a power-law spectrum, consistent with previous works. We further find that the attenuation due to γγ absorption alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low ratio between synchrotron self-Compton (SSC) and synchrotron emission outputs. This requires the magnetic field energy density to be much larger than the synchrotron photon energy density so that the SSC flux is greatly suppressed. This indicates that the jet composition of GRB 221009A is likely Poynting flux dominated.

GRB 221009A: Revealing a Hidden Afterglow during the Prompt Emission Phase with Fermi-GBM Observations

Recently, LHAASO reported the detection of the brightest-of-all-time GRB 221009A, revealing the early onset of a TeV afterglow. We analyze the spectral evolution of the X-ray/gamma-ray emission of GRB 221009A measured by the Fermi Gamma-ray Burst Monitor (GBM) during the dips of two prompt emission pulses (i.e., intervals T 0 + [300–328] s and T 0 + [338–378] s, where T 0 is the GBM trigger time). We find that the spectra at the dips transit from the Band function to a power-law function, indicating a transition from the prompt emission to the afterglow. After ∼T 0 + 660 s, the spectrum is well described by a power-law function, and the afterglow becomes dominant. Remarkably, the underlying afterglow emission at the dips smoothly connect with the afterglow after ∼T 0 + 660 s. The entire afterglow emission measured by GBM can be fitted by a power-law function F ∼ t −0.95±0.05, where t is the time since the first main pulse at T* = T 0 + 226 s, consistent with the TeV afterglow decay measured by LHAASO. The start time of this power-law decay indicates that the afterglow peak of GRB 221009A should be earlier than T 0 + 300 s. We also test the possible presence of a jet break in the early afterglow light curve, finding that both the jet break model and single power-law decay model are consistent with the GBM data. The two models cannot be distinguished with the GBM data alone because the inferred jet break time is quite close to the end of the GBM observations.

Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow

We report the discovery of GRB 221009A, the highest flux gamma-ray burst (GRB) ever observed by the Fermi Gamma-ray Burst Monitor (Fermi-GBM). This GRB has continuous prompt emission lasting more than 600 s, which smoothly transitions to afterglow emission visible in the Fermi-GBM energy range (8 keV–40 MeV), and total energetics higher than any other burst in the Fermi-GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the Fermi-GBM trigger time (t 0; 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock breakout, with significant emission up to ∼15 MeV. We characterize the onset of external shock at t 0 + 600 s and find evidence of a plateau region in the early-afterglow phase, which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the Fermi-GBM sample and find that this GRB has the highest total isotropic-equivalent energy (E γ,iso = 1.0 × 1055 erg) and second highest isotropic-equivalent luminosity (L γ,iso = 9.9 × 1053 erg s–1) based on its redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of Fermi-GBM from the beginning of the prompt emission phase through the onset of early afterglow.

Photometric and Spectroscopic Observations of GRB 190106A: Emission from Reverse and Forward Shocks with Late-time Energy Injection

Early optical observations of gamma-ray bursts can significantly contribute to the study of the central engine and physical processes therein. However, of the thousands observed so far, only a few have data at optical wavelengths in the first minutes after the onset of the prompt emission. Here we report on GRB 190106A, whose afterglow was observed in optical bands just 36 s after the Swift/BAT trigger, i.e., during the prompt emission phase. The early optical afterglow exhibits a bimodal structure followed by a normal decay, with a faster decay after ∼T 0 + 1 day. We present optical photometric and spectroscopic observations of GRB 190106A. We derive the redshift via metal absorption lines from Xinglong 2.16 m/BFOSC spectroscopic observations. From the BFOSC spectrum, we measure z = 1.861 ± 0.002. The double-peak optical light curve is a significant feature predicted by the reverse-forward external-shock model. The shallow decay followed by a normal decay in both the X-ray and optical light curves is well explained with the standard forward-shock model with late-time energy injection. Therefore, GRB 190106A offers a case study for GRB emission from both reverse and forward shocks.

