arXiv subjects

D. M. Wei

Publications and source records attributed to D. M. Wei.

36 records · Page 2Linked to original sources

arXiv2004

Prediction on the very early Afterglow of X-ray Flashes

In the past two years, tremendous progresses to understand X-ray Flashes have been made. Now it is widely believed that X-ray Flashes and Gamma-ray Bursts are intrinsically the same, their much different peak energy and flux may be just due to our different viewing angles to them. Here we analytically calculate the very early afterglow of X-ray Flashes, i.e., the reverse shock emission powered by the outflows interacting with the interstellar medium. Assuming $z\sim 0.3$, we have shown that typically the R Band flux of reverse shock emission can be bright to $16\sim 17$th magnitude (the actual values are model dependent and sensitive to the initial Lorentz factor of the viewed ejecta). That emission is bright enough to be detected by the telescope on work today such as ROTSE-III or the upcoming UVOT carried by Swift Satellite, planned for launching in later 2004.

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arXiv2004

Gamma-ray burst internal shocks with magnetization

(Please note that the abstract has been significantly shorted) We investigate Gamma-ray Burst (GRB) internal shocks with moderate magnetization, with the magnetization parameter $σ$ ranging from 0.001 to 10. Possible magnetic dissipation in the stripped magnetized shells is also taken into account through introducing a parameter $k$ ($0<k\leq 1$), which is the ratio of the electric field strength of the downstream and the upstream. By solving the general MHD jump conditions, we show that the dynamic evolution of the shock with magnetic dissipation is different from the familiar one obtained in the ideal MHD limit. In view of the possible high degree of linear polarization of GRB 021206 and the identification of a possible highly magnetized flow in GRB 990123 and GRB 021211, we suggest that a mildly magnetized internal shock model ($0.01<σ<1$) with moderate magnetic dissipation is a good candidate to explain the GRB prompt emission data.

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arXiv2003

Can the Bump be Observed in the Early Afterglow of GRBS with X-Ray Line Emission Features?

Extremely powerful emission lines are observed in the X-ray afterglow of several GRBs. The energy contained in the illuminating continuum which is responsible for the line production exceeds 10$^{51}$ erg, much higher than that of the collimated GRBs. It constrains the models which explain the production of X-ray emission lines. In this paper, We argue that this energy can come from a continuous postburst outflow. Focusing on a central engine of highly magnetized millisecond pulsar or magnetar we find that afterglow can be affected by the illuminating continuum, and therefore a distinct achromatic bump may be observed in the early afterglow lightcurves. With the luminosity of the continuous outflow which produces the line emission, we define the upper limit of the time when the bump feature appears. We argue that the reason why the achromatic bumps have not been detected so far is that the bumps should appear at the time too early to be observed.

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arXiv2003

Strong GeV Emission Accompanying TeV Blazar H1426+428

For High frequency BL Lac objects (HBLs) like H1426+428, a significant fraction of their TeV gamma-rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding $e^\pm$ pairs. The resulting $e^\pm$ pairs generate one hitherto undiscovered GeV emission by inverse Compton scattering with the cosmic microwave background photons (CMBPs). We study such emission by taking the 1998-2000 CAT data, the reanalyzed 1999 & 2000 HEGRA data and the corresponding intrinsic spectra proposed by Aharonian et al. (2003a). We numerically calculate the scattered photon spectra for different intergalactic magnetic field (IGMF) strengths. If the IGMF is about $10^{-18}{\rm G}$ or weaker, there comes very strong GeV emission, whose flux is far above the detection sensitivity of the upcoming satellite GLAST! Considered its relatively high redshift ($z=0.129$), the detected GeV emission in turn provides us a valuable chance to calibrate the poor known spectral energy distribution of the intergalactic infrared background, or provides us some reliable constraints on the poorly known IGMF strength.

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arXiv2003

X-ray flashes from off-axis nonuniform jets

It is widely believed that outflows of gamma-ray bursts are jetted. Some people also suggest that the jets may have structures like $ε(θ)\proptoθ^{-k}$. We test the possibility of X-ray flashes coming from such jets in this paper. Both qualitative and quantitative analyses have shown that this model can reproduce most observational features for both X-ray flashes and gamma-ray bursts. Using common parameters of gamma-ray bursts, we have done both uniform and nonuniform jets' numerical calculations for their fluxes, spectra and peak energies. It seems that nonuniform jets are more appropriated to these observational properties than uniform ones. We also give a spectrum and flux fit to the most significant X-ray flash, XRF971019 by our model. We also have shown that in our model the observational ratio of gamma-ray bursts to X-ray flashes is about several.

