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1.
The central compact object for some gamma-ray bursts (GRBs) may be a strongly magnetized millisecond pulsar. It can inject energy to the outer shock of the GRB by through the magnetic dipole radiation, and therefore causes the shallow decay of the early afterglow. Recently, from a large number of GRB X-ray afterglows observed by Swift/XRT(X-ray telescope), it is revealed that many of them exhibit the shallow decay about 102∼104 s after the burst prompt emission. We have fitted the X-ray afterglow light curves of 11 GRBs by using the energy injection model of a magnetar with the rotation period in the millisecond order of magnitude. The obtained result shows the validity and universality of the magnetar energy injection model in explaining the shallow decay of afterglows, and simultaneously provides some constraints on the magnetic field strength and rotation period of the central magnetar.  相似文献   

2.
We present an internal shock model with external characteristics for explaining the complicated light curves of gamma-ray bursts. Shocks produce gamma-rays in the interaction between a precessing beam of relativistic particles and the interstellar medium. Each time the particle beam passes the same line of sight with the observer the interstellar medium is pushed outward. Subsequent interactions between the medium and the beam are delayed by the extra distance to be travelled for the particles before the shock can form. This results in a natural retardation and leads to an intrinsic asymmetry in the light curves produced for gamma-ray bursts. In addition, we account for the cooling of the electron–proton plasma in the shocked region, which gives rise to an exponential decay in the gamma-ray flux. The combination of these effects and the precessing jet of ultrarelativistic particles produces light curves that can be directly compared with observed gamma-ray burst light curves. We illustrate the model by fitting a number of observed gamma-ray bursts that are difficult to explain with only a precessing jet. We develop a genetic algorithm to fit several observed gamma-ray bursts with remarkable accuracy. We find that for different bursts the observed fluence, assuming isotropic emission, easily varies over four orders of magnitude from the energy generated intrinsically.  相似文献   

3.
We present optical UBVRI photometric and spectroscopic data of the Type Ibn supernova SN 2006jc, until the onset of the dust-forming phase. The optical spectrum shows a blue continuum and is dominated by the presence of moderately narrow (velocity ∼2500 km s−1) He  i emission lines superimposed over a relatively weak supernova spectrum. The helium lines are produced in a pre-existing He-rich circumstellar shell. The observed helium line fluxes indicate the circumstellar shell is dense, with a density of  ∼109–1010 cm−3  . The helium mass in this shell is estimated to be  ≲0.07 M⊙  . The optical light curves show a clear signature of dust formation, indicated by a sharp decrease in the magnitudes around day 50, accompanied by a reddening of the colours. The evolution of the optical light curves during the early phase and that of the uvoir bolometric light curve at all phases is reasonably similar to normal Ib/c supernovae.  相似文献   

4.
The MASTER robotic telescope has obtained the first optical images of GRB 060926. We have discovered an optical flare from GRB 060926, a recurrent brightening that closely follows its behavior in the X-ray range. We have determined the spectral slope for GRB 060926 from the X-ray to optical range in the first minutes. Based on the spinar model, we show that the parameters of the optical and X-ray flares suggest that the gamma-ray burst resulted from the core collapse of a 7M ⊙ star with an initial effective Kerr parameter of 7.6 and an initial magnetic-to-gravitational energy ratio of 10?4.  相似文献   

5.
It is widely accepted that many gamma-ray bursts (GRBs) are produced by relativistic jets. Previous studies on the beaming effects in GRBs are mainly based on the conical geometry. However, some observations of the relativistic jets in radio galaxies, active galactic nuclei, and “micro-quasars” have shown that many of these outflows are cylindrical, but not conical. In this study, we assume that the jets that produce GRBs are cylindrical, and that the circum-burst environment is dense and optically thick. In the prompt burst phase, the strong X-ray emission can sublimate the circum-burst medium to form an optically thin channel, from which the optical photons are allowed to escape. As a result, the optical afterglows can be observed only for the observers who are positioned on the axes of jets. It is shown that the observed optical afterglows usually decay very rapidly (in the form of Sv oc t^v^l1 where p is the index of electron power-law distribution), due to the joint effect of the lateral expansion of the cylindrical jet and the absorption of optical photons by the dust outside the channel. Our model provides a possible explanation for the dark gamma-ray bursts.  相似文献   

