arXiv subjects

F. Y. Wang

Publications and source records attributed to F. Y. Wang.

At least 109 records · Page 6Linked to original sources

arXiv2015

Research on the redshift evolution of luminosity function and selection effect of GRBs

We study the redshift evolution of the luminosity function (LF) and redshift selection effect of long gamma-ray bursts (LGRBs). The method is to fit the observed peak flux and redshift distributions, simultaneously. To account for the complex triggering algorithm of Swift, we use a flux triggering efficiency function. We find evidence supporting an evolving LF, where the break luminosity scales as $L_b\propto (1+z)^τ$, with $τ=3.5^{+0.4}_{-0.2}$ and $τ=0.8^{+0.1}_{-0.08}$ for two kind of LGRB rate models. The corresponding local GRB rates are $\dot{R}(0)=0.86^{+0.11}_{-0.08} \yr^{-1}\Gpc^{-3}$ and $\dot{R}(0)= 0.54^{+0.25}_{-0.07} \yr^{-1}\Gpc^{-3}$, respectively. Furthermore, by comparing the redshift distribution between the observed one and our mocked one, we find that the redshift detection efficiency of the flux triggered GRBs decreases with redshift. Especially, a great number of GRBs miss their redshifts in the redshift range of $1<z<2.5$, where "redshift desert" effect may be dominated. More interestingly, our results show that the "redshift desert" effect is mainly introduced by the dimmer GRBs, e.g., $P<10^{-7}\ergs /\s/\cm^2$, but has no effect on the brighter GRBs.

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arXiv2015

The near-infrared radiation background, gravitational wave background and star formation rate of Pop III and Pop II during cosmic reionization

In this paper, we obtain the NIRB and SBGWs from the early stars, which are constrained by the observation of reionization and star formation rate. We study the transition from Pop III to Pop II stars via the star formation model of different population, which takes into account the reionization and the metal enrichment evolution. We calculate the two main metal pollution channels arising from the supernova-driven protogalactic outflows and "genetic channel". We obtain the SFRs of Pop III and Pop II and their NIRB and SBGWs radiation. We predict that the upper limit of metallicity in metal-enriched IGM (the galaxies whose polluted via "genetic channel") reaches $Z_{\rm crit}=10^{-3.5}Z_{\odot}$ at $z\sim13$ ($z\sim11$), which is consistent with our star formation model. We constrain on the SFR of Pop III stars from the observation of reionization. The peak intensity of NIRB is about $0.03-0.2~nW m^{-2}{sr}^{-1}$ at $\sim 1 μm$ for $z>6$. The prediction of NIRB signal is consistent with the metallicity evolution. We also obtain the gravitational wave background from the black holes formed by these early stars. The predicted gravitational wave background has a peak amplitude of $Ω_{GW}\simeq8\times10^{-9}$ at $ν=158$ Hz for Pop II star remnants. However, the background generated by Pop III.2 stars is much less than Pop II stars, with a peak amplitude of $Ω_{GW}\simeq1.2\times10^{-11}$ at $ν=28~Hz$. The background of Pop III.1 shifts to lower frequencies, and the amplitude of $Ω_{GW}$ for Pop III.1 stars shows a minimum value at $ν\simeq 10$ Hz, due to the lack of gravitational wave signals from the stars with $140~M_{\odot}<M_\ast<260~M_{\odot}$.

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arXiv2015

GRB Cosmology and Self-organized Criticality in GRBs

Gamma-ray bursts (GRBs), which have isotropic energy up to $10^{54}$ erg, would be the ideal tool to study the properties of early universe: including dark energy, star formation rate, and the metal enrichment history of the Universe. We will briefly review the progress on the field of GRB cosmology. Meanwhile, X-ray flares, which may have important clues to the central engine, are common phenomena in the GRB afterglows. We present statistical results of X-ray flares, i.e., energy, duration time and waiting time distributions, and compare the results with solar flares. The similarity between the two kinds of flares are found, which may indicates that the physical mechanism of GRB X-ray flares is magnetic reconnection.

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arXiv2015

Cosmological test using strong gravitational lensing systems

As one of the probes of universe, strong gravitational lensing systems allow us to compare different cosmological models and constrain vital cosmological parameters. This purpose can be reached from the dynamic and geometry properties of strong gravitational lensing systems, for instance, time-delay $Δτ$ of images, the velocity dispersion $σ$ of the lensing galaxies and the combination of these two effects, $Δτ/σ^2$. In this paper, in order to carry out one-on-one comparisons between $Λ$CDM universe and $R_h=ct$ universe, we use a sample containing 36 strong lensing systems with the measurement of velocity dispersion from the SLACS and LSD survey. Concerning the time-delay effect, 12 two-image lensing systems with $Δτ$ are also used. In addition, Monte Carlo (MC) simulations are used to compare the efficiency of the three methods as mentioned above. From simulations, we estimate the number of lenses required to rule out one model at the $99.7\%$ confidence level. Comparing with constraints from $Δτ$ and the velocity dispersion $σ$, we find that using $Δτ/σ^2$ can improve the discrimination between cosmological models. Despite the independence tests of these methods reveal a correlation between $Δτ/σ^2$ and $σ$, $Δτ/σ^2$ could be considered as an improved method of $σ$ if more data samples are available.