Self-similarities and Power Laws in the Time-resolved Spectra of GRB 190114C, GRB 130427A, GRB 160509A, and GRB 160625B

Binary-driven hypernova (BdHN) models have been adopted to explain the observed properties of long gamma-ray bursts (GRBs). Here, we perform a comprehensive data analysis (temporal and spectral analysis, GeV emission, and afterglow) on GRB 130427A, GRB 160509A, and GRB 160625B. We identify three specific episodes characterized by different observational signatures and show that these episodes can be explained and predicted to occur within the framework of the BdHNe I model, as first observed in GRB 190114C and reported in an accompanying paper. Episode 1 includes the “SN-rise” with the characteristic cutoff power-law spectrum; Episode 2 is initiated by the moment of formation of the black hole, coincident with the onset of the GeV emission and the ultrarelativistic prompt emission phase, and is characterized by a cutoff power law and blackbody spectra; Episode 3 is the “cavity,” with its characteristic featureless spectrum.

On the nature of the ultrarelativistic prompt emission phase of GRB 190114C and GRB 180720B

A profound difference has occurred in the analysis of GRBs initially analyzed in the domain of gamma-ray astronomy with the Compton Gamma Ray Observatory and the BATSE Detector with the extension to X-ray astronomy and optical astronomy introduced by Beppo-SAX and the KEK and VLT optical observatory, followed by the AGILE, Fermi mission, Niels Gehrels SWIFT mission as well as HEHSS and MAGIC observations. An authentic ``Copernican revolution" has occurred in the transition from the traditional model of GRBs originating in a single star progenitor (collapsar) to GRBs with a binary progenitor composed of a CO core and a companion NS. We evidence from 24 SN observations related to GRBs that all of them, once analyzed with the general relativistic corrections, can be identified with I Bc Sn with a common value of the luminosity and common time of occurrence of the peak of the optical emission. This can be understood in terms of a precursor composed of a CO core and a binary NS companion. By contrast, the GRBs differ profoundly in their energetics which can be expressed in terms of 3 different classes of BDHNe: BDHN I, BDHN II, BDHN III, also originating from precursors composed of a CO core and a binary companion. It is pointed out how the analysis of these systems has profoundly modified the concept of the BH and the associated fundamental physics necessary for their description since the publication of ``Introducing the Black Hole" by Ruffini and Wheeler in 1971. At first, it is illustrated how the most radical change has occurred in the introduction of the effective BH charge, with an electric field only function of the BH dimensionless spin and a background magnetic field $B0$. This definitely indicates the abandonment of the Kerr--Newman solution in favour of the Kerr metric, for energy extraction processes in BH physics. We then enter in the detailed description of the ``seven Seals", the seven Episodes characterizing GRB 190114c and the associated SN 2019jrj. Based on the article by Aimuratov et al., in preparation.

The structure of the ultrarelativistic prompt emission phase and the properties of the black hole in GRB 180720B

In analogy with GRB 190114C, we here analyze the ultrarelativistic prompt emission (UPE) of GRB 180720B observed in the rest-frame time interval t_{mathrm{rf}}=4.84–10.89 s by Fermi-GBM. We reveal the UPE hierarchical structure from the time-resolved spectral analysis performed in time sub-intervals: the spectrum in each shorter time interval is always fitted by a composite blackbody plus cutoff power-law model. We explain this structure with the inner engine of binary-driven hypernova (BdHN) model operating in a quantum electrodynamics (QED) regime. In this regime, the electric field induced by the gravitomagnetic interaction of the newborn Kerr BH with the surrounding magnetic field is overcritical, i.e., |mathbf{E}|ge E_c, where E_c=m_e^2 c^3/(ehbar ). The overcritical field polarizes the vacuum leading to an e^+~e^- pair plasma that loads baryons from the surroundings during its expansion. We calculate the dynamics of the self-acceleration of the pair-electromagnetic-baryon (PEMB) pulses to their point of transparency. We characterize the quantum vacuum polarization process in the sequences of decreasing time bins of the UPE by determining the radiation timescale, Lorentz factors, and transparency radius of the PEMB pulses. We also estimate the strength of the surrounding magnetic field sim 10^{14} G, and obtain a lower limit to the BH mass, M=2.4~M_odot , and correspondingly an upper limit to the spin, alpha = 0.6, from the conditions that the UPE is powered by the Kerr BH extractable energy and its mass is bound from below by the NS critical mass.