6.
Being the largest gravitationally bound structures in the Universe, galaxy clusters are huge reservoirs of photons generated by the bremsstrahlung of a hot cluster gas. We consider the absorption of high-energy photons from distant cosmological gamma-ray sources by the bremsstrahlung of galaxy clusters. The magnitude of this effect is the third in order of smallness after the effects of absorption by the cosmic microwave background and absorption by the extragalactic background light. Our calculations of the effect of absorption by the bremsstrahlung of galaxy clusters have shown that this effect manifests itself in the energy range ~1–100 GeV and can be τ ~ 10?5 in optical depth.  相似文献   

7.
We present gamma-ray burst afterglow light curves in X-ray, optical and radio bands for various distributions of accelerated electrons behind the shock. The effects of lateral expansion of the jet and of winds in typical Wolf-Rayet star on the evolution are discussed. The light curves in the radiative case decline more rapidly than those in the adiabatic case. Under the combined effect of jet expansion and wind environment, the light curves have the greatest deviation from those of the standard model. All these results refer to the relativistic phase.  相似文献   

8.
We present the results of X-ray and optical observations of GRB 050712 performed by Swift . The X-ray light curve of this burst exhibits episodes of flares in the first 1000 s, the same epoch at which the UVOT detected an optical counterpart. A shallow X-ray decay, with a decay slope of  α=−0.73  , followed and lasted ∼70 ks. This behaviour can be explained in terms of activity of the gamma-ray burst 'inner engine', with the possibility that the last flare is caused by the interaction of the ejecta with the surrounding medium.
We also find interesting spectral parameters for the X-ray emission. In particular, data suggest the presence of an intrinsic absorption in the first 1000 s, which can be explained if circumburst medium clouds lie along the line of sight.  相似文献   

9.
We present broadband (radio,optical,X-ray and GeV) fits to the afterglow light curves and spectra of three long-duration gamma-ray bursts (GRBs 080916C,090902B,and 090926A) detected by the Gamma-Ray Burst Monitor and Large Area Telescope (LAT) instruments on the Fermi satellite.Using the observed broadband data,we study the origin of the high energy emission,and suggest that the early-time GeV emission and the late-time radio,optical,and X-ray afterglows can be understood as being due to synchrotron emissio...  相似文献   

10.
We present the results of concurrent X-ray and optical monitoring of the Seyfert 1 galaxy Mrk 79 over a period of more than 5 yr. We find that on short to medium time-scales (days to a few tens of days) the 2–10 keV X-ray and optical u - and V -band fluxes are significantly correlated, with a delay between the bands consistent with 0 d. We show that most of these variations may be well reproduced by a model where the short-term optical variations originate from reprocessing of X-rays by an optically thick accretion disc. The optical light curves, however, also display long time-scale variations over thousands of days, which are not present in the X-ray light curve. These optical variations must originate from an independent variability mechanism and we show that they can be produced by variations in the (geometrically) thin disc accretion rate as well as by varying reprocessed fractions through changes in the location of the X-ray corona.  相似文献   

11.
We investigate the possibility of constructing a unified model for the radio and X-ray outbursts of nova CI Cam 1998 in terms of the shock interaction of the nova envelope with circumstellar gas. In a spherical model, we manage to describe the kinematics and evolution of the radio source flux and very roughly the evolution of the X-ray flux. The X-ray spectrum in this model is appreciably harder. Better agreement with observations in all respects is shown by the model for the interaction of a spherical nova envelope with a nonspherical circumstellar medium. The latter is simulated in our model by a combination of rarefied bipolar conical outflows of stationary wind with an opening angle of 120° and a dense equatorial disk. In the optimal model, the initial kinetic energy of the nova envelope is ∼5 × 1043 erg and its mass lies within the range (1–5) × 10−7 M ⊙. The energy and mass of the nova envelope as well as the mass loss rate in the nonspherical model are close to those obtained in the spherical model by Filippova et al. (2008).  相似文献   

