X. Y. Wang

Huazhong (Central China) Normal University, Wu-han-shih, Hubei, China

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Publications (23)124.82 Total impact

  • Source
    Y. W. Yu, X. Y. Wang, Z. G. Dai
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    ABSTRACT: In the popular internal shock model for the prompt emission of gamma-ray bursts (GRBs), collisions between a series of relativistic shells generate lots of paired forward and reverse shocks. We show that the synchrotron emission pro- duced by the forward and reverse shocks respectively could peak at two quite different energy bands if the Lorentz factors of these two types of shocks are significantly different with each other (e.g., one shock is relativistic and the other is Newtonian). We then investigate whether this scenario is applicable to the case of GRB 080319B and find that a bimodal distribution of the shell Lorentz factors, peaking at ∼ 400 and ∼ 105, is required. In addition, this scenario pre- dicts an accompanying inverse-Compton (IC) GeV emission with a luminosity comparable to (not much higher than) that of the synchrotron MeV emission, which can be tested with future Fermi observations.
    The Astrophysical Journal 01/2009; 692(2):1662-1668. · 6.73 Impact Factor
  • Yun-Wei Yu, X. Y. Wang, Z. G. Dai
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    ABSTRACT: In the framework of the popular internal shock model, paired forward and reverse shocks are generated simultaneously by collisions among relativistic shells. If the Lorentz factors of these two types of shocks are very different, the synchrotron emissions produced by them could peak at two different energy bands. We show that such a two-component synchrotron scenario can account for the prompt optical and MeV Γ-ray emissions of GRB 080319B. The same dynamical origin of the forward and reverse shocks is consistent with the temporal coincidence between the prompt Γ-ray and optical emission components.
    10/2008;
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    ABSTRACT: Radio afterglows have been detected following two giant flares from soft gamma repeaters (i.e. SGR1900+14, SGR 1806‐20). Recent follow‐up observations of the December 27 giant flare of SGR 1806‐20 have detected a multi‐frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from one week to about one month after the flare. The angular size of the source was also measured for the first time. Here we show that this radio afterglow gives the first piece of clear evidence that an energetic blast wave sweeps up its surrounding medium and produces a synchrotron afterglow, the same mechanism as established for GRB afterglows. © 2006 American Institute of Physics
    AIP Conference Proceedings. 05/2006; 836(1):72-75.
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    Z G Dai, X Y Wang, X F Wu, B Zhang
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    ABSTRACT: Recent observations support the suggestion that short-duration gamma-ray bursts are produced by compact star mergers. The x-ray flares discovered in two short gamma-ray bursts last much longer than the previously proposed postmerger energy-release time scales. Here, we show that they can be produced by differentially rotating, millisecond pulsars after the mergers of binary neutron stars. The differential rotation leads to windup of interior poloidal magnetic fields and the resulting toroidal fields are strong enough to float up and break through the stellar surface. Magnetic reconnection-driven explosive events then occur, leading to multiple x-ray flares minutes after the original gamma-ray burst.
    Science 03/2006; 311(5764):1127-9. · 31.20 Impact Factor
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    ABSTRACT: We analyze several recently detected gamma-ray bursts (GRBs) with late X-ray flares in the context of late internal shock and late external shock models. We find that the X-ray flares in GRB 050421 and GRB 050502B originate from late internal shocks, while the main X-ray flares in GRB 050406 and GRB 050607 may arise from late external shocks. We also constrain the lower limit on the Lorentz factor of X-ray flares in the context of the late internal shock model.
    Advances in Space Research 01/2006; · 1.18 Impact Factor
  • X. Y. Wang, Z. G. Dai, T. Lu
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    ABSTRACT: Large-scale, decelerating, relativistic X-ray jets from microquasar XTE J1550−564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the approaching jet at the late time is consistent with the well-known Sedov evolutionary phase R∝ t 2/5. 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 proper-motion data of the approaching jet reasonably well. The inferred interstellar medium density around the source is well below the canonical value n ISM∼1 cm−3. 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.
    Astrophysics and Space Science 05/2005; 297(1):155-166. · 2.06 Impact Factor
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    K. S. Cheng, T. Harko, X Y Wang
