arXiv subjects

Z. G. Dai

Publications and source records attributed to Z. G. Dai.

At least 181 records · Page 10Linked to original sources

arXiv2006

Constraining the Cosmological Parameters and Transition Redshift with Gamma-Ray Bursts and Supernovae

A new method of measuring cosmology with gamma-ray bursts(GRBs) has been proposed by Liang and Zhang recently. In this method, only observable quantities including the rest frame peak energy of the νF_νspectrum (E'_p), the isotropic energy of GRB (E_{γ,iso}), and the break time of the optical afterglow light curves in the rest frame (t'_b) are used. By considering this method we constrain the cosmological parameters and the redshift at which the universe expanded from the deceleration to acceleration phase. We add five recently-detected GRBs to the sample and derive E_{γ, iso}/10^{52} ergs=(0.93\pm 0.25)\times (E'_p/100 keV)^{1.91\pm 0.32}\times (t'_b/day)^{-0.93\pm 0.38} for a flat cosmology with Ω_M=0.28 and H_0=71.0 km s^{-1} Mpc^{-1}. This relation is independent of the medium density around bursts and the efficiency of conversion of the explosion energy to gamma-ray energy that are difficult to measure. We regard the E_{γ,iso}(E'_p, t'_b) relationship as a standard candle and find 0.05<Ω_M<0.48 and Ω_Λ<1.15 (at the 1σconfidence level). In a flat universe with the cosmological constant we obtain 0.25<Ω_M <0.46 and 0.54<Ω_Λ<0.78 at the 1σconfidence level. The transition redshift is z_T=0.69_{-0.12}^{+0.11}. Combining 20 GRBs and 157 type Ia supernovae, we find Ω_M=0.29 \pm0.03 for a flat universe and the transition redshift is z_T=0.61_{-0.05}^{+0.06}, which is slightly larger than the value found by considering SNe Ia alone. In particular, We also discuss several dark-energy models in which the equation of state w(z) is arameterized and investigate constraints on the cosmological parameters in detail.

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arXiv2005

X-ray flares: late internal and late external shocks

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. Under the assumption that the central engine has two periods of activities, we get four basic types of X-ray light curves. The classification of these types depends on which period of activities produces the prompt gamma-ray emission (Type 1 and Type 2: the earlier period; Type 3 and Type 4: the late period), and on whether the late ejecta catching up with the early ejecta happens earlier than the deceleration of the early ejecta (Type 1 and Type 3) or not (Type 2 and Type 4). We find that the X-ray flare caused by a late external shock is a special case of Type 1. Our analysis reveals that the X-ray light curves of GRBs 050406, 050421, and 050607 can be classified as Type 1, while the X-ray light curve of GRB 050502B is classified as Type 2. However, the X-ray light curve of GRB 050406 is also likely to be Type 2. We also predict a long-lag short-lived X-ray flare caused by the inner external shock, which forms when a low baryon-loading long-lag late ejecta decelerates in the non-relativistic tail of an outer external shock driven by an early ejecta.

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arXiv2005

Early Afterglows in Wind Environments Revisited

When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyze the reverse-forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g., an optical flash may emerge. The reverse shock behaves differently in two approximations: relativistic and Newtonian cases, which depend on the parameters, e.g., the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than $114 E_{53}^{1/4} Δ_{0,12}^{-1/4} A_{*,-1}^{-1/4}$, the early reverse shock is Newtonian. This may take place for the wider of a two-component jet, an orphan afterglow caused by a low initial Lorentz factor, and so on. The synchrotron self absorption effect is significant especially for the Newtonian reverse shock case, since the absorption frequency $ν_a$ is larger than the cooling frequency $ν_c$ and the minimum synchrotron frequency $ν_m$ for typical parameters. For the optical to X-ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities, and compare them with these in the ISM model.

