Statistical Analyses of the Energies of X-Ray Plateaus and Flares in Gamma-Ray Bursts

Distinct X-ray plateau and flare phases have been observed in the afterglows of gamma-ray bursts (GRBs), and most of them should be related to central engine activities. In this paper, we collect 174 GRBs with X-ray plateau phases and 106 GRBs with X-ray flares. There are 51 GRBs that overlap in the two selected samples. We analyze the distributions of the proportions of the plateau energy E plateau and the flare energy E flare relative to the isotropic prompt emission energy E γ,iso. The results indicate that they well meet the Gaussian distributions and the medians of the logarithmic ratios are ∼−0.96 and −1.39 in the two cases. Moreover, strong positive correlations between E plateau (or E flare ) and E γ,iso with slopes of ∼0.95 (or ∼0.80) are presented. For the overlapping sample, the slope is ∼0.80. We argue that most of X-ray plateaus and flares might have the same physical origin but appear with different features because of the different circumstances and radiation mechanisms. We also test the applicabilities of two models, i.e., black holes surrounded by fractured hyperaccretion disks and millisecond magnetars, on the origins of X-ray plateaus and flares.

Photospheric Prompt Emission From Long Gamma-ray Burst Simulations. I. Optical Emission

A complete understanding of gamma-ray bursts (GRBs) has been difficult to achieve, due to our incomplete knowledge of the radiation mechanism that is responsible for producing the prompt emission. This emission, which is detected in the first tens of seconds of the GRB, is typically dominated by hard X-ray and gamma-ray photons, although there have also been a few dozen prompt optical detections. These optical detections have the potential to discriminate between plausible prompt emission models, such as the photospheric and synchrotron shock models. In this work, we use an improved MCRaT code, which includes cyclo-synchrotron emission and absorption, to conduct radiative transfer calculations from optical to gamma-ray energies under the photospheric model. The calculations are conducted using a set of two-dimensional relativistic hydrodynamic long GRB jet simulations, consisting of a constant and a variable jet. We predict the correlations between the optical and gamma-ray light curves as functions of observer angle and jet variability, and find that there should be extremely dim optical prompt precursors for large viewing angles. Additionally, the detected optical emission originates from dense regions of the outflow, such as shock interfaces and the jet-cocoon interface. Our results also show that the photospheric model is unable to account for the current set of optical prompt detections that have been made and therefore additional radiative mechanisms are needed to explain these prompt optical observations. These findings show the importance of conducting global radiative transfer simulations using hydrodynamically calculated jet structures.

A Powerful e ± Outflow Driven by a Proto-strange Quark Star

An electron–positron layer can cover the surface of a bare strange star (SS), the electric field in which can excite the vacuum and drive a pair wind by taking away the heat of the star. In order to investigate the pair-emission ability of a proto-SS, we establish a toy model to describe its early thermal evolution, where the initial trapping of neutrinos is specially taken into account. It is found that the early cooling of the SS is dominated by the neutrino diffusion rather than the conventional Urca processes, which leads to the appearance of an initial temperature plateau. During this plateau phase, the surface e ± pair emission can maintain a constant luminosity of 1048 − 1050erg s−1 for about a few to a few tens of seconds, which is dependent on the value of the initial temperature. The total energy released through this e ± wind can reach as high as ∼1051 erg. In principle, this pair wind could be responsible for the prompt emission or extended emission of some gamma-ray bursts.

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

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.

A Comprehensive Study of Multiflare GRB Spectral Lag

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.

GRB Polarization: A Unique Probe of GRB Physics

Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emissions are highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field.

Lomonosov GRB Catalogue: The First Experience of Prompt Emission Multi-Wavelength Observations

This paper presents a catalogue of gamma-ray bursts (GRBs) that were detected by the instruments onboard the Lomonosov space observatory. The Lomonosov mission gave the first experience of not only multi-wavelength (from optical to gamma) observations of GRBs but also multi-messenger observations of extreme phenomena and GRBs. The detailed light curves and energy spectra of the detected GRBs are presented. The results of the prompt, early an afterglow optical observations of several GRBs are discussed.

Gravitational waves from GRB core spindown

We investigate long Gamma-Ray Bursts (GRB) which manifest a sharp linear rise followed by an exponential decay in their γ-ray prompt emission observed with the BAT instrument on board the Swift satellite. We offer a simple electrodynamic model that may account for these particular characteristics. We associate the sharp rise with the winding of the magnetic field by the fast rotating core that formed in the interior of the stellar precursor. We also associate the subsequent exponential decay with the electromagnetic spindown of the core following the release of the electromagnetic jet from the stellar interior. Any non-axisymmetric distortion in the rotating core will generate gravitational waves with exponentially decreasing frequency, a so-called ‘down-chirp’. We obtain a detailed estimate of the gravitational wave profile if the distortion of spacetime is due to the winding of a non-axisymmetric component of the magnetic field during that particular phase of the burst. We offer 7 particular time intervals during which one may look into LIGO archival data for the presence of our particular predicted waveforms in order to test our interpretation.