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arXiv2003

Are there cosmological evolution trends on Gamma-Ray Burst features?

The variability of gamma-ray burst (GRB) is thought to be correlated with its absolute peak luminosity, and this relation had been used to derive an estimate of the redshifts of GRBs. Recently Amati et al. presented the results of spectral and energetic properties of several GRBs with known redshifts. Here we analyse the properties of two group GRBs, one group with known redshift from afterglow observation, and another group with redshift derived from the luminosity - variability relation. We study the redshift dependence of various GRBs features in their cosmological rest frames, including the burst duration, the isotropic luminosity and radiated energy, and the peak energy $E_p$ of $νF_ν$ spectra. We find that, for these two group GRBs, their properties are all redshift dependent, i.e. their intrinsic duration, luminosity, radiated energy and peak energy $E_p$, are all correlated with the redshift, which means that there are cosmological evolution effects on gamma-ray bursts features, and this can provide an interesting clue to the nature of GRBs. If this is true, then the results also imply that the redshift derived from the luminosity - variability relation may be reliable.

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arXiv2003

The afterglow of GRB021211: Another case of reverse shock emission

GRB021211 was first detected by HETE II and its early afterglow has been observed. There is a break in its afterglow light curve at about 12 minutes after the bursts, before the break the optical flux decays with a power-law index of about -1.6, while at late time the power-law slope is about -1 (Chornock et al. 2002). Here we will show that the afterglow light curve of GRB021211 can be explained within the framework of the standard fireball model. We show that the afterglow emission before the break time is the contribution of the emission from both the reverse shock and the forward shock, while the afterglow emission after the break time is mainly due to the forward shock emission. From the fitting we can give constraints on the parameters: the initial Lorentz factor $250\leq γ_0 \leq 900$, the surrounding medium density $n\geq 1.6\times 10^{-3}$ atoms ${\rm cm^{-3}}$, and $0.077ε_{\rm B}^{3/16} \leq ε_{\rm B}^{1/4}(\frac{ε_{\rm e}}{0.1}) \leq 0.14$, where $ε_{\rm B}$ and $ε_{\rm e}$ are the fractions of the shock energy given to the magnetic field and electrons. We propose that since the values of $ε_{\rm B}$ and $ε_{\rm e}$ are somewhat smaller for GRB021211, so the peak energy of the reverse shock emission is well below the optical band, and thus it is substantially fainter than 990123 at similar epochs. Also we suggest that such a break might be a common feature in early optical afterglows.

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arXiv2003

GRB afterglow light curves from uniform and non-uniform jets

Here we calculate the GRB afterglow light curves from a relativistic jet as seen by observers at a wide range of viewing angles from the jet axis, and the jet is uniform or non-uniform. We find that, for uniform jet the afterglow light curves for different viewing angles are somewhat different: in general, there are two breaks in the light curve, corresponding to the time $γ\sim (θ_j-θ_v)^{-1}$ and $γ\sim (θ_j+θ_v)^{-1}$ respectively. However, for non-uniform jet, the things become more complicated. For the case $θ_v=0$, we can obtain the analytical results, for $k<8/(p+4)$ there should be two breaks in the light curve correspond to $γ\simθ_c^{-1}$ and $γ\simθ_j^{-1}$ respectively, while for $k>8/(p+4)$ there should be only one break corresponds to $γ\simθ_c^{-1}$, and this provides a possible explanation for some rapidly fading afterglows whose light curves have no breaks since the time at which $γ\simθ_c^{-1}$ is much earlier than our first observation time. For the case $θ_v\neq 0$, our numerical results show that, the afterglow light curves are strongly affected by the values of $θ_v$, $θ_c$ and $k$. If the values of $θ_v/θ_c$ and $k$ are larger, there will be a prominent flattening in the afterglow light curve, which is quite different from the uniform jet, and after the flattening a very sharp break will be occurred at the time $γ\sim (θ_v + θ_c)^{-1}.

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arXiv2002

Can all breaks in GRB afterglows be explained by jet effects?

