Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts (GRBs) presented by Preece et al., we show that the relation between the isotropic-equivalent luminosity (Liso) and the peak energy (E′p of the vFv, spectrum in the cosmological rest frame, Liso ∝ E′p², holds within these bursts and also holds among these GRBs, assuming that the burst rate as a function of redshift is proportional to the star formation rate. The possible implications of this relation for the fireball models are discussed by defining a parameter ω Ξ (Liso/10⁵² ergs s ^-1)^0.5/(E′p/200 keV). It is found that ω is narrowly clustered in 0.1-1. We constrain some parameters for both the internal shock and external shock models from the requirement of ω 0.1-1, assuming that these model parameters are uncorrelated. The distributions of the parameters suggest that if the prompt gamma rays are produced from kinetic-energy-dominated internal shocks, they may be radiated from a region around R ∼ 10¹²-10¹³ cm (or Lorentz factor ∼130-410) with a combined internal shock parameter ζi ∼ 0.1-1 during the prompt gamma-ray phase, which is consistent with the standard internal shock model; if the prompt gamma rays of these GRBs are radiated from magnetic-dissipation-dominated external shocks, the narrow cluster of ω requires σ ∼ 1-470, Γ ∼ 216-511, E ∼ 10 ⁵¹-10⁵⁴ ergs, n ∼ 0.5-470 cm^-3, and ζe, ∼ 0.36-3.6, where σ is the ratio of the cold to hot luminosity components, Γ is the bulk Lorentz factor of the fireball, E is the total energy release in the gamma-ray band, n is the medium number density, and ζe, is a combined external shock parameter; these values are also in a good agreement with the fittings to the afterglow data. These results indicate that both the kinetic-energy-dominated internal shock model and the magnetic-dissipation-dominated external shock model can well interpret the Liso ∝ E′p relation and the value of ω.

Liang E. W.;Dai Z. G.;Wu X. F.

Astrophysical Journal

2004

We study the peak energy (Ep) distribution of the vFv spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by High Energy Transient Explorer (HETE-2) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed Ep distribution of HETE-2 GRBs/XRFs with that of BATSE GRBs, we find that the observed Ep distribution of GRBs/XRFs is a bimodal one with peaks of ≲30 keV and ∼ 160-250 keV. According to the recently discovered equivalent-isotropic energy-Ep relationship, such a bimodal distribution implies a two-component structure of GRB/XRF jets. A simple simulation analysis shows that this structured jet model does roughly reproduce a bimodal distribution with peaks of ∼15 and ∼200 keV. We argue that future observations of the peak of ∼15 keV in the Ep distribution would be evidence supporting this model. Swift, which covers an energy band of 0.2-150 keV, is expected to provide a key test for our results.

Liang E. W.;Dai Z. G.

Astrophysical Journal

2004

An E′γ,jet E′p^1.5 relationship with a small scatter for current gamma-ray burst (GRB) data was recently reported, where Eγ,jet, is the beaming-corrected gamma-ray energy and E′p is the vFV, peak energy in the local observer frame. By considering this relationship for a sample of 12 GRB s with known redshift, peak energy, and break time of afterglow light curves, we constrain the mass density of the universe and the nature of dark energy. We find that the mass density ΩM = 0.35 -0.15^+0.15 (at the 1 σ confidence level) for a flat universe with a cosmological constant, and the w parameter of an assumed static dark energy equation of state w = -0.84-0.83^+0.57 (1 σ). Our results are consistent with those from Type la supernovae. A larger sample established by the upcoming Swift satellite is expected to provide further constraints.

Dai Z. G.;Liang E. W.;Xu D.

Astrophysical Journal

2004

The polarization behaviour of optical afterglows from two-component gamma-ray burst jets are investigated, assuming various configurations for the two components. In most cases, the observed polarization is dominated by the inner narrow component for a long period. Interestingly, it is revealed that different assumptions concerning the lateral expansion of the jet can lead to different evolutions of the position angle of polarization. The observed afterglow light curve and polarization behaviour of GRB 020813 can be well explained by the two-component jet model. In particular, the model is able to explain the constancy of the observed position angle in this event, given that the line of sight is slightly outside the narrow component. © 2005 RAS.