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    ABSTRACT: The transport equations for polarized radiation transfer in non-Riemannian, Weyl-Cartan type space-times are derived, with the effects of both torsion and non-metricity included. To obtain the basic propagation equations we use the tangent bundle approach. The equations describing the time evolution of the Stokes parameters, of the photon distribution function and of the total polarization degree can be formulated as a system of coupled first order partial differential equations. As an application of our results we consider the propagation of the cosmological gamma ray bursts in spatially homogeneous and isotropic spaces with torsion and non-metricity. For this case the exact general solution of the equation for the polarization degree is obtained, with the effects of the torsion and non-metricity included. The presence of a non-Riemannian geometrical background in which the electromagnetic fields couple to torsion and/or non-metricity affect the polarization of photon beams. Consequently, we suggest that the observed polarization of prompt cosmological gamma ray bursts and of their optical afterglows may have a propagation effect component, due to a torsion/non-metricity induced birefringence of the vacuum. A cosmological redshift and frequency dependence of the polarization degree of gamma ray bursts also follows from the model, thus providing a clear observational signature of the torsional/non-metric effects. On the other hand, observations of the polarization of the gamma ray bursts can impose strong constraints on the torsion and non-metricity and discriminate between different theoretical models. Comment: 12 pages, 3 figures, accepted for publication in PRD
    Physical Review D 05/2005; · 4.69 Impact Factor
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    ABSTRACT: The cylindrical jet model of Gamma-ray Burst (GRB) afterglows has been suggested. The idea of massive stars as progenitors of GRBs is widely accepted. Within this notion, a density boundary probably exists along the way of the relativistic jet. We find that a cylindrical jet interacting with this boundary can explain the optical lightcurves of GRB 970508 and GRB 000301C. On the other hand, the observing prospect for cylindrical jet model for short bursts is shown to be promising. Thus in the near future, the cylindrical jet model can be tested by the coming data.
    Il Nuovo Cimento C 04/2005; 28:447.
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    ABSTRACT: Recent follow-up observations of the 2004 December 27 giant flare of SGR 180620 have detected a multiple- frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from a week to about a month after the flare. The angular size of the source has also been measured for the first time. Here we show that this radio afterglow provides the first clear evidence of an energetic blast wave sweeping up its surrounding medium and producing a synchrotron afterglow, the same mechanism as has been established for gamma-ray burst afterglows. The optical afterglow is expected to have been intrinsically as bright as mR 13 at t 0.1 days after the flare, but very heavy extinction due to the low Galactic latitude of the source would have made detection difficult. Rapid infrared follow-up observations of giant flares are therefore crucial for low-latitude soft gamma-ray repeaters (SGRs), while for high-latitude SGRs (e.g., SGR 052666), rapid follow-up should result in the identification of possible optical afterglows. Rapid multiwavelength follow-up will also provide more detailed information on the early evolution of the fireball, as well as its composition. Subject headings: gamma rays: bursts — ISM: jets and outflows — stars: individual (SGR 180620)
    The Astrophysical Journal 01/2005; 623(1). · 6.73 Impact Factor
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    ABSTRACT: The brightest giant flare from the soft gamma-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least 1 order of magnitude more energetic than those of the two other SGR giant flares. Starting from about 1 week after the burst, a very bright (~80 mJy), fading radio afterglow was detected. Follow-up observations revealed the multifrequency light curves of the afterglow and the temporal evolution of the source size. Here we show that these observations can be understood in a two-component explosion model. In this model, one component is a relativistic collimated outflow responsible for the initial giant flare and the early afterglow, and the other component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss the triggering mechanisms of these two components within the framework of the magnetar model.
    The Astrophysical Journal 01/2005; 629. · 6.73 Impact Factor
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    ABSTRACT: The different temporal behaviors of the high energy component and the hundreds of keV emission from GRB 941017 suggest that they come from different emission regions. The nearly constant flux of this high energy component is consistent with being produced in the region of the early external shock formed when the ejecta hit the surrounding medium. Here we show that the relatively hard spectrum of this component can be numerically modelled as the synchrotron self-Compton emission from external reverse shock when the ejecta is decelerated by a typical interstellar medium. Constraints on the parameters of the fireball, such as the initial Lorentz factor $\eta$, the burst energy, and the magnetic field equipartition fraction are obtained. Very high initial Lorentz factor ($\eta\ga 10^3$) inferred for this burst may explain the rarity of this kind of high-energy component in gamma-ray bursts (GRBs). Large GeV–TeV flux from the reverse shock is predicted for GRBs with high initial Lorentz factors.
