Modeling the quasi-periodic oscillation of Swift J1644+57

A 200-second X-ray quasi-periodicity in the 2-8 ke V band from Swift J1644+57 was found by Reis et al.From the onset time of quasi-periodic oscillation(QPO),we show that Swift J1644+57 is a plunging event.This QPO may be related to discrete clumps from the accretion disk falling into a supermassive black hole,then the outflow in the jet may be also discontinuous.We estimate the lifetime of clumps to be about several hundreds seconds and the fraction of clumpy ejecta to be about 30% from the QPO.The other possible model involves the interface between the inflow and jet magnetosphere in the magnetically choked accretion flow.Theory and numerical simulations indicate that a magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface can produce a jet-disk QPO mechanism.This event may be the first evidence of jet-disk QPO.From observations,the two models are comparable.

Early afterglows from radially structured outflows and the application to X-ray shallow decays

In the fireball model, it is more physically realistic that gamma-rayburst (GRB) ejecta have a range of bulk Lorentz factors (assuming M ∝Γ-s). The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases: the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sari & Me′sza′ros (2000). However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.

Extending the correlation of L_R-L_X to gamma-ray bursts

The well-known correlation between radio luminosity(LR) and X-ray luminosity(LX), LR/LX≌ 10-5, holds for a variety of objects, such as active galactic nuclei, Galactic black holes, solar flares and cool stars. Here we extend the relation to gamma-ray bursts(GRBs) and find that the GRBs also obey a similar LR- LX relation, with a slightly different slope of L1R∝ L.1X. This relation implies that the explosions that occur on different scales may have a common underlying origin.

Searching for events in Chinese ancient records to explain the increase in ^(14)C from AD 774–775 and AD 993–994

According to analysis of the14 C content in two Japanese trees, that grew over a period of approximately 3000 years, with high time resolution, Miyake et al.found a rapid increase at AD 774–775 and another one at AD 993–994. These increases correspond to high-energy events that happened within those years and radiated γ-ray energy of about 7×1024erg toward the Earth. The origin of these events is a mystery. Such strong events should have an unusual optical counterpart, and have been recorded in historical literatures. We searched Chinese historical materials around AD 744–775 and AD 993–994, but no remarkable event was found except for a violent thunderstorm in AD 775. However, the possibility of a thunderstorm containing so much energy is unlikely. We conclude that the events, which caused the14 C increase, are still unclear. These events most probably had no optical counterpart, and a short gamma-ray burst, giant flare of a soft gamma-ray repeater or a terrestrial γ-ray flash could all be candidates.

A two-component jet model based on the Blandford-Znajek and Blandford-Payne processes

We propose a two-component jet model consistent with the observations of several gamma ray bursts (GRBs) and active galactic nuclei (AGNs). The jet consists of inner and outer components, which are supposed to be driven by the Blandford- Znajek (BZ) and Blandford-Payne (BP) processes, respectively. The baryons in the BP jet are accelerated centrifugally via the magnetic field anchored in the accretion disk. The BZ jet is assumed to be entrained in a fraction of accreting matter leaving the inner edge of the accretion disk, and the baryons are accelerated in the conversion from electromagnetic energy to kinetic energy. By fitting the Lorentz factors of some GRBs (GRB 030329, GRB 051221A and GRB 080413B) and AGNs (Cen A, Mkn 501 and Mkn 421) with this model, we constrain the physical parameters related to the accretion and outflow of these two kinds of objects. We conclude that the spine/sheath structure of the jet from these sources can be interpreted naturally by the BZ and BP processes.

Revisiting gamma-ray burst afterglows with time-dependent parameters

The relativistic external shock model of gamma-ray burst(GRB) afterglows has been established with five free parameters, i.e., the total kinetic energy E, the equipartition parameters for electrons ε_e and for the magnetic field ε_B, the number density of the environment n and the index of the powerlaw distribution of shocked electrons p. A lot of modified models have been constructed to consider the variety of GRB afterglows, such as: the wind medium environment by letting n change with radius,the energy injection model by letting kinetic energy change with time and so on. In this paper, by assuming all four parameters(except p) change with time, we obtain a set of formulas for the dynamics and radiation, which can be used as a reference for modeling GRB afterglows. Some interesting results are obtained. For example, in some spectral segments, the radiated flux density does not depend on the number density or the profile of the environment. As an application, through modeling the afterglow of GRB 060607A, we find that it can be interpreted in the framework of the time dependent parameter model within a reasonable range.

The correlations among variability,optical peak time and spectral time lag of long gamma-ray bursts

Statistical relations are useful tools to comprehend the intrinsic physics processes of gamma-ray bursts(GRBs). In this work we collect spectral lag(τ), variability(V) and optical peak time(tp,o). We find that there is a correlation between variabilities and spectral lags, reading as V =-0.0075(±0.0007) ×log10τ +0.0351(±0.0024). There may also exist a relatively weak positive tendency between GRBs optical band peak times and their spectral time lags. Its Pearson coefficient is 0.398, which indicates a weak linear correlation. If we contain some "negative spectral lag" samples, then the latter relation would be worse due to two outlying points. The τ-V relation is consistent with previous studies, and the positive trend betweenτ and tp,oindicates the spectral lag of GRB might be caused by the curvature effect, but this conclusion is not significant.

The updated Bulk Lorentz Factors of Gamma-Ray Burst X-Ray Flares

X-ray flares are the most common phenomena in the afterglow phase of gamma-ray bursts(GRBs) in the Swift era,and are known as a canonical component in X-ray afterglows.In this work,we constrain the Lorentz factor of X-ray flares with an updated sample.We extensively search for X-ray light curves showing flare and jet break simultaneously.A smooth broken power law function is used to fit the jet breaks in 11 GRBs.We also use a smooth broken power law function to fit the profile of X-ray flares,and the total number of the flares is 20.We obtain the lower and upper limits of Lorentz factor(Γ_X) with the timescale,half-opening angle and mean luminosity of the X-ray flares for interstellar medium(ISM) and wind cases.The lower limits on Γ_X range from tens to a few hundred,and the upper limits are mainly about a few hundred.We also apply the limited Lorentz factor to test correlations of Γ_0-E_(γ,iso) and Γ_0-L_(γ,iso) for GRBs,and find X-ray flares in the ISM case are much more consistent with those of prompt emission than the wind case in a statistical sense for both correlations.X-ray flares are almost consistent with the trend in the correlations of Γ_0-E__(γ,iso)(L_(γ,iso)) for prompt GRBs,indicating X-ray flares and prompt bursts may have the same physical origin.

Implication from GRB 221009A:Can TeV emission come from the GRB prompt phase?

Recently,the B.O.A.T.(“brightest of all time”)gamma-ray burst,dubbed GRB 221009A,was detected by various instruments.Unprecedentedly,the GRB presented very-high-energy(VHE,energy above 0.1 Te V)gamma-ray emission with energy extending above 10 Te V,as reported by the Large High Altitude Air Shower Observatory(LHAASO).We here demonstrate that the VHE and especially>10 Te V emission may originate from the internal hadronic dissipation of the GRB,without the need of invoking any exotic processes as suggested by some previous studies.The possible prompt origin of LHAASO photons may imply the first detection of the GRB prompt phase in the VHE regime.We also discuss the constraints on the properties of the GRB ejecta from multiwavelength and multi-messenger observations,which favors a magnetically dominated GRB ejecta.The suggested Poynting-flux-dominated GRB ejecta in this work supports the Blandford&Znajek(BZ)mechanism as the possible central engine model of GRB,as well as the possible strong magnetic dissipation and acceleration.