Wu X. F.;Dai Z. G.;Huang Y. F.;Lu T.

Monthly Notices of the Royal Astronomical Society

2005

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. © Springer Science + Business Media, Inc. 2005.

Wang X. Y.;Dai Z. G.;Lu T.

Astrophysics and Space Science

2005

Similar to the case of pulsars the magnetic axis and the spin axis of gamma-ray burst sources may not lie on the same line. This may cause the formation of a ring-like jet due to collimation of the precessing magnetic axis. We analyze the tail emission from such a jet, and find that it has a shallow decay phase with a temporal index of -1/2 if the Lorentz factor of the ejecta is not very high, which is consistent with the shallow decay phase of some early X-ray afterglow detected by Swift. The ring-like jet has a tail cusp with sharp rising and very sharp decay. This effect can provide an explanation for the re-brightening and sharp decay of the X-ray afterglow of GRB 050709.

Zou Yuan-Chuan;Dai Zi-Gao

Chinese Journal of Astronomy and Astrophysics

2006

Swift BAT has detected ∼200 long-duration GRBs, with redshift measurements for ∼50 of them. We derive the luminosity function (Φ^HL) and the local event rate (ρ0^HL) of the conventional high-luminosity (HL) GRBs by using the z-known Swift GRBs. Our results are generally consistent with that derived from the CGRO BATSE data. However, the fact that Swift detected a low-luminosity (LL) GRB, GRB 060218, at z = 0.033 within ∼2 years of operation, together with the previous detection of the nearby GRB 980425, suggests a much higher local rate for these LL-GRBs. We explore the possibility that LL-GRBs are a distinct GRB population from the HL-GRBs. We find that ρ0^LL is ∼325 -177⁺³⁵² Gpc^-3 yr^-1, which is much higher than ρ0^HL (1.12-0.20^+0.43 Gpc^-3 yr^-1). This rate is ∼0.7% of the local Type Ib/c SNe. Our results, together with the finding that less than 10% of Type Ib/c SNe are associated with off-beam GRBs, suggest that LL-GRBs have a beaming factor typically less than 14, or a jet angle typically wider than 31°. The high local GRB rate, small beaming factor, and low-luminosity make the LL-GRBs distinct from the HL-GRBs. Although the current data could not fully rule out the possibility that both HL- and LL-GRBs are the same population, our results suggest that LL-GRBs are likely a unique GRB population and that the observed low-redshift GRB sample is dominated by the LL-GRBs. © 2007. The American Astronomical Society. All rights reserved.

Liang Enwei;Zhang Bing;Virgili Francisco;Dai Z. G.

Astrophysical Journal

2007

From an analysis of radio and X observations of PSR 0656+14, we found its rotation axis to be inclined at 30° to its magnetic axis and at 14° to the line of sight. These values will make the theoretical X light curve coincide with the observed curve by the ROSAT satellite. According to the cooling processes in neutron stars and strange stars we found that PSR 0656+14 may either be a proton-rich neutron star (in which direct URCA processes are present) or a strange star. © 1994.

Dai Zi-gao;Lu Tan;Song Li-ming;Wei Da-ming

Chinese Astronomy and Astrophysics

1994

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 produced 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 from 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 predicts 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. © 2009. The American Astronomical Society. All rights reserved.

Yu Y. W.;Wang X. Y.;Dai Z. G.

Astrophysical Journal

2009

It was widely shown that the cosmological parameters and dark energy can be constrained by using data from type-Ia supernovae (SNe Ia), cosmic microwave background (CMB) anisotropy, cosmic large scale structure, and so on. Recently gamma-ray bursts (GRBs) were also argued to be promising standard candles for cosmography. in this paper, we summarize the results of Wang et al. (2007). We present constraints on the cosmological parameters and dark energy by combining a recent GRB sample with other cosmological probes. We find that for the ACDM cosmology this combination makes the constraints stringent and the best fit is close to the flat universe. We also find that several dark energy models used usually are consistent with the ACDM cosmology. Furthermore, we reconstruct the dark energy equation-of-state parameter w(z) and the deceleration parameter q(z), and obtain the transition redshift of cosmic past deceleration to present acceleration. © 2008 American Institute of Physics.

Dai Z. G.;Wang F. Y.

Aip Conference Proceedings

2008