arXiv subjects

Y. Z. Fan

Publications and source records attributed to Y. Z. Fan.

90 records · Page 5Linked to original sources

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

Curvature effect in structured GRB jets

We study the influence of jet structure on the curvature effect in a GRB-lightcurve. Using a simple model of jet emissivity, we numerically calculate lightcurves for a short flash from a relativistic outflow having various profiles of the Lorentz factor and outflowing energy density (gaussian, core+power-law). We find that for "on-beam" viewing geometry, with the line of sight passing through the bright core of the outflow, the shape of the lightcurve practically does not depend on the jet structure, initially following the temporal slope 2+delta, where delta is the spectral index. When the viewing angle is larger than the core, the light curve decaying slope is shallower. We discuss the implications of our results for the Swift data.

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arXiv2005

Linearly polarized X-ray flares following short gamma-ray bursts

Soft X-ray flares were detected to follow the short-duration gamma-ray burst GRB 050724. The temporal properties of the flares suggest that they are likely due to the late time activity of the central engine. We argue that if short GRBs are generated through compact star mergers, as is supported by the recent observations, the jet powering the late X-ray flares must be launched via magnetic processes rather than via neutrino-antineutrino annihilations. As a result, the X-ray flares following short GRBs are expected to be linearly polarized. The argument may also apply to the X-ray flares following long GRBs. Future observations with the upcoming X-ray polarimeters will test this prediction.

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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

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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arXiv2005

Optical afterglows of short Gamma-ray Bursts and GRB 040924

Short-duration Gamma-ray bursts (GRBs) (\leq 2{\rm s}) have remained a mystery due to the lack of afterglow detection until recently. The models to interpret short GRBs invoke distinct progenitor scenarios. Here we present a generic analysis of short GRB afterglows, and calculate the optical lightcurves of short GRBs within the framework of different progenitor models. We show that all these optical afterglows are bright enough to be detected by the Ultraviolet and Optical Telescope (UVOT) on board the {\em Swift} observatory, and that different models could be distinguished with a well-monitored lightcurve. We also model the afterglow data of the recently discovered short burst GRB 040924. We find that the limited data are consistent with a low medium density environment which is consistent with the pre-concept of the compact-star merger progenitor model, although the models with a collapsar progenitor are not ruled out.

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arXiv2005

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

Recent follow-up observations of the December 27 giant flare of SGR 1806-20 have detected a multiple-frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from a week to about a month after the flare. The angular size of the source was also measured for the first time. Here we show that this radio afterglow gives the first piece of clear evidence that an energetic blast wave sweeps up its surrounding medium and produces a synchrotron afterglow, the same mechanism as established for gamma-ray burst afterglows. The optical afterglow is expected to be intrinsically as bright as $m_R\simeq13$ at $t\la 0.1$ days after the flare, but very heavy extinction makes the detection difficult because of the low galactic latitude of the source. Rapid infrared follow-up observations to giant flares are therefore crucial for the low-latitude SGRs, while for high-latitude SGRs (e.g. SGR 0526-66), rapid follow-ups should result in identification of their possible optical afterglows. Rapid multi-wavelength follow-ups will also provide more detailed information of the early evolution of a fireball as well as its composition.

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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

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

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

Optical Flash of GRB 990123: constraints on the physical parameter of the reverse shock

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 $ε_e$, $ε_B$ should be 0.61, 0.39 respectively, which are much different from those in the late afterglow.

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