Whether gamma-ray bursts are highly beamed or not is a very important question, since it has been pointed out that the beaming will lead to a sharp break in the afterglow light curves during the ultra-relativistic phase, with the breaking point determined by $Γ\sim 1/θ_{0}$, where $Γ$ is the bulk Lorentz factor and $θ_{0}$ is the initial half opening angle of the ejecta, and such a break is claimed to be present in the light curves of some GRBs. In this paper we will examine whether all the observed breaks in GRB afterglow light curves can be explained by jet effects. Here we present a detailed calculation of the jet evolution and emission, and have obtained a simple formula of bulk Lorentz factor evolution. We show that the light curves are very smoothly steepened by jet effect, and the shape of the light curve is determined by only one parameter -- $(E/n)^{1/8} θ_{0}^{3/4}$, where E and n are the fireball energy and surrounding medium density. We find that for GRB990123 and GRB991216, the jet model can approximately fit their light curves, and the values of $(E_{51}/n_1)^{1/8} θ_{0}^{3/4}$ are about 0.17 and 0.22 respectively. While for GRB990510, GRB000301c, GRB000926 and GRB010222, their light curves cannot be fitted by the jet model, which suggests that the breaks may be caused by some other reasons, jet should be not the unique reason.

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arXiv2001

Are some breaks in GRB afterglows caused by their spectra?

Sharp breaks have been observed in the afterglow light curves of several GRBs; this is generally explained by the jet model. However, there are still some uncertainties concerning this interpretation due to the unclear hydrodynamics of jet sideways expansion. Here we propose an alternative explanation to these observed breaks. If we assume that the multiwavelength spectra of GRB afterglows are not made of exact power law segments but their slope changes smoothly, i.e. $dβ/d logν<0$, where $β$ is the spectral index, we find that this fact can very nicely explain the afterglow light curves showing breaks. Therefore we suggest that some breaks in the afterglow light curves may be caused by their curved spectra. The main feature of this interpretation is that the break time is dependent on the observed frequency, while the jet model produces achromatic breaks in the light curves. In addition, it is very important to know the position of the characteristic frequency $ν_{c}$ in the multiwavelength spectrum at the time of the break, since it is a further discriminant between our model and the jet model. We find that although the optical light curves of seven GRB afterglows can be well fitted by the model we propose, in fact only one of them (i.e. GRB000926) can be explained in this framework, since for other ones the characteristic frequency $ν_{c}$ is either above the optical after the break or below the optical before the break.

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arXiv2000

The evolution of beamed GRB afterglow: non-relativistic case

There has been increasing evidence that at least some GRBs are emission beamed. The beamed GRB afterglow evolution has been discussed by several authors in the ultra-relativistic case. It has been shown that the dynamics of the blast wave will be significantly modified by the sideways expansion, and there may be a sharp break in the afterglow light curves under certain circumstances. However, this is true only when the fireball is still relativistic. Here we present an analytical approach to the evolution of the beamed GRB blast wave expanding in the surrounding medium (density $n\propto r^{-s}$) in the non-relativistic case, our purpose is to explore whether the sideways expansion will strongly affect the blast wave evolution as in the relativistic case. We find that the blast wave evolution is strongly dependent on the speed of the sideways expansion. If it expands with the sound speed, then the jet angle $θ$ increases with time as $θ\propto ln t$, which means that the sideways expansion has little effect on the afterglow light curves, the flux $F\propto t^{-\frac{3(5α-1)}{5}}$ for $s=0$ and $F\propto t^{-\frac{7α+1}{3}}$ for $s=2$.It is clear that the light curve of $s=2$ is not always steeper than that of $s=0$, as in the relativistic case.We also show that if the expansion speed is a constant, then the jet angle $θ\propto t$, and the radius $r \propto t^{0}$, in this case the sideways expansion has the most significant effect on the blast wave evolution, the flux $F\propto t^{-(5α-1)}$ independent of $s$, and we expect that there should be a smooth and gradual break in the light curve.

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arXiv2000

The influence of inverse Compton scattering on GRB afterglows: one possible way to flatten and steepen the light curves

The fireball model of gamma-ray bursts predicted that when the energetic blast wave encountered the surrounding medium, there will be afterglow emission, and the subsequent afterglow observations appeared to confirm this prediction. In this simplest fireball model, the electrons have been accelerated to a power law energy distribution in a relativistic blast wave, then they give afterglow emission through synchrotron radiation. Up to now synchrotron radiation is believed to be the main mechanism of GRB emission, however, here we will show that under some circumstances, the inverse Compton scattering (ICS) may play an important role, and can change the light curves of GRB afterglows. Here we investigate the effects of ICS in the relativistic case (the surrounding medium density $ρ\propto r^{-2}$) and in the non-relativistic case (for both $ρ=constant$ and $ρ\propto r^{-2}$), we find that in the relativistic case the effect of ICS is usually important, while in the non-relativistic case, this effect is usually unimportant, unless the surrounding medium density is very high. We show that if ICS is important, then it can flatten and steepen the light curves of GRB afterglows, and this may provide the explanation for some afterglow observations.

