arXiv subjects

X. Y. Wang

Publications and source records attributed to X. Y. Wang.

46 records · Page 3Linked to original sources

arXiv2003

The radio afterglow from the giant flare of SGR 1900+14: the same mechanism as afterglows from classic gamma-ray bursts?

A radio afterglow was detected following the 1998 August 27 giant flare from the soft gamma repeater (SGR) 1900+14. Its short-lived behavior is quite different from the radio nebula of SGR 1806-20, but very similar to radio afterglows from classic gamma-ray bursts (GRBs). Motivated by this, we attempt to explain it with the external shock model as invoked in the standard theory of GRB afterglows. We find that the light curve of this radio afterglow is not consistent with the forward shock emission of an ultra-relativistic outflow, which is suggested to be responsible for the initial hard spike of the giant flare. Nevertheless, shock emission from a mildly or sub-relativistic outflow expanding into the interstellar medium could fit the observations. The possible origin for this kind of outflow is discussed, based on the magnetar model for SGRs. Furthermore, we suggest that the presence of an ultra-relativistic fireball from SGR giant flares could be tested by rapid radio to optical follow-up observations in the future.

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arXiv2003

External shock model for the large-scale, relativistic X-ray jets from the microquasar XTE J1550-564

Large-scale, decelerating, relativistic X-ray jets due to material ejected from the black-hole candidate X-ray transient and microquasar XTE J1550-564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the eastern jet at the late time is consistent with the well-known Sedov evolutionary phase. A trans-relativistic external shock dynamic model by analogy with the evolution of gamma-ray burst remnants, is shown to be able to fit the observation data reasonably well. The inferred interstellar medium density around the source is well below the canonical value $n_ISM \sim 1 cm^{-3}$. We find that the emission from the continuously shocked interstellar medium (forward shock region) decays too slowly to be a viable mechanism for the eastern X-ray jet. However, the rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta. To ensure the dominance of the emission from the shocked ejecta over that from the forward shock region during the period of the observations, the magnetic field and electron energy fractions in the forward shock region must be far below equipartition. Future continuous, follow-up multi-wavelength observations of new ejection events from microquasars up to the significant deceleration phase should provide more valuable insight into the nature of the interaction between the jets and external medium.

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arXiv2002

Possible geometries of afterglow generation in the gamma-ray burst GRB990705

The absorption feature detected in the prompt X-ray emission of GRB990705 has important consequences for its circum-burst environment and therefore on its afterglows. Here we investigate whether the circum-burst environment constrained by the absorption feature could be consistent with the observed H-band afterglow, which exhibits an earlier power law decay but a much faster decay about one day after the burst. Two possible geometries of the afterglow-emitting regions are suggested: 1) afterglow emission produced by the impact of the fireball on the surrounding torus, which serves as the absorbing material of the X-ray feature, as would be expected in the models involving that a supernova explosion precedes the gamma-ray burst by some time; 2)afterglow emission produced in the dense circum-burst medium inside the torus. In case 1), the faster decay at the later time is attributed to the disappearance of the shock due to the counter-pressure in the hot torus illuminated by the burst and afterglow photons. For case 2), the circum-burst medium density is found to be very high ($n\ga 10^4-10^5 {\rm cm^{-3}}$) if the emitting plasma is a jet or even higher if it is spherical. Future better observations of afterglows of GRBs that have absorption features might make it possible to make a more definite choice between these two scenarios.

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arXiv2002

Emission Processes of High Energy Gamma Rays from Gamma-Ray Bursts

Synchrotron self-Compton (SSC) process in the reverse shocks of gamma-ray bursts is suggested to be responsible for the observed prompt high-energy gamma-ray emissions from several gamma-ray bursts. We find that the SSC emission from the reverse shocks dominates over other emission processes in energy bands from tens of MeV to tens of GeV, for a wide range of shock parameters. This model is favorable for escape of energetic photons from the emitting regions due to a lower internal pair-production optical depth, as the characteristic size of the reverse shock region is much larger than that of internal shocks. We predict that, in this model, the prompt high-energy emissions are correlated with the prompt optical flashes, which can be test in the forthcoming GLAST era.

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arXiv2002

Early GeV afterglows from gamma-ray bursts in pulsar wind bubbles

Gamma-ray bursts may occur within pulsar wind bubbles (PWBs) under a number of scenarios, such as the supranova-like models in which the progenitor pulsar drives a powerful wind shocking against the ambient medium before it comes to death and produces a fireball. We here study the early afterglow emission from GRBs expanding into such a PWB environment. Different from the usual cold GRB external medium, the PWBs consist of a hot electron-positron medium with typical 'thermal' Lorentz factor of the order of gamma_w, the Lorentz factor of the pulsar particle wind. After GRB blast waves shock these hot electron-positron pairs, they will emit synchrotron radiation peaking at GeV bands. It is shown that GeV photons suffer negligible absorption by the soft photons radiation field in PWBs. Thus, strong GeV emissions in the early afterglow phases are expected, providing a plausible explanation for the long-duration GeV emission from GRB940217 detected by EGRET. Future GLAST may have the potential to test this GRB-PWB interaction model.

