X Y Wang

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

Are you X Y Wang?

Claim your profile

Publications (31)174.46 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years ago. However, considerable progress unraveling their mysteries has been made in recent years. Developments in observations, theory, and instrumentation have prepared the way so that the next decade can be the one in which we finally answer the question, "What are gamma-ray bursts?" This question encompasses not only what the progenitors are that produce the GRBs, but also how the enormous luminosity of the GRBs, concentrated in gamma rays, is achieved. Observations across the electromagnetic spectrum, from both the ground and space, will be required to fully tackle this important question. This white paper, mostly distilled from a recent study commissioned by the Division of Astrophysics of the American Physical Society, focuses on what very high energy (~100 GeV and above) gamma-ray observations can contribute. Very high energy gamma rays probe the most extreme high energy particle populations in the burst environment, testing models of lepton and proton acceleration in GRBs and constraining the bulk Lorentz factor and opacity of the outflow. Sensitivity improvements of more than an order of magnitude in the very high energy gamma-ray band can be achieved early in the next decade, in order to contribute to this science.
    03/2009;
  • Source
    Y. W. Yu, X. Y. Wang, Z. G. Dai
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This is a report on the findings of the gamma ray burst working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the overall version has also been released and can be found on astro-ph. This detailed section of the white paper discusses the status of past and current attempts to observe gamma ray bursts at GeV-TeV energies. We concentrate on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that major advances are possible and that the detection of very high energy emission would have strong implications for GRB models, as well as cosmic ray origin.
    11/2008;
  • Yun-Wei Yu, X. Y. Wang, Z. G. Dai
    [Show abstract] [Hide abstract]
    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;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Massive stars end their short lives in spectacular explosions-supernovae-that synthesize new elements and drive galaxy evolution. Historically, supernovae were discovered mainly through their `delayed' optical light (some days after the burst of neutrinos that marks the actual event), preventing observations in the first moments following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of the explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the `break-out' of the supernova shock wave from the progenitor star, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We predict that future wide-field X-ray surveys will catch each year hundreds of supernovae in the act of exploding.
    Nature 08/2008; 454(7201):246. · 38.60 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The prompt emission of gamma-ray bursts (GRBs) is generally thought to be produced by internal shocks within the relativistic outflow, while the longer-wavelength afterglow is produced by external shocks resulting from the interaction between the outflow and external medium. This picture has gained support from the clear separation between the prompt and afterglow phases seen in most Swift GRBs. However, no separation is seen in GRB 070318. The prompt X-ray emission extrapolated from the observed Swift BAT flux matches well the early X-ray afterglow emission in both temporal and spectral properties, and the combined 0.3-10 keV light curve can be fit with a single power-law with an index typical of late-time afterglows. The UV-optical light curve displays a similar power-law decay after the broad maximum. In addition, the gamma-ray emission displays a single, FRED(fast rise, exponential decay)-like peak and unusually smooth light curve, consistent with a single impulsive event interacting with the external medium.
    01/2008;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: ABSTRACTGRB 061007 was the brightest gamma-ray burst (GRB) to be detected by Swift and was accompanied by an exceptionally luminous afterglow that had a V-band magnitude <11.1 at 80 s after the prompt emission. From the start of the Swift observations the afterglow decayed as a power law with a slope of αX= 1.66 ± 0.01 in the X-ray and αopt= 1.64 ± 0.01 in the UV/optical, up to the point that it was no longer detected above background in the optical or X-ray bands. The brightness of this GRB and the similarity in the decay rate of the X-ray, optical and γ-ray emission from 100 s after the trigger distinguish this burst from others and present a challenge to the fireball model. The lack of a cooling or jet break in the afterglow up to ∼105 s constrains any model that can produce the large luminosity observed in GRB 061007, which we found to require either an excessively large kinetic energy or highly collimated outflow. Analysis of the multiwavelength spectral and high-resolution temporal data taken with Swift suggests an early time jet break to be a more plausible scenario than a highly energetic GRB. This must have occurred within 80 s of the prompt emission, which places an upper limit on the jet opening angle of θj= 0.8°. Such a highly collimated outflow resolves the energy budget problem presented in a spherical emission model, reducing the isotropic-equivalent energy of this burst to Ecorrγ= 1050 erg, consistent with other GRBs.
    Monthly Notices of the Royal Astronomical Society 09/2007; 380(3):1041 - 1052. · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: GRB061007 is the brightest gamma-ray burst (GRB) to be detected by Swift and is accompanied by an exceptionally luminous afterglow that had a V-band magnitude <11.1 at 80s after the prompt emission. From the start of the Swift observations the afterglow decayed as a power law with a slope of \alpha_X=1.66+/-0.01 in the X-ray and \alpha_{opt}=1.64+/-0.01 in the UV/optical, up to the point that it was no longer detected above background in the optical or X-ray bands. The brightness of this GRB and the similarity in the decay rate of the X-ray, optical and gamma-ray emission from 100s after the trigger distinguish this burst from others and present a challenge to the fireball model. The lack of a cooling or jet break in the afterglow up to \~10^5s constrains any model that can produce the large luminosity observed in GRB061007, which we found to require either an excessively large kinetic energy or highly collimated outflow. Analysis of the multi-wavelength spectral and high-resolution temporal data taken with Swift suggest an early time jet-break to be a more plausible scenario. This must have occurred within 80s of the prompt emission, which places an upper limit on the jet opening angle of \theta_j=0.8deg. Such a highly collimated outflow resolves the energy budget problem presented in a spherical emission model, reducing the isotropic equivalent energy of this burst to E_{\gamma}^{corr}=10^{50} ergs; consistent with other GRBs. Comment: Accepted by MNRAS, 11 pages, 3 figures
    Monthly Notices of the Royal Astronomical Society 11/2006; · 5.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: %auto-ignore This paper has been withdrawn by the authors due to dublicate submission. To download the paper please go to astro-ph/0611081 Comment: This paper has been withdrawn
    11/2006;
  • P. Mészáros, S. Razzaque, X. Y. Wang
    [Show abstract] [Hide abstract]
    ABSTRACT: Gamma‐Ray Bursts (GRB) are powerful sources of MeV gamma‐rays, whose prompt emission has been detected in some case up to tens of GeV. Leptonic emission mechanisms could produce also TeV gamma‐rays, and if proton acceleration takes place, hadronic and photopion processes are expected to produce TeV neutrino emission as well. Current models for the production of GeV‐TeV photons and neutrinos are discussed. © 2006 American Institute of Physics
    AIP Conference Proceedings. 07/2006; 842(1):1007-1009.
  • [Show abstract] [Hide abstract]
    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.
  • Source
    Z G Dai, X Y Wang, X F Wu, B Zhang
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
  • Source
    K. S. Cheng, T. Harko, X Y Wang
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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

Publication Stats

410 Citations
174.46 Total Impact Points

Institutions

  • 2009
    • Huazhong (Central China) Normal University
      Wu-han-shih, Hubei, China
  • 2007
    • Stanford University
      • Department of Physics
      Palo Alto, California, United States
  • 2006–2007
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, MD, United States
  • 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