Probing the Progenitor of High-z Short-duration GRB 201221D and its Possible Bulk Acceleration in Prompt Emission

The growing observed evidence shows that the long- and short-duration gamma-ray bursts (GRBs) originate from massive star core-collapse and the merger of compact stars, respectively. GRB 201221D is a short-duration GRB lasting ∼0.1 s without extended emission at high redshift z = 1.046. By analyzing data observed with the Swift/BAT and Fermi/GBM, we find that a cutoff power-law model can adequately fit the spectrum with a soft keV, and isotropic energy . In order to reveal the possible physical origin of GRB 201221D, we adopted multi-wavelength criteria (e.g., Amati relation, ε-parameter, amplitude parameter, local event rate density, luminosity function, and properties of the host galaxy), and find that most of the observations of GRB 201221D favor a compact star merger origin. Moreover, we find that is larger than in the prompt emission phase which suggests that the emission region is possibly undergoing acceleration during the prompt emission phase with a Poynting-flux-dominated jet.

GRB 210121A: Observation of Photospheric Emissions from Different Regimes and the Evolution of the Outflow

GRB 210121A was observed by Insight-HXMT, by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM), and by the Fermi Gamma-ray Burst Monitor (Fermi/GBM) on 2021 January 21. In this work, photospheric emission from a structured jet is preferred to interpret the prompt emission phase of GRB 210121A, and emissions from different regimes are observed on-axis. Particularly, the emission from the intermediate photosphere is first observed in the first 1.3 s of the prompt emission, while emissions from the other part are dominant by the emissions from the saturated regime. This offers an alternative explanation compared with previous work. Moreover, the emissions that consider the intermediate photosphere can well interpret the changes in the low-energy photon index α during the pulses. In addition, the evolution of the outflow is extracted from a time-resolved analysis, and a correlation of is obtained, which implies that the jet may be mainly launched by neutrino annihilation in a hyper-accretion disk.

The GRB Prompt Emission: An Unsolved Puzzle

The recent multi-messenger and multi-wavelength observations of gamma-ray bursts (GRBs) have encouraged renewed interest in these energetic events. In spite of the substantial amount of data accumulated during the past few decades, the nature of the prompt emission remains an unsolved puzzle. We present an overview of the leading models for their prompt emission phase, focusing on the perspective opened by future missions.

A Comprehensive Consistency Check between Synchrotron Radiation and the Observed Gamma-Ray Burst Spectra

Abstract We performed a time-resolved spectral analysis of 53 bright gamma-ray bursts (GRBs) observed by Fermi/GBM. Our sample consists of 1117 individual spectra extracted from the finest time slices in each GRB. We fitted them with the synchrotron radiation model by considering the electron distributions in five different cases: monoenergetic, single power law, Maxwellian, traditional fast cooling, and broken power law. Our results were further qualified through the Bayesian information criterion (BIC) by comparing with the fit by empirical models, namely, the so-called Band function and cutoff power-law models. Our study showed that the synchrotron models, except for the fast-cooling case, can successfully fit most observed spectra, with the single power-law case being the most preferred. We also found that the electron distribution indices for the single power-law synchrotron fit in more than half of our spectra exhibit flux-tracking behavior, i.e., the index increases/decreases with the flux increasing/decreasing, implying that the distribution of the radiating electrons is increasingly narrower with time before the flux peaks and becomes more spreading afterward. Our results indicate that the synchrotron radiation is still feasible as a radiation mechanism of the GRB prompt emission phase.

Prevalence of Extra Power-Law Spectral Components in Short Gamma-Ray Bursts

Abstract A prompt extra power-law (PL) spectral component that usually dominates the spectral energy distribution below tens of keV or above ∼10 MeV has been discovered in some bright gamma-ray bursts (GRBs). However, its origin is still unclear. In this paper, we present a systematic analysis of 13 Fermi short GRBs, as of 2020 August, with contemporaneous keV–MeV and GeV detections during the prompt emission phase. We find that the extra PL component is a ubiquitous spectral feature for short GRBs, showing up in all 13 analyzed GRBs. The PL indices are mostly harder than −2.0, which may be well reproduced by considering the electromagnetic cascade induced by ultrarelativistic protons or electrons accelerated in the prompt emission phase. The average flux of these extra PL components positively correlates with that of the main spectral components, which implies they may share the same physical origin.