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arXiv2005

Testing Gamma-Ray Burst Jet Structure with the Distribution of Gamma-Ray Energy Release

We present a general method for testing gamma-ray burst (GRB) jet structure and carry out a comprehensive analysis about the prevalent jet structure models. According to the jet angular energy distribution, we can not only derive the expected distribution of the GRB isotropic-equivalent energy release for any possible jet structure, but also obtain a two-dimensional distribution including redshift z. By using the Kolmogorov-Smirnov test we compare the predicted distribution with the observed sample, and find that the power-law structured jet model is most consistent with the current sample and that the uniform jet model is also plausible. However, this conclusion is tentative because of the small size and the inhomogeneity of this sample. Future observations (e.g., Swift) will provide a larger and less biased sample for us to make a robust conclusion by using the procedure proposed in this paper.

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arXiv2005

Can Gamma-Ray Bursts Be Used to Measure Cosmology? A Further Analysis

Three different methods of measuring cosmology with gamma-ray bursts (GRBs) have been proposed since a relation between the $γ$-ray energy $E_γ $ of a GRB jet and the peak energy $E_p$ of the $νF_ν$ spectrum in the burst frame was reported by Ghirlanda and coauthors. In Method I, to calculate the probability for a favored cosmology, only the contribution of the $E_γ-E_p$ relation that is already best fitted for this cosmology is considered. We apply this method to a sample of 17 GRBs, and obtain the mass density $Ω_M=0.15^{+0.45}_{-0.13}$ ($1σ$) for a flat $Λ$CDM universe. In Method II, to calculate the probability for some certain cosmology, contributions of all the possible $E_γ-E_p$ relations that are best fitted for their corresponding cosmologies are taken into account. With this method, we find a constraint on the mass density $0.14<Ω_M<0.69$ ($1 σ$) for a flat universe. In Method III, to obtain the probability for some cosmology, contributions of all the possible $E_γ-E_p$ relations associated with their unequal weights are considered. With this method, we obtain an inspiring constraint on the mass density $0.16<Ω_M<0.45$ ($1 σ$) for a flat universe, and a $χ^2_{dof}=19.08/15=1.27$ for the concordance model of $Ω_M=0.27$. (abridged)

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arXiv2005

A Reverse-Shock Model for the Early Afterglow of GRB 050525A

The prompt localization of gamma-ray burst (GRB) 050525A by {\em Swift} allowed the rapid follow-up of the afterglow. The observations revealed that the optical afterglow had a major rebrightening starting at $\sim 0.01$ days and ending at $\sim 0.03$ days, which was followed by an initial power-law decay. Here we show that this early emission feature can be interpreted as the reverse shock emission superposed by the forward shock emission in an interstellar medium environment. By fitting the observed data, we further constrain some parameters of the standard fireball-shock model: the initial Lorentz factor of the ejecta $γ_0>120$, the magnetic energy fraction $ε_B>4\times10^{-6}$, and the medium density $n<2 {\rm cm^{-3}}$. These limits are consistent with those from the other very-early optical afterglows observed so far. In principle, a wind environment for GRB 050525A is disfavored.

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arXiv2005

A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR1806-20

The brightest giant flare from the soft $γ$-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least one order of magnitude more energetic than those of the two other SGR giant flares. Starting from about one week after the burst, a very bright ($\sim 80$ mJy), fading radio afterglow was detected. Follow-up observations revealed the multi-frequency 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 another component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss triggering mechanisms of these two components within the framework of the magnetar model.

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arXiv2005

An energetic blast wave from the December 27 giant flare of the soft gamma-ray repeater 1806-20

Recent follow-up observations of the December 27 giant flare of SGR 1806-20 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 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 gamma-ray burst afterglows. The optical afterglow is expected to be intrinsically as bright as $m_R\simeq13$ at $t\la 0.1$ days after the flare, but very heavy extinction makes the detection difficult because of the low galactic latitude of the source. Rapid infrared follow-up observations to giant flares are therefore crucial for the low-latitude SGRs, while for high-latitude SGRs (e.g. SGR 0526-66), rapid follow-ups should result in identification of their possible optical afterglows. Rapid multi-wavelength follow-ups will also provide more detailed information of the early evolution of a fireball as well as its composition.

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arXiv2005

Fireball/Blastwave Model and Soft Gamma-ray Repeaters

Soft gamma-ray repeaters are at determined distances and their positions are known accurately. If observed, afterglows from their soft gamma-ray bursts will provide important clues to the study of the so called "classical gamma-ray bursts". On applying the popular fireball/blastwave model of classical gamma-ray bursts to soft gamma-ray repeaters, it is found that their X-ray and optical afterglows are detectable. Monitoring of the three repeaters is solicited.

