ABSTRACT X-ray flares are generally supposed to be produced by later activities of the central engine, and may share a similar physical origin with the prompt emission of gamma-ray bursts (GRBs). In ...this paper, we have analyzed all significant X-ray flares from the GRBs observed by Swift from 2005 April to 2015 March. The catalog contains 468 bright X-ray flares, including 200 flares with redshifts. We obtain the fitting results of X-ray flares, such as start time, peak time, duration, peak flux, fluence, peak luminosity, and mean luminosity. The peak luminosity decreases with peak time, following a power-law behavior . The flare duration increases with peak time. The 0.3-10 keV isotropic energy of the distribution of X-ray flares is a log-normal peaked at erg. We also study the frequency distributions of flare parameters, including energies, durations, peak fluxes, rise times, decay times, and waiting times. Power-law distributions of energies, durations, peak fluxes, and waiting times are found in GRB X-ray flares and solar flares. These distributions could be well explained by a fractal-diffusive, self-organized criticality model. Some theoretical models based on magnetic reconnection have been proposed to explain X-ray flares. Our result shows that the relativistic jets of GRBs may be dominated by Poynting flux.
Abstract
We present a time-resolved spectral analysis of bright X-ray flares observed by Swift X-ray telescope from 2004 December to 2021 December. We selected 58 bright flares from 56 gamma-ray ...burst X-ray afterglows to perform time-resolved spectral analysis. We find that there is a strong spectral evolution in X-ray flares. Spectral evolution exists in all the X-ray flares. We have not found flares without spectral evolutions in our samples. The spectrum of an X-ray afterglow is dominated by the flare when it exists. Four spectral evolution patterns were found, i.e., hard-to-soft, intensity tracking, soft-to-hard, and anti-intensity tracking. Most of the flares (46 flares, 79.3%) show a spectral evolution from hard to soft. Five (8.6%) flares are intensity tracking. Two (3.5%) flares show soft-to-hard. Five (8.6%) flares show anti-intensity tracking. Therefore, hard-to-soft spectral evolution patterns dominate the spectral evolutions of X-ray flares. In other words, the hard-to-soft spectral evolution pattern is the main evolution pattern of X-ray flares. Hard-to-soft, intensity tracking, and soft-to-hard spectral evolution patterns are the same as prompt emission spectral evolutions, indicating that the origin of flares should be the same as prompt emissions and the flares are the lower energy band emissions of the prompt emissions. The spectral evolution intensities are independent of the peak time and FWHM of the flares. In other words, the spectral evolution intensity is irrelevant to the flare occurrence time and its duration but related to the peak fluxes of flares. This means that the more luminous the flares, the stronger the spectral evolutions.
ABSTRACT We present time-resolved spectral analysis of the steep decay segments of 29 bright X-ray flares of gamma-ray bursts (GRBs) observed with the Swift/X-ray telescope, and model their light ...curves and spectral index evolution behaviors with the curvature effect model. Our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fitted with this model, and the derived characteristic timescales (tc) are in the range of 23 ∼ 264 s. Using an empirical relation between the peak luminosity and the Lorentz factor derived from the prompt gamma-rays, we estimate the Lorentz factors of the flares (ΓX). We obtain ΓX = 17 ∼ 87 with a median value of 52, which is smaller than the initial Lorentz factors of prompt gamma-ray fireballs. With the derived tc and ΓX, we constrain the radiating regions of 13 X-ray flares, yielding RX = (0.2 ∼ 1.1) × 1016 cm, which are smaller than the radii of the afterglow fireballs at the peak times of the flares. A long evolution feature from prompt gamma-ray phase to the X-ray epoch is found by incorporating our results with a sample of GRBs whose initial Lorentz factors are available in the literature, i.e., . These results may shed light on the long-term evolution of GRB central engines.
We jointly analyze the gamma-ray burst (GRB) data observed with Burst Alert Telescope (BAT) and X-ray Telescope on board the Swift mission to present a global view on the internal energy dissipation ...processes in GRBs, including precursors, prompt gamma-ray emission, extended soft gamma-ray emission, and late X-ray flares. The Bayesian block method is utilized to analyze the BAT light curves to identify various emission episodes. Our results suggest that these emission components likely share the same physical origin, which is the repeated activation of the GRB central engine. What we observe in the gamma-ray band may be a small part of more extended underlying activities. The precursor emission, which is detected in about 10% of Swift GRBs, is preferably detected in those GRBs that have a massive star core-collapse origin. The soft extended emission tail, on the other hand, is preferably detected in those GRBs that have a compact star merger origin. Bright X-ray emission is detected during the BAT quiescent phases prior to subsequent gamma-ray peaks, implying that X-ray emission may be detectable prior the BAT trigger time. Future GRB alert instruments with soft X-ray capability are essential for revealing the early stages of GRB central engine activities, and shedding light on jet composition and the jet launching mechanism in GRBs.