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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

Can the jet steepen the light curves of GRB afterglow?

Beaming of relativistic ejecta in GRBs has been postulated by many authors in order to reduce the total GRB energy, thus it is very important to look for the observational evidence of beaming. Rhoads (1999) has pointed out that the dynamics of the blast wave, which is formed when the beamed ejecta sweeping the external medium, will be significantly modified by the sideways expansion due to the increased swept up matter. He claimed that shortly after the bulk Lorentz factor ($Γ$) of the blast wave drops below the inverse of the initial opening angle ($θ_{0}$) of the beamed ejecta, there will be a sharp break in the afterglow light curves. However, some other authors have performed numerical calculations and shown that the break of the light curve is weaker and much smoother than the one analytically predicted. In this paper we reanalyse the dynamical evolution of the jet blast wave, calculate the jet emission analytically, we find that the sharp break predicted by Rhoads will actually not exist, and for most cases the afterglow light curve will almost not be affected by sideways expansion unless the beaming angle is extremely small. We demonstrate that only when $θ_{0}<0.1$, the afterglow light curves may be steepened by sideways expansion, and in fact there cannot be two breaks as claimed before. We have also constructed a simple numerical code to verify our conclusion.

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arXiv1998

Diverse Temporal Properties of GRB Afterglow

The detection of delayed X-ray, optical and radio emission, "afterglow", associated with $γ$-ray bursts (GRBs) is consistent with fireball models, where the emission are produced by relativistic expanding blast wave, driven by expanding fireball at cosmogical distances. The emission mechanisms of GRB afterglow have been discussed by many authors and synchrotron radiation is believed to be the main mechanism. The observations show that the optical light curves of two observed gamma-ray bursts, GRB970228 and GRB GRB970508, can be described by a simple power law, which seems to support the synchrotron radiation explanation. However, here we shall show that under some circumstances, the inverse Compton scattering (ICS) may play an important role in emission spectrum and this may influence the temporal properties of GRB afterglow. We expect that the light curves of GRB afterglow may consist of multi-components, which depends on the fireball parameters.

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arXiv1998

Gamma-ray bursts: postburst evolution of fireballs

The postburst evolution of fireballs that produce $γ$-ray bursts is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Due to adoption of a new relation among $t$, $R$ and $γ$ (see the text), our results differ markedly from the previous studies. Synchrotron radiation from the shocked interstellar medium is attentively calculated, using a convenient set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory approach to observations. It is also found that the expansion will no longer be highly relativistic about 4 days after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.

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arXiv1998

Is GRO J1744-28 a Strange Star?

The unusal hard x-ray burster GRO J1744-28 recently discovered by the Compton Gamma-ray Observatory (GRO) can be modeled as a strange star with a dipolar magnetic field $\le 10^{11} $Gauss. When the accreted mass of the star exceeds some critical mass, its crust may break, resulting in conversion of the accreted matter into strange matter and release of energy. Subsequently, a fireball may form and expand relativistically outward. The expanding fireball may interact with the surrounding interstellar medium, causing its kinetic energy to be radiated in shock waves, producing a burst of x-ray radiation. The burst energy, duration, interval and spectrum derived from such a model are consistent with the observations of GRO J1744-28.

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arXiv1997

The evolution of GRB remnants

The detection of the delayed emission in X-ray, optical and radio band, i.e. the afterglow of $γ$-ray bursts (GRBs), suggestes that the sources of GRBs are likely to be at cosmological distances. Here we explore the interaction of a relativistic shell with a uniform interstellar medium (ISM) and obtain the exact solution of the evolution of $γ$-ray burst remnants, including the radiative losses. We show that in general the evolution of bulk Lorentz factor $γ$ satisfies $γ\propto t^{-α_{t}}$ when $γ\gg 1$, here $α_{t}$ is mainly in the range $9/22 \sim 3/8$, the latter corresponds to adiabatic expansion. So it is clear that adiabatic expansion is a good approximation even when radiative loss is considered. However, in fact, $α_{t}$ is slightly larger than 3/8, which may have some effects on a detailed data analysis. Synchrotron-self-absorption is also calculated and it is demonstrated that the radio emission may become optically thin during the afterglow. Our solution can also apply to the nonrelativistic case ($γ\sim 1$), at that time the observed flux decrease more rapidly than that in the relativistic case.

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