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arXiv2004

Afterglow Light Curves of Jetted Gamma-ray Burst Ejecta in Stellar Winds

Optical and radio afterglows arising from the shocks by relativistic conical ejecta running into pre-burst massive stellar winds are revisited. Under the homogeneous thin-shell approximation and a realistic treatment for the lateral expansion of jets, our results show that a notable break exists in the optical light curve in most cases we calculated in which the physical parameters are varied within reasonable ranges. For a relatively tenuous wind which cannot decelerate the relativistic jet to cause a light curve break within days, the wind termination shock due to the ram pressure of the surrounding medium occurs at a small radius, namely, a few times 10^{17} cm. In such a structured wind environment, the jet will pass through the wind within several hours and run into the outer uniform dense medium. The resulting optical light curve flattens with a shallower drop after the jet encounters the uniform medium, and then declines deeply, triggered by runaway lateral expansion.

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arXiv2004

Gamma-Ray Bursts: Polarization of Afterglows from Two-Component Jets

Polarization behaviors 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 about the lateral expansion of the jet can lead to different evolutions of the position angle of polarization. The observed afterglow light curve and polarization behaviors of GRB 020813 can be well explained by the two-component jet model. Particularly, 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.

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arXiv2004

Jet break time -- flux density relationship and constraints on physical parameters of GRB afterglows

We derive a relation between the flux density $F_{ν,j}$ at the light-curve break of a gamma-ray burst (GRB) afterglow and the break time $t_{j}$. The break is due to the transition from the spherical-like to jet-like evolution of the afterglow, when the Lorentz factor of the jet equals the inverse of the initial half-opening angle, i.e., $γ=1/θ_0$. We show that this relation indeed behaves as $F_{ν,j}\propto t_{j}^{-p}$ among GRBs for the slow-cooling case, where $p$ is the power-law index of electron distribution. A statistical analysis of the optical jet breaks of nine GRBs gives $p=2.10\pm 0.21$, which is consistent with the shock acceleration theory. The value of $p$ derived in this way is different from the observed temporal index $α_2$ ($F_ν\propto t^{-α_{2}}$) of the late-time light curve after $t_{j}$, which suffers several uncertainties from the unclear hydrodynamics of the sideways expansion and exhibits a large dispersion. Our results not only confirm that the remnants of GRBs are standard candles, but also provide the first evidence that the physical parameters of relativistic shocks are universal, with the favored values $ε_{e}\sim 0.1$ and $ε_{B}\sim 10^{-3}$.

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arXiv2004

Analytical light curves in the realistic model for GRB afterglows

(abridged) Afterglow light curves are constructed analytically for realistic gamma-ray burst remnants decelerating in either a homogeneous interstellar medium or a stellar wind environment, taking into account the radiative loss of the blast wave, which affects the temporal behaviors significantly. Inverse Compton scattering is considered. The inverse Compton effect prolongs the fast-cooling phase markedly, during which the relativistic shock is semi-radiative and the radiation efficiency is approximately constant. It is further shown that the shock is still semi-radiative for quite a long time after it transits into the slow-cooling phase. The temporal decaying index of the X-ray afterglow light curve in this semi-radiative phase is more consistent with the observed $<α_{X}> \sim 1.3$ than the commonly used adiabatic one. To manifest as a bump or even dominant in the X-ray afterglows during the relativistic stage, it is required that the density should be larger than about 1-10 cm$^{-3}$ in the interstellar medium case, or the wind parameter $A_{\ast}$ should be larger than unity in the stellar wind case.