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arXiv2001

The Inverse Compton Emission Spectra in the Very Early Afterglows of Gamma-Ray Bursts

We calculate the spectra of inverse Compton (IC) emissions in gamma-ray burst (GRB) shocks produced when relativistic ejecta encounters the external interstellar medium, assuming a broken power-law approximation to the synchrotron seed spectrum. Four IC processes, including the synchrotron self-Compton (SSC) processes in GRB forward and reverse shocks, and two combined-IC processes (i.e. scattering of reverse shock photons on the electrons in forward shocks and forward shock photons on the electrons in reverse shocks), are considered. We find that the SSC emission from reverse shocks dominates over other emission processes in energy bands from tens of MeV to tens of GeV, for a wide range of shock parameters. This mechanism may be responsible for the prompt high energy gamma-rays detected by the Energetic Gamma Ray Experiment Telescope (EGRET). At TeV energy bands, however, the combined-IC emissions and/or the SSC emission from the forward shocks become increasingly dominant for a moderately steep distribution of shocked electrons.

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arXiv2000

Prompt High Energy $γ$-Ray Emission From the Synchrotron Self-Compton Process in the Reverse Shocks of $γ$-Ray Bursts

In the standard scenario of the fireball model of gamma-ray bursts(GRBs), the huge initial energy release produces a relativistic blast wave expanding into the external medium and a reverse shock moving into and heating the fireball ejecta. We calculate the high energy gamma-ray emission due to inverse Compton scattering of the synchrotron photons from relativistic electrons in the reverse shock. Under the favorable values of the physical parameters of the GRBs and the interstellar medium, our result shows that during the prompt phase, this emission dominates over the component from the forward shock at high energy gamma-ray bands. This mechanism can excellently account for the observations of the prompt high energy gamma-rays detected by EGRET, such as from GRB930131.

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arXiv2000

Intrinsic Parameters of GRB990123 from Its Prompt Optical Flash and Afterglow

We have constrained the intrinsic parameters, such as the magnetic energy density fraction ($ε_{B}$), the electron energy density fraction ($ε_e$), the initial Lorentz factor ($Γ_0$) and the Lorentz factor of the reverse external shock ($Γ_{rs}$), of GRB990123, in terms of the afterglow information (forward shock model) and the optical flash information (reverse shock model). Our result shows: 1) the inferred values of $ε_e$ and $ε_B$ are consistent with the suggestion that they may be universal parameters, comparing to those inferred for GRB970508; 2) the reverse external shock may have become relativistic before it passed through the ejecta shell. Other instrinsic parameters of GRB990123, such as energy contained in the forward shock $E$ and the ambient density $n$ are also determined and discussed in this paper.

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arXiv2000

A Possible Model for the Supernova/Gamma-Ray Burst Connection

Conversion from neutron stars to strange stars as a possible mechanism of cosmological gamma-ray bursts (GRBs) has been discussed in previous works, although the existence of strange stars is still an open question. On the basis of this mechanism, we here outline an explanation of the connection between supernovae (SNe) and GRBs, which has got increasing evidence recently. An asymmetric but normal SN explosion leaves a massive ($\geq1.8{\rm M_\odot}$) and rapidly rotating neutron star, which then converts to a strange star few days later, due to its rapid spindown. The accompanied fireball, which can be accelerated to ultra-relativistic velocity ($Γ_0\sim 100$) due to the very low baryon contamination of the strange star, flows out along the direction of the high-velocity SN jet and subsequently produces a GRB and the following low energy afterglows by interacting with the surrounding stellar wind. We will also expect a very luminous supernova like SN1998bw, if a large fraction of the conversion energy finally turns into the kinetic energy of the supernova ejecta.

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arXiv2000

A Possible Explanation of the Radio Afterglow of GRB980519: The Dense Medium Effect

GRB{980519} is characterized by its rapidly declining optical and X-ray afterglows. Explanations of this behavior include models invoking a dense medium environment which makes the shock wave evolve quickly into the sub-relativistic phase, a jet-like outflow, and a wind-shaped circumburst medium environment. Recently, Frail {et al}. (1999a) found that the latter two cases are consistent with the radio afterglow of this burst. Here, by considering the trans-relativistic shock hydrodynamics, we show that the dense medium model can also account for the radio light curve quite well. The potential virtue of the dense medium model for GRB{980519} is that it implies a smaller angular size of the afterglow, which is essential for interpreting the strong modulation of the radio light curve. Optical extinction due to the dense medium is not important if the prompt optical-UV flash accompanying the $γ$-ray emission can destroy dust by sublimation out to an appreciable distance. Comparisons with some other radio afterglows are also discussed.

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