arXiv subjects

D. M. Wei

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

At least 19 recordsLinked to original sources

arXiv2009

The X-ray afterglow of GRB 081109A: clue to the wind bubble structure

We present the prompt BAT and afterglow XRT data of Swift-discovered GRB081109A up to ~ 5\times 10^5 sec after the trigger, and the early ground-based optical follow-ups. The temporal and spectral indices of the X-ray afterglow emission change remarkably. We interpret this as the GRB jet first traversing the freely expanding supersonic stellar wind of the progenitor with density varying as $ρ\propto r^{-2}$. Then after approximately 300 sec the jet traverses into a region of apparent constant density similar to that expected in the stalled-wind region of a stellar wind bubble or the interstellar medium (ISM). The optical afterglow data are generally consistent with such a scenario. Our best numerical model has a wind density parameter {$A_{*} \sim 0.02$, a density of the stalled wind $n\sim 0.12 {\rm cm}^{-3}$, and a transition radius $ \sim 4.5 \times 10^{17}$ cm}. Such a transition radius is smaller than that predicted by numerical simulations of the stellar wind bubbles and may be due to a rapidly evolving wind of the progenitor close to the time of its core-collapse.

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arXiv2009

Very High Energy Gamma-ray Afterglow Emission of Nearby Gamma-ray Bursts

The synchrotron self-Compton (SSC) emission from Gamma-ray Burst (GRB) forward shock can extend to the very-high-energy (VHE; $E_γ> $100 GeV) range. Such high energy photons are rare and are attenuated by the cosmic infrared background before reaching us. In this work, we discuss the prospect to detect these VHE photons using the current ground-based Cherenkov detectors. Our calculated results are consistent with the upper limits obtained with several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during the afterglow phase. For 5 bursts in our nearby GRB sample (except for GRB 030329), current ground-based Cherenkov detectors would not be expected to detect the modeled VHE signal. Only for those very bright and nearby bursts like GRB 030329, detection of VHE photons is possible under favorable observing conditions and a delayed observation time of $\la$10 hours.

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arXiv2008

VHE Gamma-ray Afterglow Emission from Nearby GRBs

Gamma-ray Bursts (GRBs) are among the potential extragalactic sources of very-high-energy (VHE) gamma-rays. We discuss the prospects of detecting VHE gamma-rays with current ground-based Cherenkov instruments during the afterglow phase. Using the fireball model, we calculate the synchrotron self-Compton (SSC) emission from forward-shock electrons. The modeled results are compared with the observational afterglow data taken with and/or the sensitivity level of ground-based VHE instruments (e.g. STACEE, H.E.S.S., MAGIC, VERITAS, and Whipple). We find that modeled SSC emission from bright and nearby bursts such as GRB 030329 are detectable by these instruments even with a delayed observation time of ~10 hours.

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arXiv2008

On Dust Extinction of Gamma-ray Burst Host Galaxies

Although it is well recognized that gamma-ray burst (GRB) afterglows are obscured and reddened by dust in their host galaxies, the wavelength-dependence and quantity of dust extinction are still poorly known. Current studies on this mostly rely on fitting the afterglow spectral energy distributions (SEDs) with template extinction models. The inferred extinction (both quantity and wavelength-dependence) and dust-to-gas ratios are often in disagreement with that obtained from dust depletion and X-ray spectroscopy studies. We argue that this discrepancy could result from the prior assumption of a template extinction law. We propose an analytical formula to approximate the GRB host extinction law. With the template extinction laws self-contained, and the capability of revealing extinction laws differing from the conventional ones, it is shown that this is a powerful approach in modeling the afterglow SEDs to derive GRB host extinction.