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arXiv2004

Constraining $Ω_M$ and Dark Energy with Gamma-Ray Bursts

An $E_{γ,{\rm jet}}\propto {E'_p}^{1.5}$ relationship with a small scatter for current $γ$-ray burst (GRB) data was recently reported, where $E_{γ,{\rm jet}}$ is the beaming-corrected $γ$-ray energy and $E'_p$ is the $νF_ν$ peak energy in the local observer frame. By considering this relationship for a sample of 12 GRBs 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 $Ω_M=0.35\pm^{0.15}_{0.15}$ (at the $1σ$ confident level) for a flat universe with a cosmological constant, and the $w$ parameter of an assumed static dark-energy equation of state $w=-0.84\pm^{0.57}_{0.83}$ ($1σ$). Our results are consistent with those from type Ia supernovae. A larger sample established by the upcoming {\em Swift} satellite is expected to provide further constraints.

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arXiv2004

Testing the Predictions of the Universal-Structured Jet Model of Gamma-Ray Bursts by Simulations

Based on the standard energy reservoir of gamma-ray burst (GRB) jets and the relationship between peak energy ($E_p$) of $νF_ν$ spectrum and the equivalent-isotropic energy ($E_{\rm{iso}}$), we test the GRB probability distributions as a function of viewing angle ($θ$) and of viewing angle together with redshift $z$, $P(θ)$ and $P(θ,z)$, predicted by the universal-structured jet (USJ) model with simulations on observational bases for a detection threshold of $S>4 \times 10^{-5}$ erg cm$^{-2}$, where $S$ is the total fluence in an energy band $>20$ keV. We simulate a sample including $10^{6}$ GRBs, and then derive $P(θ)$ and $P(θ,z)$. We show that both simulated and theoretical $P(θ)$ and $P(θ,z)$ are consistent with each other. This consistency seems to be regarded as support for the USJ model. A small difference between the theoretical and simulated results might be due to the observed $E_p$ distribution used in our simulations, which perhaps suffers a little of biases for $E_p<100$ keV.

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arXiv2004

The Peak Energy Distribution of the nu F_nu Spectra and the Implications for the Jet Structure Models of Gamma-Ray Bursts

We study the peak energy ($E_{\rm{p}}$) distribution of the $νF_ν$ spectra of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) with a sample of 57 bursts observed by {\em High Energy Transient Explorer 2} ({\em HETE-2}) French Gamma Telescope and discuss its implications for the jet structure models. Combining the observed $E_{\rm{p}}$ distribution of {\em HETE--2} GRBs/XRFs with that of BATSE GRBs, we find that the observed $E_{\rm{p}}$ distribution of GRBs/XRFs is a bimodal one with peaks of $\lesssim 30$ keV and $\sim 160-250$ keV. According to the recently-discovered equivalent-isotropic energy-$E_{\rm{p}}$ 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 $\sim 15$ and $\sim 200$ keV. We argue that future observations of the peak of $\sim 15$ keV in the $E_{\rm{p}}$ distribution would be evidence supporting this model. {\em Swift}, which covers an energy band of 0.2--150 keV, is expected to provide a key test for our results.

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arXiv2004

The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models

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 (L_iso) and the spectral peak energy (E_p) in the cosmological rest frame, L_iso \propto E_p^2, not only holds within these bursts, but 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 emission models of GRBs are discussed. We suggest that both the kinetic-energy-dominated internal shock model and the magnetic-dissipation-dominated external shock model can well interpret this relation. We constrain the parameters for these two models, and find that they are in a good agreement with the parameters from the fittings to the afterglow data (abridged).

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arXiv2004

Relativistic Wind Bubbles and Afterglow Signatures

Highly magnetized, rapidly rotating compact objects are widely argued as central energy sources of $γ$-ray bursts (GRBs). After the GRB, such a magnetar-like object may directly lose its rotational energy through some magnetically-driven processes, which produce an ultrarelativistic wind dominated possibly by the energy flux of electron-positron pairs. The interaction of such a wind with an outward-expanding fireball leads to a relativistic wind bubble, being regarded as a relativistic version of the well-studied Crab Nebula. We here explore the dynamics of this wind bubble and its emission signatures. We find that when the injection energy significantly exceeds the initial energy of the fireball, the bulk Lorentz factor of the wind bubble decays more slowly than before, and more importantly, the reverse-shock emission could dominate the afterglow emission, which yields a bump in afterglow light curves. In addition, high polarization of the bump emission would be expected if a toroidal magnetic field in the shocked wind dominates over the random component.

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