arXiv subjects

X. F. Wu

Publications and source records attributed to X. F. Wu.

52 records · Page 3Linked to original sources

arXiv2006

X-Ray Flares from Postmerger Millisecond Pulsars

Recent observations support the suggestion that short-duration gamma-ray bursts are produced by compact star mergers. The X-ray flares discovered in two short gamma-ray bursts last much longer than the previously proposed postmerger energy release time scales. Here we show that they can be produced by differentially rotating, millisecond pulsars after the mergers of binary neutron stars. The differential rotation leads to windup of interior poloidal magnetic fields and the resulting toroidal fields are strong enough to float up and break through the stellar surface. Magnetic reconnection--driven explosive events then occur, leading to multiple X-ray flares minutes after the original gamma-ray burst.

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arXiv2005

X-ray flares: late internal and late external shocks

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.

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arXiv2005

Early Afterglows in Wind Environments Revisited

When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyze the reverse-forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g., an optical flash may emerge. The reverse shock behaves differently in two approximations: relativistic and Newtonian cases, which depend on the parameters, e.g., the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than $114 E_{53}^{1/4} Δ_{0,12}^{-1/4} A_{*,-1}^{-1/4}$, the early reverse shock is Newtonian. This may take place for the wider of a two-component jet, an orphan afterglow caused by a low initial Lorentz factor, and so on. The synchrotron self absorption effect is significant especially for the Newtonian reverse shock case, since the absorption frequency $ν_a$ is larger than the cooling frequency $ν_c$ and the minimum synchrotron frequency $ν_m$ for typical parameters. For the optical to X-ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities, and compare them with these in the ISM model.

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arXiv2005

Testing Gamma-Ray Burst Jet Structure with the Distribution of Gamma-Ray Energy Release

We present a general method for testing gamma-ray burst (GRB) jet structure and carry out a comprehensive analysis about the prevalent jet structure models. According to the jet angular energy distribution, we can not only derive the expected distribution of the GRB isotropic-equivalent energy release for any possible jet structure, but also obtain a two-dimensional distribution including redshift z. By using the Kolmogorov-Smirnov test we compare the predicted distribution with the observed sample, and find that the power-law structured jet model is most consistent with the current sample and that the uniform jet model is also plausible. However, this conclusion is tentative because of the small size and the inhomogeneity of this sample. Future observations (e.g., Swift) will provide a larger and less biased sample for us to make a robust conclusion by using the procedure proposed in this paper.

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arXiv2005

A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR1806-20

The brightest giant flare from the soft $γ$-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least one order of magnitude more energetic than those of the two other SGR giant flares. Starting from about one week after the burst, a very bright ($\sim 80$ mJy), fading radio afterglow was detected. Follow-up observations revealed the multi-frequency light curves of the afterglow and the temporal evolution of the source size. Here we show that these observations can be understood in a two-component explosion model. In this model, one component is a relativistic collimated outflow responsible for the initial giant flare and the early afterglow, and another component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss triggering mechanisms of these two components within the framework of the magnetar model.

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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

Afterglow Light Curves of Jetted Gamma-ray Burst Ejecta in Stellar Winds

Optical and radio afterglows arising from the shocks by relativistic conical ejecta running into pre-burst massive stellar winds are revisited. Under the homogeneous thin-shell approximation and a realistic treatment for the lateral expansion of jets, our results show that a notable break exists in the optical light curve in most cases we calculated in which the physical parameters are varied within reasonable ranges. For a relatively tenuous wind which cannot decelerate the relativistic jet to cause a light curve break within days, the wind termination shock due to the ram pressure of the surrounding medium occurs at a small radius, namely, a few times 10^{17} cm. In such a structured wind environment, the jet will pass through the wind within several hours and run into the outer uniform dense medium. The resulting optical light curve flattens with a shallower drop after the jet encounters the uniform medium, and then declines deeply, triggered by runaway lateral expansion.

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arXiv2004

Gamma-Ray Bursts: Polarization of Afterglows from Two-Component Jets

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.