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arXiv2006

The GRB early optical flashes from internal shocks: application to GRB990123, GRB041219a and GRB060111b

With the successful launch of the Swift Gamma-Ray Burst Explorer, people expected the prompt optical flash like GRB990123 would be easily detected. However the fact that early optical flash have not been detected for a number of GRBs indicates the reverse shock must be suppressed. Here we explore the possibility that the optical flash may arise from the internal shock. We find that, under certain circumstance, the optical flash of GRB990123 and GRB060111b can really be explained by the internal shock. For GRB041219a, the prompt optical emission was correlated with the gamma-ray emission, we explain this feature also in the internal shock scenario, the optical emission is the low energy extension of the gamma-ray emission, and we can restrict its redshift $z\sim 0.2$. As for GRB050904, we have shown in previous paper that the optical flash was produced by synchrotron radiation and the X-ray flare was produced by the synchrotron-self-Compton mechanism. Therefore we conclude that the early optical flash of GRBs can usually come from the internal shock. Meanwhile since the condition to produce the optical flash is not easily satisfied, so the optical flash like GRB990123 should not be common in GRBs. In addition, we also discussed the synchrotron-self-Compton effect in the internal shock model, and find that for different values of parameters, there would be soft gamma-ray (100 KeV), hard gamma-ray (10 MeV) and GeV flare accompanying the optical flash. For GRB like GRB990123, a GeV flare with fluence about 10^{-8} erg cm^{-2} s^{-1} is expected, which may be detected by the GLAST satellite.

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arXiv2005

The Very Early Gamma-ray Burst Afterglows Powered by Structured Jets

If X-ray flashes (XRFs) and X-ray rich Gamma-ray Bursts(XRRGs) have the same origin with Gamma-ray Bursts (GRBs) but are viewed from larger angles of structured jets, their early afterglows may differ from those of GRBs. When the ultra-relativistic outflow interact with the surrounding medium, there are two shocks formed, one is a forward shock, the other is a reverse shock. In this paper we calculate numerically the early afterglow powered by uniform jet, Gaussian jet and power-law jet in the forward-reverse shock scenario. A set of differential equations are used to govern the dynamical evolution and synchrotron self-Compton effect has been taken into account to calculate the emission. In uniform jets, the very early afterglows of XRRGs and XRFs are significantly lower than GRBs and the observed peak times of RS emission are longer in interstellar medium environment. The RS components in XRRGs and XRFs are difficult to be detected. But in stellar wind, the reduce of very early flux and the delay of RS peak time are not so remarkable. In nonuniform jet(Gaussian jet and power-law jet), where there are emission materials on the line of sight, the very early light curve resembles isotropic-equivalent ejecta in general although the RS flux decay index shows notable deviation if the RS is relativistic(in stellar wind).

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arXiv2005

The optical flare and afterglow light curve of GRB 050904 at redshift z=6.29

GRB050904 is very interesting since it is by far the most distant GRB event known to date($z=6.29$). It was reported that during the prompt high energy emission phase, a very bright optical flare was detected, and it was temporal coincident with an X-ray flare. Here we use two models to explain the optical flare, One is the "late internal shock model", in which the optical flare is produced by the synchrotron radiation of the electrons accelerated by the late internal shock, and the X-ray flare is produced by the synchrotron-self-Compton mechanism. The other is the external forward-reverse shock model, in which the optical flare is from the reverse shock emission and the X-ray flare is attributed to the central engine activity. We show that with proper parameters, a bright optical flare can appear in both models. We think the "late internal shock model" is more favored since in this model the optical flash and the X-ray flare have the same origin, which provides a natural explanation of the temporal coincidence of them. In the forward-reverse shock scenario, fits to the optical flare and the late afterglow suggests that the physical parameters of the reverse shock are much different from that of forward shock, as found in modeling the optical flash of GRB 990123 previously.

