Search

The influence of fast neutron irradiation on the structure and spatial distribution of Ge nanocrystals (NC) embedded in an amorphous SiO2 matrix has been studied. The investigation was conducted by means of laser Raman Scattering (RS), High Resolution Transmission Electron Microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The irradiation of NC-Ge samples by a high dose of fast neutrons lead to a partial destruction of the nanocrystals. Full reconstruction of crystallinity was achieved after annealing the radiation damage at 800 deg. C, which resulted in full restoration of the RS spectrum. HR-TEM images show, however, that the spatial distributions of NC-Ge changed as a result of irradiation and annealing. A sharp decrease in NC distribution towards the SiO2 surface has been observed. This was accompanied by XPS detection of Ge oxides and elemental Ge within both the surface and subsurface region.

Extensive multi-band afterglow data are available for GRB 980703. Especially, its radio afterglow was very bright and was monitored until more than 1000 days after the trigger time. Additionally, there is no obvious special feature, i.e., no rebrightenings, no plateau, and no special steep decay or slow decay in the multi-band afterglow light curves. All these conditions make GRB 980703 a precious sample in gamma-ray burst research. Here we use the observational data of GRB 980703 to test the standard fireball model in depth. It is found that the model can give a satisfactory explanation to the multi-band and overall afterglow light curves. The beaming angle of GRB 980703 is derived as ~0.23 radian, and the circum-burst medium density is ~ 27 cm-3. The total isotropic equivalent kinetic energy of the ejecta is ~ 3.8E52 ergs. A rest-frame extinction of Av ~ 2.5 mag in the host galaxy is also derived., Comment: 10 pages, 3 figures, PDF version only; typos corrected, to appear in: Science in China Series G

We study the optimal use of third order statistics in the analysis of weak lensing by large-scale structure. These higher order statistics have long been advocated as a powerful tool to break measured degeneracies between cosmological parameters. Using ray-tracing simulations, incorporating important survey features such as a realistic depth-dependent redshift distribution, we find that a joint two- and three-point correlation function analysis is a much stronger probe of cosmology than the skewness statistic. We compare different observing strategies, showing that for a limited survey time there is an optimal depth for the measurement of third-order statistics, which balances statistical noise and cosmic variance against signal amplitude. We find that the chosen CFHTLS observing strategy was optimal and forecast that a joint two- and three-point analysis of the completed CFHTLS-Wide will constrain the amplitude of the matter power spectrum $\sigma_8$ to 10% and the matter density parameter $\Omega_m$ to 17%, a factor of ~2.5 improvement on the two-point analysis alone. Our error analysis includes all non-Gaussian terms, finding that the coupling between cosmic variance and shot noise is a non-negligible contribution which should be included in any future analytical error calculations., Comment: 27 pages, 13 figures, 3 tables

In this paper, we revisit the weak gravitational lensing of CMB. The key point is that the deflection angle of CMB photon varies with the redshift. From this we obtain a set of modified power spectra. Compared with the ones calculated before, the temperature, the polarization E mode and the cross power spectra correction are of the order $0.01 %$, while the polarization B mode correction is about $0.4 %$ (to $l=2000$)., Comment: Accepted for publication in the Astrophysical Journal. 18 pages, 6 figures

The usual inflationary scenario predicts a Gaussian random primordial density fluctuation, different Fourier modes of which do not correlate with each other. In this paper we propose a correlation between these different modes. A simple case is that these different Fourier modes correlate with each other following a Gaussian function. For such a primordial density fluctuation we calculate the CMB angular power spectrum and find that its amplitude decreases but the decrease is different for different $l$. This feature can be used to constrain the the correlation strength from the real data., Comment: accepted by PRD, 5 pages, 6 figures

Aims. The high-redshift (z = 4:048) gamma-ray burst GRB 060206 showed unusual behavior, with a significant rebrightening by a factor of ~ 4 at about 3000 s after the burst. We argue that this rebrightening implies that the central engine became active again after the main burst produced by the first ejecta, then drove another more collimated jet-like ejecta with a larger viewing angle. The two ejecta both interacted with the ambient medium, giving rise to forward shocks that propagated into the ambient medium and reverse shocks that penetrated into the ejecta. The total emission was a combination of the emissions from the reverse- and forward- shocked regions. We discuss how this combined emission accounts for the observed rebrightening. Methods. We apply numerical models to calculate the light curves from the shocked regions, which include a forward shock originating in the first ejecta and a forward-reverse shock for the second ejecta. Results. We find evidence that the central engine became active again 2000 s after the main burst. The combined emission produced by interactions of these two ejecta with the ambient medium can describe the properties of the afterglow of this burst. We argue that the rapid rise in brightness at ~ 3000 s in the afterglow is due to the off-axis emission from the second ejecta. The precession of the torus or accretion disk of the central engine is a natural explanation for the departure of the second ejecta from the line of sight., Comment: 6 pages, 2 figures