We have studied two events in which relatively superdense plasma appeared near geosynchronous orbit after the interplanetary magnetic field (IMF) changed from a long period of northward condition to ...southward condition. We used the magnetosphere MHD model Open Geospace General Circulation Model to simulate these two events and compared the simulation results with observations. The observations and the simulation results show that an extended period of northward IMF caused a cold dense plasma sheet (CDPS) to form in the near tail, and a subsequent southward IMF caused a highly stretched near‐tail dipole field, which enhanced the earthward convection and eventually set off reconnection. Some of the cold dense plasma was thus consequently transported to geosynchronous orbit and formed a superdense plasma sheet (SDPS). CDPS is the main source of SDPS in its early stage after the IMF southward turning.
Key Points
An event with both CDPS and SDPS observations is reproduced in an OpenGGCM simulation
CDPS is the main source of SDPS in its early stage
CDPS and a subsequent IMF southward turning causes the formation of SDPS
Due to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the interplanetary magnetic field (IMF) directs ...northward. This simulation study uses two different global magnetosphere magnetohydrodynamics (MHD) models to simulate two northward IMF events and study the evolution of the magnetotail. Both models show that the magnetotail may form a structure that is composed of a dawnside tail lobe and a duskside tail lobe, under northward IMF conditions with significant By, instead of a northern tail lobe and a southern tail lobe under southward IMF conditions. In this magnetic configuration, a tail lobe extends a domain from northern (southern) cusp to southern (northern) IMF. The larger the magnitude of IMF clock angle, the longer and wider the magnetotail. Such magnetic configuration suggests that magnetotail reconnection is possible to occur when the dawnside tail lobe contacts with the duskside tail lobe and thus a substorm is also possible to occur under northward IMF conditions with significant By.
Plain Language Summary
The Earth's magnetosphere is a vast space with certain magnetic structure and plasma material and is directly affected by solar wind and its embedded magnetic field named interplanetary magnetic field (IMF). Due to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetosphere looks like and how it evolves, especially when the IMF arriving the magnetosphere directs northward with a significant dawn‐dusk component, By. This simulation study uses two different global magnetosphere models to simulate the magnetosphere for two periods with northward IMF and show that the magnetotail (tail of magnetosphere) may form a structure that is composed of a dawnside tail lobe and a duskside tail lobe instead of a northern tail lobe and a southern tail lobe under southward IMF conditions. In this magnetic configuration, a tail lobe extends a domain from northern (southern) cusp to southern (northern) IMF. The larger the IMF By, the longer and wider the magnetotail. Such magnetic configuration suggests that magnetotail reconnection is possible to occur when the dawnside tail lobe contacts with the duskside tail lobe, and thus a substorm is also possible to occur under northward IMF conditions with significant By.
Key Points
A dawnside tail lobe and a duskside tail lobe may form in the magnetotail under northward interplanetary magnetic field (IMF) conditions with significant By
The larger the magnitude of IMF clock angle, the longer and wider the magnetotail
Magnetotail reconnection may occur when the dawnside tail lobe contacts with the duskside tail lobe, and is possible to cause substorm
We continue our systematic statistical study of various components of gamma-ray burst (GRB) optical light curves. We decompose the early onset bump and the late re-brightening bump with empirical ...fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical light curves, the onset and re-brightening bumps are observed in 38 and 26 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are ~1.5 and ~ - 1.15, respectively. No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time Lp is proportional to t super(-1.81+ or -0.32)p for the onset bumps and Lp is proportional to t super(-0.83+ or -0.17)p for the re-brightening bumps. Both Lp and the isotropic energy release of the onset bumps are correlated with E sub(Gamma, iso), whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as nis proportional tor super(-1) within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between the initial Lorentz factor (Gamma sub(0)) and E sub(iso, Gamma) in our previous work. The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component. Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component, but with a lower Gamma sub(0), typically several tens.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted)Well-sampled optical light curves of 146 gamma-ray bursts (GRBs) are compiled from the literature. By empirical fitting, we identify ...eight possible emission components and summarize the results in a "synthetic" light curve. Both optical flare and early shallow-decay components are likely related to long-term central engine activities. We focus on their statistical properties in this paper. Twenty-four optical flares are obtained from 19 GRBs. The isotropic R-band energy is smaller than 1% of E sub( gamma ,iso). The relation between the isotropic luminosities of the flares and gamma rays follows ... Later flares tend to be wider and dimmer, i.e., ... and ... The detection probability of the optical flares is much smaller than that of X-ray flares. An optical shallow-decay segment is observed in 39 GRBs. The relation between the break time and break luminosity is a power law, with an index of -0.78 + or - 0.08, similar to that derived from X-ray flares. The X-ray and optical breaks are usually chromatic, but a tentative correlation is found. We suggest that similar to the prompt optical emission that tracks gamma -rays, the optical flares are also related to the erratic behavior of the central engine. The shallow-decay component is likely related to a long-lasting spinning-down central engine or piling up of flare materials onto the blast wave. Mixing of different emission components may be the reason for the diverse chromatic afterglow behaviors.
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