    Astronomy and Astrophysics 01/2005; · 5.08 Impact Factor
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    ABSTRACT: It has been suggested that electromagnetic cascade of very high energy gamma-rays from gamma-ray bursts (GRBs) in the infrared/microwave background can produce delayed MeV-GeV photons. This delay could be caused by the angular spreading effect of the scattered microwave photons or deflection of the secondly pairs due to intergalactic magnetic field. Very high energy TeV photons of GRBs could be produced by a few mechanisms including the proton-synchrotron radiation and electron inverse Compton emission from GRB internal shocks as well as external shocks. We suggest that the information provided by the delayed emission could give constraints on models for TeV gamma-rays. A more accurate calculation of the delayed time caused by the angular spreading effect is presented by considering recent observations of the extragalactic infrared background and the theoretic high-redshift infrared background. We also suggest that the dependence of the maximum time delay of scattered photons on their energies, if determined by future GLAST detector, could differentiate the two mechanisms causing the time delay. Comment: 16 pages including 1 table, accepted for publication in ApJ
    The Astrophysical Journal 11/2003; · 6.73 Impact Factor
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    K. S. Cheng, X Y Wang
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    ABSTRACT: 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.
    The Astrophysical Journal 08/2003; · 6.73 Impact Factor
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    X Y Wang, Z. G. Dai, T. Lu
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    ABSTRACT: 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. Comment: Accepted for publication in ApJ, minor changes; 11 pages, including 5 figures
    The Astrophysical Journal 03/2003; · 6.73 Impact Factor
  • X. Y. Wang, Z. G. Dai, T. Lu
    Proceedings of the International Astronomical Union 01/2003; 214.
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    X. Y. Wang, Z. G. Dai, T. Lu
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    ABSTRACT: The absorption feature detected in the prompt X-ray emission of GRB 990705 has important consequences for its circum-burst environment and therefore on its afterglow. 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 (F~ t-1.68) but a much faster decay (alpha >2.6; F~ t-alpha ) 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>~ 104-105 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.
    Astronomy and Astrophysics 01/2003; 401:593-597. · 5.08 Impact Factor
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    X Y Wang, Z. G. Dai, T. Lu
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    ABSTRACT: 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.
    12/2002;
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    X Y Wang, Z. G. Dai, T. Lu
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    ABSTRACT: 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. Comment: 12 pages, two missing references added, MNRAS, in press
    Monthly Notices of the Royal Astronomical Society 06/2002; · 5.52 Impact Factor
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    X Y Wang, Z. G. Dai, T. Lu
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    ABSTRACT: 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. Comment: 15 pages, 4 EPS figures, Latex, accepted for publication in ApJ, scheduled for the v556 n2 Aug 1, 2001 issue
    The Astrophysical Journal 04/2001; · 6.73 Impact Factor
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    X. Y. Wang, Z. G. Dai, T. Lu
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    ABSTRACT: We have constrained the intrinsic parameters, such as the magnetic energy density fraction (εB), the electron energy density fraction (εe), the initial Lorentz factor (Γ0) and the Lorentz factor of the reverse external shock (Γrs), of GRB 990123, 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 εe and εB are consistent with the suggestion that they may be universal parameters, comparing to those inferred for GRB 970508; (2) the reverse external shock may have become relativistic before it passed through the ejecta shell. Other intrinsic parameters of GRB 990123, such as the energy contained in the forward shock E and the ambient density n, are also determined and discussed in this paper.
    Monthly Notices of the Royal Astronomical Society 11/2000; 319(4):1159 - 1162. · 5.52 Impact Factor

Publication Stats

96 Citations
1 Download
766 Views
124.82 Total Impact Points

Institutions

  • 2009
    • Huazhong (Central China) Normal University
      Wu-han-shih, Hubei, China
  • 2000–2006
    • Nanjing University
      • Department of Astronomy
      Nanjing, Jiangsu Sheng, China
    • Northeast Institute of Geography and Agroecology
      • Institute of High Energy Physics
      Beijing, Beijing Shi, China