Since the inhomogeneous instrument noise can produce extra non-Gaussianity in the CMB anisotropy, its effect should be carefully subtracted in the primordial non-Gaussianity estimation. We calculate the probability distribution function of the CMB anisotropy for local type of non-Gaussianity, from which the optimal estimator in the general case (inhomogeneous noise and cut sky) is obtained. The new estimator obtained here is different from the popular one, since the inhomogeneous noise and cut sky effects are completely accounted. The CMB anisotropy in the new estimator is noise weighted. The noise weight is different from that used by WMAP Group in their 5-year data analysis. Although it is still difficult to calculate the new estimator rigorously, for the case of the slightly inhomogeneous noise, there exists a series expansion method to compute the new estimator. Each order in the series is suppressed by two factors, $(\frac{\sigma^2}{\sigma^2_{i}}-1)$ and $\frac{C_l}{C^{tot}_{l}}$, which make the method feasible. Through the Edgeworth expansion we can generalize our discussion to other types of non-Gaussianity., Comment: 11 pages.no figures

An integrated piecewise thermal equilibrium approach based on the first-principles calculation method has been developed to calculate bias dependent electronic structures and current- and differential conductance-voltage characteristics of the gold-benzene-1,4-dithiol-gold molecular junction. The calculated currents and differential conductance have the same order of magnitude as experimental ones. An electron transfer was found between the two electrodes when a bias is applied, which renders the two electrodes to have different local electronic structures. It was also found that when Au 5d electrons were treated as core electrons the calculated currents were overestimated, which can be understood as an underestimate of the Au-S covalent bonding and consequently the contact potential barrier and the replacement of delocalized Au 5d carriers by more itinerant delocalized Au 6sp carriers in the electrodes.

The discovery of multiband afterglows definitely shows that most $\gamma$-ray bursts are of cosmological origin. $\gamma$-ray bursts are found to be one of the most violent explosive phenomena in the Universe, in which astonishing ultra-relativistic motions are involved. In this article, the multiband observational characteristics of $\gamma$-ray bursts and their afterglows are briefly reviewed. The standard model of $\gamma$-ray bursts, i.e. the fireball model, is described. Emphasis is then put on the importance of the nonrelativistic phase of afterglows. The concept of deep Newtonian phase is elaborated. A generic dynamical model that is applicable in both the relativistic and nonrelativistic phases is introduced. Based on these elaborations, the overall afterglow behaviors, from the very early stages to the very late stages, can be conveniently calculated., Comment: A review paper accepted for publication in a Chinese journal of: Progress in Natural Science. 6 figures, 21 pages

We have investigated the valley splitting of two-dimensional electrons in high quality Si/Si$_{1-x}$Ge$_x$ heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor $\nu=3$ ($\Delta_3$) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear density dependence of $\Delta_3$ on the electron density was observed, while the slope of these two configurations differs by more than a factor of two. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of $\Delta_3$ before and after the coincidence., Comment: REVTEX 4 pages, 4 figures

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., Comment: 15 pages, 4 figures, ApJ submitted

Gamma-ray burst afterglows have been observed for months or even years in a few cases. It deserves noting that at such late stages, the remnants should have entered the deep Newtonian phase, during which the majority of shock-accelerated electrons will no longer be highly relativistic. To calculate the afterglows, we must assume that the electrons obey a power-law distribution according to their kinetic energy, not simply the Lorentz factor., Comment: Poster at the 4th workshop "Gamma-Ray Bursts in the Afterglow Era" (Rome, 2004), accepted for publication in the proceedings. 4 pages, with 3 figures inserted

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., Comment: 9 pages including 6 figures, accepted for publication in MNRAS

(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 $<\alpha_{X}> \sim 1.3$ 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., Comment: 28 pages including 2 figures, accepted by ApJ on October 11