12.
Observational parameters of the optical and gamma-ray emissions from 58 gamma-ray bursts (GRBs) with discovered afterglows and known redshifts are analyzed. The distributions of these parameters and pair correlations between them are studied. Approximately half of the objects exhibit a relatively slow decrease in the optical flux at initial afterglow phases (with a power-law index in the decay law α < 1). Correlations have been found between the luminosities, energies, and durations of the optical and gamma-ray emissions, which can be explained by the presence of universal features in the light curves. A correlation of the peak luminosity for afterglows with the redshift and an anticorrelation of their durations with the redshift have been found for the first time. Against the background of a weak z dependence of the total afterglow energy, this effect can be explained by cosmological evolution of the GRB environment, which determines the rate of optical energy release.  相似文献   

13.
We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～104 K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? a = 10?8-10?6 M ⊙yr?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10?6 M ⊙yr?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images.  相似文献   

14.
The physico-chemical origin of the hydrogenated carbon clusters (cumulenes, PAHs, graphite or amorphous carbon) in space is still an open question. We have worked out a numerical simulation code in order to build up planar (graphite-like) carbon clusters. We assume that hydrogen atoms can fix on the carbon skeleton following a random process allowing forH 2 formation. The structures we have found are very complex. In a given cluster, several molecular entities can simultaneously be present: (sp 2) carbon chains, rings or compact formations (aromatic structures or small PAHs). We argue that these very contorted hydrogenated structures could be ubiquitous in the interstellar medium, in carbon-rich circumstellar regions and PNe.  相似文献   

15.
We present optical photometry of the X-ray transient XTE J2123−058, obtained in 1998 July–October. The light curves are strongly modulated on the 5.95-h orbital period, and exhibit dramatic changes in amplitude and form during the decline. We used synthetic models, which include the effect of partial eclipses and X-ray heating effects, to estimate the system parameters, and we constrain the binary inclination to be i =73°±4. The model is successful in reproducing the light curves at different stages of the decay by requiring the accretion disc to become smaller and thinner by 30 per cent as the system fades by 1.7 mag in the optical. From August 26 the system reaches quiescence with a mean magnitude of R =21.7±0.1 and our data are consistent with the optical variability being dominated by the ellipsoidal modulation of the companion.  相似文献   

16.
The initial stages of deceleration in the circumstellar medium of a stellar envelope, thrown off by a shock wave, are investigated. The equations of spherical-symmetric adiabatic hydrodynamics are shown to have a similarity solution in the case of the density of the expanding envelope being approximated by a reasonable power law. The overall flow pattern has such a form that the stellar material is decelerated in the internal shock wave while another shock propagates through the circumstellar matter. Between the shocks there is a contact discontinuity separating the circumstellar and stellar matter. The characteristics of the similarity solution are calculated for various exponents in the density laws of an expanding envelope and circumstellar matter and for two values of the adiabatic index (=5/3, 4/3). Some parts of the flow exhibit Rayleigh-Taylor instability.Special attention is paid to the validity of the hydrodynamics. In full agreement with D'yachenkoet al. (1969), we conclude that the kinetic and collisionless processes are of great importance if the initial stages of stellar envelope deceleration are to be properly monitored.The results obtained can also be employed to describe the interaction between the exploding core of a red giant star and its rarefied envelope. This is of interest for explosive nucleosynthesis.The similarity solution is applied to the envelopes expelled both by type-II supernovae and by rapid novae. In particular, the thermalization time-scale of circumstellar plasma is estimated. For SNii this time-scale proves to be of the order of 60 yr. This confirms with the observational data on the moment of the maximum radio-emission of young SNRs. In the case of rapid novae, this time is less by a factor of 10. Therefore, the peak radio and X-ray (2 keV) lumnosity may occur several years after the rapid nova outburst. The explosion of a degenerate carbon core is found to result in the heating of the hydrogen-helium envelope of a red giant star up to 3×106 K.  相似文献   