Evolution patterns of the peak energy in the GRB prompt emission

Context. The peak energy (Ep) exhibited during the prompt emission phase of gamma-ray bursts (GRBs) shows two different evolution patterns, namely hard-to-soft and intensity-tracking, of which the physical origin remains unknown. In addition to low-energy indices of GRB prompt spectra, the evolution patterns of Ep may be another crucial indicator with which to discriminate radiation mechanisms (e.g., synchrotron or photosphere) for GRBs. Aims. We explore the parameter space to find conditions that could generate different evolution patterns in the peak energy in the framework of synchrotron radiation. Methods. We have developed a code to calculate the synchrotron emission from a simplified shell numerically, considering: three cooling processes (synchrotron, synchrotron self-Compton (SSC), and adiabatic) of electrons, the effect of decaying magnetic field, the effect of the bulk acceleration of the emitting shell, and the effect of a variable source function that describes electrons accelerated in the emitting region. Results. After exploring the parameter space of the GRB synchrotron scenario, we find that the intensity-tracking pattern of Ep could be achieved in two situations. One is that the cooling process of electrons is dominated by adiabatic cooling or SSC+adiabatic cooling at the same time. The other is that the emitting region is under acceleration in addition to the cooling process being dominated by SSC cooling. Otherwise, hard-to-soft patterns of Ep are normally expected. Moreover, a chromatic intensity-tracking pattern of Ep could be induced by the effect of a variable source function.

Nature of the ultrarelativistic prompt emission phase of GRB 190114C

We address the physical origin of the ultrarelativistic prompt emission (UPE) phase of GRB 190114C observed in the interval 1.9-3.99 s, by the Fermi-GBM in 10 keV-10 MeV . Thanks to high S/N ratio of Fermi-GBM data, a time resolved spectral analysis has evidenced a sequence of similar blackbody plus cutoff power-law spectra, on ever decreasing time intervals during the entire UPE phase. We assume that during the UPE phase, the inner engine of the GRB, composed of a Kerr black hole and a uniform test magnetic field B0, aligned with the BH rotation axis, operates in an overcritical field. We infer an $e^+e^-$ pair electromagnetic plasma in presence of a baryon load, a PEMB pulse, originating from a vacuum polarization quantum process in the inner engine. This initially optically thick plasma self-accelerates, giving rise at the transparency radius to the MeV radiation observed by Ferm-GBM. At trf > 3.99 s, the electric field becomes undercritical, and the inner engine operates in the classical electrodynamics regime and generate the GeV emission. During both the quantum and the classical electrodynamics processes, we determine the time varying mass and spin of the Kerr BH in the inner engine, fulfilling the Christodoulou-Hawking-Ruffini mass-energy formula. For the first time, we quantitatively show how the inner engine, by extracting the rotational energy of the Kerr BH, produces a series of PEMB pulses. We follow the quantum vacuum polarization process in sequences with decreasing time bins. We compute the Lorentz factors, the baryon loads and the radii at transparency, as well as the value of the magnetic field, assumed to be constant in each sequence. The fundamental hierarchical structure, linking the quantum electrodynamics regime to the classical electrodynamics regime, is characterized by the emission of blackholic quanta with a timescale $t=10^{-9}$s, and energy $E=10^{45}$ erg.

X-Ray Afterglows from the Gamma-Ray Burst “Large-angle” Emission

Abstract We derive basic analytical results for the timing and decay of the gamma-ray burst (GRB) counterpart and delayed afterglow light curves for a brief emission episode from a relativistic surface endowed with angular structure, consisting of a uniform core of size (Lorentz factor and surface emissivity are angle independent) and an axially symmetric power-law envelope ( ). In this “large-angle emission” model, radiation produced during the prompt emission phase (GRB) at angles arrives at the observer well after the burst (delayed emission). The dynamical time range of the very fast decaying GRB “tail” and of the flat afterglow “plateau” and the morphology of the GRB counterpart/afterglow are all determined by two parameters: the core's parameter and the envelope's Lorentz factor index g, leading to three types of light curves that display three post-GRB phases (type 1: tail, plateau/slow decay, post-plateau/normal decay), two post-GRB phases (type 2: tail and fast decay), or just one (type 3: normal decay). We show how X-ray light-curve features can be used to determine core and envelope dynamical and spectral parameters. Testing of the large-angle emission model is done using the Swift/XRT X-ray emission of two afterglows of type 1 (GRB 060607A, GRB 061121), one of type 2 (GRB 061110A), and one of type 3 (GRB 061007). We find that the X-ray afterglows with plateaus require an envelope Lorentz factor and a comoving-frame emissivity ; thus, for a typical afterglow spectrum , the lab-frame energy release is uniform over the emitting surface.