The overall dynamical evolution and radiation mechanism of $\gamma$-ray burst jets are briefly introduced. Various interesting topics concerning beaming in $\gamma$-ray bursts are discussed, including jet structures, orphan afterglows and cylindrical jets. The possible connection between $\gamma$-ray bursts and neutron star kicks is also addressed., Comment: 10 Pages, 4 figures, to appear in a special issue of ApSS. Oral report presented at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero)

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

In many GRBs, afterglows have been observed for months or even years. It deserves noting that at such late stages, the remnants should have entered the deep Newtonian phase, during which the majority of shock-accelerated electrons will no longer be highly relativistic. However, a small portion of electrons are still ultra-relativistic and capable of emitting synchrotron radiation. Under the assumption that the electrons obey a power-law distribution according to their kinetic energy (not simply the Lorentz factor), we calculate optical afterglows from both isotropic fireballs and beamed ejecta, paying special attention to the late stages. In the beamed cases, it is found that the light curves are universally characterized by a flattening during the deep Newtonian phase. Implication of our results on orphan afterglows is also addressed., Comment: Shortened to 4 pages, to appear in the proceedings of the 2003 GRB conference (Santa Fe, Sept 8 -- 12, 2003)

Optical afterglows from two-component jets under various configurations are investigated numerically. Generally, the light curve is characterized by a rapid rebrightening when the observer is off-axis with respect to the narrow component, with the amplitude and peak time depending on detailed parameters. We further show that the optical afterglow of XRF 030723, especially its notable and rapid rebrightening, can be well explained by a typical two-component jet. This X-ray flash, together with GRB 030329, strongly hints the two-component jet model as a unified picture for X-ray flashes and gamma-ray bursts. With a narrow but ultra-relativistic inner outflow and a wide but less energetic outer ejecta, a two-component jet will be observed as a typical gamma-ray burst if our line of sight is within the angular scope of the narrow outflow. Otherwise, if the line of sight is within or slightly beyond the cone of the wide component, an X-ray flash will be detected., Comment: 1 figure inserted and references added, 19 pages, ApJ in press

Gamma-ray bursts (GRBs) are believed to originate from ultra-relativistic winds/fireballs to avoid the "compactness problem". However, the most energetic photons in GRBs may still suffer from $\gamma-\gamma$ absorption leading to electron/positron pair production in the winds/fireballs. We show here that in a wide range of model parameters, the resulting pairs may dominate those electrons associated with baryons. Later on, the pairs would be carried into a reverse shock so that a shocked pair-rich fireball may produce a strong flash at lower frequencies, i.e. in the IR band, in contrast with optical/UV emission from a pair-poor fireball. The IR emission would show a 5/2 spectral index due to strong self-absorption. Rapid responses to GRB triggers in the IR band would detect such strong flashes. The future detections of many IR flashes will infer that the rarity of prompt optical/UV emissions is in fact due to dust obscuration in the star formation regions., Comment: 8 pages, 2 figures, ApJ accepted

The shock model has successfully explained the observed behaviors of afterglows from long gamma-ray bursts (GRBs). Here we use it to investigate the so-called early afterglows from short GRBs, which arises from blast waves that are not decelerated considerably by their surrounding medium. We consider a nearby medium loaded with $e^{\pm}$ pairs (Beloborodov 2002). The temporal behaviors show first a soft-to-hard spectral evolution, from the optical to hard X-ray, and then a usual hard-to-soft evolution after the blast waves begin to decelerate. The light curves show variability, and consist of two peaks. The first peak, due to the pair effect, can be observed in the X-ray, though too faint and too short in the optical. The second peak will be easily detected by {\it Swift}. We show that detections of the double-peak structure in the light curves of early afterglows are very helpful to determine all the shock parameters of short GRBs, including both the parameters of the relativistic source and the surroundings. Besides, from the requirement that the forward-shock emission in short GRBs should be below the BATSE detection threshold, we give a strong constraint on the shock model parameters. In particular, the initial Lorentz factor of the source is limited to be no more than $\sim 10^3$, and the ambient medium density is inferred to be low, $n\la 10^{-1}$ cm$^{-3}$., Comment: 5 pages, 1 figure, minor changes to match the publish in MNRAS