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arXiv2004

Jet break time -- flux density relationship and constraints on physical parameters of GRB afterglows

We derive a relation between the flux density $F_{ν,j}$ at the light-curve break of a gamma-ray burst (GRB) afterglow and the break time $t_{j}$. The break is due to the transition from the spherical-like to jet-like evolution of the afterglow, when the Lorentz factor of the jet equals the inverse of the initial half-opening angle, i.e., $γ=1/θ_0$. We show that this relation indeed behaves as $F_{ν,j}\propto t_{j}^{-p}$ among GRBs for the slow-cooling case, where $p$ is the power-law index of electron distribution. A statistical analysis of the optical jet breaks of nine GRBs gives $p=2.10\pm 0.21$, which is consistent with the shock acceleration theory. The value of $p$ derived in this way is different from the observed temporal index $α_2$ ($F_ν\propto t^{-α_{2}}$) of the late-time light curve after $t_{j}$, which suffers several uncertainties from the unclear hydrodynamics of the sideways expansion and exhibits a large dispersion. Our results not only confirm that the remnants of GRBs are standard candles, but also provide the first evidence that the physical parameters of relativistic shocks are universal, with the favored values $ε_{e}\sim 0.1$ and $ε_{B}\sim 10^{-3}$.

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arXiv2004

Analytical light curves in the realistic model for GRB afterglows

(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 $<α_{X}> \sim 1.3$ 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.

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arXiv2004

Testing the Predictions of the Universal-Structured Jet Model of Gamma-Ray Bursts by Simulations

Based on the standard energy reservoir of gamma-ray burst (GRB) jets and the relationship between peak energy ($E_p$) of $νF_ν$ spectrum and the equivalent-isotropic energy ($E_{\rm{iso}}$), we test the GRB probability distributions as a function of viewing angle ($θ$) and of viewing angle together with redshift $z$, $P(θ)$ and $P(θ,z)$, predicted by the universal-structured jet (USJ) model with simulations on observational bases for a detection threshold of $S>4 \times 10^{-5}$ erg cm$^{-2}$, where $S$ is the total fluence in an energy band $>20$ keV. We simulate a sample including $10^{6}$ GRBs, and then derive $P(θ)$ and $P(θ,z)$. We show that both simulated and theoretical $P(θ)$ and $P(θ,z)$ are consistent with each other. This consistency seems to be regarded as support for the USJ model. A small difference between the theoretical and simulated results might be due to the observed $E_p$ distribution used in our simulations, which perhaps suffers a little of biases for $E_p<100$ keV.

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arXiv2004

The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models

Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts (GRBs) presented by Preece et al., we show that the relation between the isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in the cosmological rest frame, L_iso \propto E_p^2, not only holds within these bursts, but also holds among these GRBs, assuming that the burst rate as a function of redshift is proportional to the star formation rate. The possible implications of this relation for the emission models of GRBs are discussed. We suggest that both the kinetic-energy-dominated internal shock model and the magnetic-dissipation-dominated external shock model can well interpret this relation. We constrain the parameters for these two models, and find that they are in a good agreement with the parameters from the fittings to the afterglow data (abridged).

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arXiv2003

Rebrightening of XRF 030723: further evidence for a two-component jet in gamma-ray burst

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.

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arXiv2003

Gamma-Ray Bursts from Neutron Star Kicks

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.

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arXiv2003

GRB 030226 in a Density-Jump Medium

We present an explanation for the unusual temporal feature of the GRB 030226 afterglow. The R-band afterglow of this burst faded as ~ t^{-1.2} in ~ 0.2 days after the burst, rebrightened during the period of ~ 0.2 - 0.5 days, and then declined with ~ t^{-2.0}. To fit such a light curve, we consider an ultrarelativistic jetted blast wave expanding in a density-jump medium. The interaction of the blast wave with a large density jump produces relativistic reverse and forward shocks. In this model, the observed rebrightening is due to emissions from these newly forming shocks, and the late-time afterglow is caused by sideways expansion of the jet. Our fitting implies that the progenitor star of GRB 030226 could have produced a stellar wind with a large density jump prior to the GRB onset.

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arXiv2003

Optical Flashes and Very Early Afterglows in Wind Environments

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.

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