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arXiv2005

The X-ray emission lines in GRB afterglows: the evidence for the two-component jet model

Recently, X-ray emission lines have been observed in X-ray afterglows of several $γ$-ray bursts. It is a major breakthrough for understanding the nature of the progenitors. It is proposed that the X-ray emission lines can be well explained by the Geometry-Dominated models, but in these models the illuminating angle is much larger than that of the collimated jet of the $γ$-ray bursts(GRBs). For GRB 011211, we obtain the illuminating angle is about $θ\sim45^{\circ}$, while the angle of GRB jet is only $3.6^{\circ}$, so we propose that the outflow of the GRBs with emission lines should have two distinct components. The wide component illuminates the reprocessing material, and produces the emission lines, while the narrow one produces the $γ$-ray bursts. The observations show that the energy for producing the emission lines is higher than that of the GRBs. In this case, when the wide component dominates the afterglows, a bump will appear in the GRBs afterglows. For GRB 011211, the emergence time of the bump is less than 0.05 days after the GRB, it is obviously too early for the observation to catch it. With the presence of the X-ray emission lines there should also be a bright emission component between the UV and the soft X-rays. These features can be tested by the $Swift$ satellite in the near future.

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arXiv2005

Late internal shock model for bright X-ray flares in Gamma-ray Burst afterglows and GRB 011121

We explore two possible models which might give rise to bright X-ray flares in GRBs afterglows. One is an external forward-reverse shock model, in which the shock parameters of forward/reverse shocks are taken to be quite different. The other is a so called "late internal shock model", which requires a refreshed unsteady relativistic outflow generated after the prompt $γ-$ray emission. In the forward-reverse shock model, after the time $t_\times$ at which the RS crosses the ejecta, the flux declines more slowly than $(t_\oplus/t_\times)^{-(2+β)}$, where $t_\oplus$ denotes the observer's time and $β$ is the spectral index of the X-ray emission. In the ``late internal shock model", decaying slopes much steeper than $(t_\oplus/t_{\rm e, \oplus})^{-(2+β)}$ are possible if the central engine shuts down after $t_{\rm e, \oplus}$ and the observed variability timescale of the X-ray flare is much shorter than $t_{\rm e, \oplus}$. The sharp decline of the X-ray flares detected in GRB 011121, XRF 050406, GRB 050502b, and GRB 050730 rules out the external forward-reverse shock model directly and favors the "late internal shock model". These X-ray flares could thus hint that the central engine operates again and a new unsteady relativistic outflow is generated just a few minutes after the intrinsic hard burst.

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arXiv2005

Early optical afterglow lightcurves of neutron-fed Gamma-ray bursts

In this paper, within the popular internal shock scenario of GRBs, we calculate the early optical afterglow lightcurves of a neutron-fed GRB fireball for different assumed neutron fractions in the fireball and for both ISM- and wind-interaction models. The cases for both long and short GRBs are considered. We show that as long as the neutron fraction is significant (e.g. the number of neutrons is comparable to that of protons), rich afterglow signatures would show up. For a constant density (ISM) model, a neutron-rich early afterglow is characterized by a slowly rising lightcurve followed by a sharp re-brightening bump caused by collision between the leading neutron decay trail ejecta and the trailing ion ejecta. For a massive star stellar-wind model, the neutron-rich early afterglow shows an extended plateau lasting for about 100 seconds before the lightcurve starts to decay. The plateau is mainly attributed to the emission from the unshocked neutron decay trail. When the overlapping of the initial prompt $γ-$rays with the shocks and the trail is important, as is common for the wind model and is also possible in the ISM model under some conditions, the IC cooling effect suppresses the very early optical afterglow significantly, making the neutron-fed signature dimmer. For short GRBs powered by compact star mergers, a neutron-decay-induced step-like re-brightening is predicted, although the amplitude is not large. All these neutron-fed signatures are likely detectable by the Ultraviolet Optical Telescope (UVOT) on board the {\em Swift} observatory if GRB fireballs are indeed baryonic and neutron-rich. Close monitoring of early afterglows from 10s to 1000s of seconds, when combined with detailed theoretical modeling, could be used to potentially diagnose the existence of the neutron component in GRB fireballs.