The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best fitted physical parameters for GRB 990123 (Panaitescu & Kumar 2001) and the assumption that the parameters in the optical flash are the same as those in the afterglow, we show that: 1) the shell is thick but not thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3\times10^{-4} of the observed. In order to compensate this divergency, the physical parameters the electron energy ration and the magnetic ratio $\epsilon_e$, $\epsilon_B$ should be 0.61, 0.39 respectively, which are much different from those in the late afterglow., Comment: 7 pages, no figure

The interaction of a relativistic fireball with its ambient medium is described through two shocks: a reverse shock that propagates into the fireball, and a forward shock that propagates into the medium. The observed optical flash of GRB 990123 has been considered to be the emission from such a reverse shock. The observational properties of afterglows suggest that the progenitors of some GRBs may be massive stars and their surrounding media may be stellar winds. We here study very early afterglows from the reverse and forward shocks in winds. An optical flash mainly arises from the relativistic reverse shock while a radio flare is produced by the forward shock. The peak flux densities of optical flashes are larger than 1 Jy for typical parameters, if we do not take into account some appropriate dust obscuration along the line of sight. The radio flare always has a long lasting constant flux, which will not be covered up by interstellar scintillation. The non-detections of optical flashes brighter than about 9th magnitude may constrain the GRBs isotropic energies to be no more than a few $10^{52}$ ergs and wind intensities to be relatively weak., Comment: 21 pages, 6 figures, accepted by MNRAS on March 7, 2003

The idea that gamma-ray bursts might be a kind of phenomena associated with neutron star kicks was first proposed by Dar & Plaga (1999). Here we study this mechanism in more detail and point out that the neutron star should be a high speed one (with proper motion larger than $\sim 1000$ km/s). It is shown that the model agrees well with observations in many aspects, such as the energetics, the event rate, the collimation, the bimodal distribution of durations, the narrowly clustered intrinsic energy, and the association of gamma-ray bursts with supernovae and star forming regions. We also discuss the implications of this model on the neutron star kick mechanism, and suggest that the high kick speed were probably acquired due to the electromagnetic rocket effect of a millisecond magnetar with an off-centered magnetic dipole., Comment: 12 pages, no figures, ApJ in press (minor revision with respect to Version 2)

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

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., Comment: 9 pages, 3 eps figures embedded. Based on the talk presented at the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17, 2002)

In previous works, a generic dynamical model has been suggested by Huang et al., which is shown to be correct for both adiabatic and radiative blastwaves, and in both ultra-relativistic and non-relativistic phases. In deriving their equations, Huang et al. have assumed that the radiative efficiency of the fireball is constant. They then applied their model directly to realistic cases where the radiative efficiency evolves with time. In this paper, we abandon the above assumption and re-derive a more accurate dynamical equation for gamma-ray burst remnants. Numerical results show that Huang et al.'s model is accurate enough in general cases., Comment: 10 pages, 5 embedded eps figures, to appear in: Chin. J. Astron. Astrophys. (2002), Vol 2

It is widely believed that multiwavelength afterglows of gamma-ray bursts (GRBs) originate from relativistic blast waves. We here show that in such blast waves, a significant fraction of the energy of shock-accelerated protons would be lost due to pion production by interactions with afterglow photons. This could lead to long-term production of $10^{16}$--$10^{18}$ eV neutrinos and sub-TeV $\gamma$-rays that accompany with usual afterglows, provided that the protons are accelerated to $10^{19}$ eV in the blast waves., Comment: 10 pages, A&A accepted

Based on a refined generic dynamical model, we investigate afterglows from jetted gamma-ray burst (GRB) remnants numerically. In the relativistic phase, the light curve break could marginally be seen. However, an obvious break does exist at the transition from the relativistic phase to the non-relativistic phase, which typically occurs at time 10 to 30 days. It is very interesting that the break is affected by many parameters, especially by the electron energy fraction (xi_e), and the magnetic energy fraction (xi_B^2). Implication of orphan afterglow surveys on GRB beaming is investigated. The possible existence of a kind of cylindrical jets is also discussed., Comment: Minor changes; 10 pages, with 9 eps figures embedded. Talk given at the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17, 2002). A slightly revised version will appear in the proceedings

The absorption feature detected in the prompt X-ray emission of GRB990705 has important consequences for its circum-burst environment and therefore on its afterglows. 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 but a much faster decay 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 ($n\ga 10^4-10^5 {\rm cm^{-3}}$) 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., Comment: 5 pages, 1 figures, final version, Astronomy & Astrophysics, in press

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