17.
We propose that gamma-ray bursts (GRBs) are produced by a shower of heavy blobs running into circumstellar material at highly relativistic speeds. The gamma-ray emission is produced in the shocks these bullets drive into the surrounding medium. The short-term variability seen in GRBs is set by the slowing-down time of the bullets, while the overall duration of the burst is set by the lifetime of the central engine. A requirement of this model is that the ambient medium be dense, consistent with a strong stellar wind. The efficiency of the burst can be relatively high.  相似文献   

18.
We study the evolution of the circumstellar medium of massive stars. We pay particular attention to Wolf-Rayet stars that are thought to be the progenitors of some long gamma-ray bursts (GRBs). We detail the mass-loss rates we use in our stellar evolution models and how we estimate the stellar wind speeds during different phases. With these details we simulate the interactions between the wind and the interstellar medium to predict the circumstellar environment around the stars at the time of core-collapse. We then investigate how the structure of the environment might affect the GRB afterglow. We find that when the afterglow jet encounters the free-wind/stalled-wind interface, rebrightening occurs and a bump is seen in the afterglow light curve. However, our predicted positions of this interface are too distant from the site of the GRB to reach while the afterglow remains observable. The values of the final wind density,   A *  , from our stellar models are of the same order (≲1) as some of the values inferred from observed afterglow light curves. We do not reproduce the lowest   A *  values below 0.5 inferred from afterglow observations. For these cases, we suggest that the progenitors could have been a WO-type Wolf–Rayet (WR) star or a very low-metallicity star. Finally, we turn our attention to the matter of stellar wind material producing absorption lines in the afterglow spectra. We discuss the observational signatures of two WR stellar types, WC and WO, in the afterglow light curve and spectra. We also indicate how it may be possible to constrain the initial mass and metallicity of a GRB progenitor by using the inferred wind density and wind velocity.  相似文献   

19.
We present optical and infrared observations of BQ Cam, the optical counterpart to the Be/X-ray transient system V0332+53. BQ Cam is shown to be an O8–9Ve star, which places V0332+53 at a distance of ∼7 kpc. H α spectroscopy and infrared photometry are used to discuss the evolution of the circumstellar envelope. Owing to the low inclination of the system, parameters are strongly constrained. We find strong evidence for a tilt of the orbital plane with respect to the circumstellar disc (presumably on the equatorial plane). Even though the periastron distance is only ≈10 R *, during the present quiescent state the circumstellar disc does not extend to the distance of periastron passage. Under these conditions, X-ray emission is effectively prevented by centrifugal inhibition of accretion. The circumstellar disc is shown to be optically thick at optical and infrared wavelengths, which, together with its small size, is taken as an indication of tidal truncation.  相似文献   

20.
We present the first statistical analysis of 27 Ultraviolet Optical Telescope (UVOT) optical/ultraviolet light curves of gamma-ray burst (GRB) afterglows. We have found, through analysis of the light curves in the observer's frame, that a significant fraction rise in the first 500 s after the GRB trigger, all light curves decay after 500 s, typically as a power law with a relatively narrow distribution of decay indices, and the brightest optical afterglows tend to decay the quickest. We find that the rise could be either produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising light curve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8 per cent confidence, there is a correlation, in the observed frame, between the apparent magnitude of the light curves at 400 s and the rate of decay after 500 s. However, in the rest frame, a Spearman rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle θ or within a core of a uniform energy density  θc  . We also produced logarithmic luminosity distributions for three rest-frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT light curves with the X-ray Telescope (XRT) light-curve canonical model. The range in decay indices seen in UVOT light curves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model, there is no indication of the rising behaviour observed in the UVOT light curves.  相似文献   

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