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arXiv2005

Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs

For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock crosses the ejecta, usually the shocked regions are still precipitated by the prompt MeV γ-ray emission. Because of the tight overlapping of the MeV photon flow with the shocked regions, the optical depth for the GeV photons produced in the shocks is very large. These high energy photons are absorbed by the MeV photon flow and generate relativistic e^\pm pairs. These pairs re-scatter the soft X-ray photons from the forward shock as well as the prompt γ-ray photons and power detectable high energy emission, significant part of which is in the sub-GeV energy range. Since the total energy contained in the forward shock region and the reverse shock region are comparable, the predicted sub-GeV emission is independent on whether the GRB ejecta are magnetized (in which case the reverse shock IC and synchrotron self-Compton emission is suppressed). As a result, a sub-GeV flash is a generic signature for the GRB wind model, and it should be typically detectable by the future {\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission powered by the interaction of the shock accelerated protons with the synchrotron photons in both the forward and reverse shock regions, there comes another $10^{14}$eV neutrino emission component powered by protons interacting with the MeV photon flow. This last component has a similar spectrum to the one generated in the internal shock phase, but the typical energy is slightly lower.

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arXiv2005

Early optical-IR emission from GRB 041219a: neutron-rich internal shocks and a mildly magnetized external reverse shock

Very early optical and near infrared (IR) emission was discovered accompanying the long gamma-ray burst (GRB) 041219a. We show that the optical/IR flash tracking the gamma-ray lightcurve during the prompt emission could be understood as emission from neutron-rich internal shocks, as has been suggested by Fan & Wei. The early K_s-band afterglow lightcurve after the prompt phase could be well-modeled as the superposition of a reverse shock and a forward shock component. The radio data also support the reverse shock interpretation.

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arXiv2005

High Energy Afterglow Emission from Giant Flares of Soft Gamma-Ray Repeaters: The Case of the 2004 December 27 Event from SGR 1806-20

We discuss the high enegry afterglow emission (including high energy photons, neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Γ_0\sim {\rm tens}, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the intial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentially detectable by GLAST if a similar giant flare occurs in the GLAST era. With the available 10 MeV data, we constrain that Γ_0 < 50 if the electron distribution is a single power law. For a broken power law distribution of electrons, a higher Γ_0 is allowed. At energies higher than 1 GeV, the flux is lower because of a high energy cut off of the synchrotron emission component. The synchrotron self-Compton emission component and the inverse Compton scattering component off the photons in the giant flare oscillation tail are also considered, but they are found not significant given a moderate Γ_0 (e.g. \leq 10). The forward shock also accelerates cosmic rays to the maximum energy 10^{17}eV, and generate neutrinos with a typical energy 10^{14}eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.

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arXiv2005

A New Model for Iron Emission Lines and Re-Burst in GRB X-Ray Afterglows

Recently iron emission features have been observed in several X-ray afterglows of GRBs. It is found that the energy obtained from the illuminating continuum which produces the emission lines is much higher than that of the main burst.The observation of SN-GRB association indicates a fallback disk should be formed after the supernovae explosion. The disk is optically thick and advection-dominated and dense. We suggest that the delayed injection energy after the initial main burst, much higher than energy of the main burst, causes the re-burst appearance in GRB afterglow and illuminates the region of the disk surface with $τ\approx1$($τ$ is the optical depth for the Thomson scatter ) and produces the iron emission line whose luminosity can be up to $10^{45}$ erg$s^{-1}$. The duration of the iron line emission can be $10^{4}-10^{5}$ s. This model can explain the appearance of re-burst and emission lines in GRB afterglow and disappearance of the iron emission lines, and also can naturally solve the problem of higher energy of the illuminating continuum than that of the main burst. This scenario is different from the models put forward to explain the emission lines before, that can be tested by SWIFT satellite.

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arXiv2005

The very early optical afterglow of GRBs, revealing the nature of the ejecta

We show that if the GRB ejecta itself is magnetized or neutron-rich, the very early afterglow of GRBs is very different from that powered by a pure ion fireball. In the Swift era, with the well monitored early afterglow data, we can potentially diagnose the ejecta composition and reveal the nature of the central GRB engine.

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arXiv2004

The Ultraviolet flash accompanying GRBs from neutron-rich internal shocks

In the neutron-rich internal shocks model for Gamma-ray Burts (GRBs), the Lorentz factors (LFs) of ions shells are variable, so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF tens, the typical beta-decay radius reads R_{β,s} several 10^{14} cm, which is much larger than the typical internal shocks radius 10^{13} cm, so their impact on the internal shocks may be unimportant. However, as GRBs last long enough (T_{90}>20(1+z) s), one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range 10^{13}-10^{15} cm, where slow neutrons have decayed significantly. We show in this work that ion shells interacting with the beta-decay products of slow neutron shells can power a ultraviolet (UV) flash bright to 12th magnitude during the prompt gamma-ray emission phase or slightly delayed, which can be detected by the upcoming Satellite SWIFT in the near future.

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arXiv2004

The very early afterglow powered by the ultra-relativistic mildly magnetized outflows

In the Poynting Flux dominated outflow (the initial ratio of the electromagnetic energy flux to the particle energy flux $σ_0\gg1$) model for Gamma-ray bursts, nearly half of the internally dissipated magnetic energy is converted into the prompt $γ-$ray energy emission and the rest is converted into the kinetic energy of the outflow. Consequently, at the end of the $γ-$ray burst, $σ$ decreases significantly ($σ\sim 1$ or even smaller). We numerically investigate the very early reverse shock emission powered by such mildly magnetized outflows interacting with medium--uniform interstellar medium (ISM) or stellar wind (WIND). We show that for $σ\sim0.05-1$ and typical parameters of Gamma-ray bursts, both the ISM-ejecta interaction and the WIND-ejecta interaction can power very strong optical emission ($m_{\rm R}\sim 10-12{\rm th}$ magnitude or even brighter). Similar to the very early afterglow powered by the non-magnetized ejecta interacting with the external medium, the main difference between the ISM-ejecta interaction case and the WIND-ejecta interaction case is that, before the reverse shock crosses the ejecta, the R-band emission flux increases rapidly for the former, but for the latter it increases only slightly. (The abstract has been shortened). We suggest that the linear polarization detection of the early multi-wavelength afterglow is highly needed to see whether the outflows powering GRBs are magnetized or not.

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arXiv2004

UV/Optical Emission Accompanying Gamma-ray Burst

We discuss the possible simultaneously UV/optical emission accompanying Gamma-ray bursts (GRBs). We show that as long as the intrinsic spectrum of GRB can extend to $\sim$10 GeV or higher, there is a large amount of relativistic $e^\pm$ pairs generated due to the annihilation of the soft $γ-$rays with the very energetic photons, which dominates over the electrons/positrons associated with the fireball, no matter the fireball is highly magnetized or not (For the highly magnetized fireball, the magnetic field is ordered, the high linear polarization of the multi-wavelength emission is expected). We find that these $e^\pm$ pairs can power an UV flash with $m\simeq 12-13{\rm th}$ magnitude, and the corresponding optical emission can be up to $m_{\rm R}\simeq15-16{\rm th}$ magnitude. Such bright UV emission can be detected by the upcoming satellite Swift, planned for launch in early 2004. The behavior of the optical-UV spectrum ($F_ν\propto ν^{5/2}$) differs significantly from that of the reverse shock emission ($F_ν\propto ν^{-β/2}$, $β\simeq 2.2$), which is a signature of the emission accompanying with GRB. The mild optical emission can be detected with the ROTSE-IIIa telescope system, if the response to the GRB alert is fast enough.

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