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arXiv2015

Cosmological model of the interaction between dark matter and dark energy

In this paper, we test the dark matter-dark energy interacting cosmological model with a dynamic equation of state $w_{DE}(z)=w_{0}+w_{1}z/(1+z)$, using type Ia supernovae (SNe Ia), Hubble parameter data, baryonic acoustic oscillation (BAO) measurements, and the cosmic microwave background (CMB) observation. This interacting cosmological model has not been studied before. The best-fitted parameters with $1 σ$ uncertainties are $δ=-0.022 \pm 0.006$, $Ω_{DM}^{0}=0.213 \pm 0.008$, $w_0 =-1.210 \pm 0.033$ and $w_1=0.872 \pm 0.072$ with $χ^2_{min}/dof = 0.990$. At the $1 σ$ confidence level, we find $δ<0$, which means that the energy transfer prefers from dark matter to dark energy. We also find that the SNe Ia are in tension with the combination of CMB, BAO and Hubble parameter data. The evolution of $ρ_{DM}/ρ_{DE}$ indicates that this interacting model is a good approach to solve the coincidence problem, because the $ρ_{DE}$ decrease with scale factor $a$. The transition redshift is $z_{tr}=0.63 \pm 0.07$ in this model.

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arXiv2015

An unexpectedly low-redshift excess of Swift gamma-ray burst rate

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe and can be used to explore the properties of high-redshift universe. It is believed that the long GRBs are associated with the deaths of massive stars. So it is possible to use GRBs to investigate the star formation rate (SFR). In this paper, we use Lynden-Bell's $c^-$ method to study the luminosity function and rate of \emph{Swift} long GRBs without any assumptions. We find that the luminosity of GRBs evolves with redshift as $L(z)\propto g(z)=(1+z)^k$ with $k=2.43_{-0.38}^{+0.41}$. After correcting the redshift evolution through $L_0(z)=L(z)/g(z)$, the luminosity function can be expressed as $ψ(L_0)\propto L_0^{-0.14\pm0.02}$ for dim GRBs and $ψ(L_0)\propto L_0^{-0.70\pm0.03}$ for bright GRBs, with the break point $L_{0}^{b}=1.43\times10^{51}~{\rm erg~s^{-1}}$. We also find that the formation rate of GRBs is almost constant at $z<1.0$ for the first time, which is remarkably different from the SFR. At $z>1.0$, the formation rate of GRB is consistent with the SFR. Our results are dramatically different from previous studies. Some possible reasons for this low-redshift excess are discussed. We also test the robustness of our results with Monte Carlo simulations. The distributions of mock data (i.e., luminosity-redshift distribution, luminosity function, cumulative distribution and $\log N-\log S$ distribution) are in good agreement with the observations. Besides, we also find that there are remarkable difference between the mock data and the observations if long GRB are unbiased tracers of SFR at $z<1.0$.

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arXiv2015

Gamma-ray Burst Cosmology

Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the Universe, which emit up to $8.8\times10^{54}$ erg isotropic equivalent energy in the hard X-ray band. The high luminosity makes them detectable out to the largest distances yet explored in the Universe. GRBs, as bright beacons in the deep Universe, would be the ideal tool to probe the properties of high-redshift universe: including the cosmic expansion and dark energy, star formation rate, the reionization epoch and the metal enrichment history of the Universe. In this article, we review the luminosity correlations of GRBs, and implications for constraining the cosmological parameters and dark energy. Observations show that the progenitors of long GRBs are massive stars. So it is expected that long GRBs are tracers of star formation rate. We also review the high-redshift star formation rate derived from GRBs, and implications for the cosmic reionization history. The afterglows of GRBs generally have broken power-law spectra, so it is possible to extract intergalactic medium (IGM) absorption features. We also present the capability of high-redshift GRBs to probe the pre-galactic metal enrichment and the first stars.

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arXiv2015

Constraining the bulk Lorentz factor from the photosphere emission

We propose a direct and model-independent method to constrain the Lorentz factor of a relativistically expanding object, like gamma-ray bursts. Only the measurements, such as thermal component of the emission, the distance and the variable time scale of the light curve, are used. If the uncertainties are considered, we will obtain lower limits of the Lorentz factor instead. We apply this method to GRB 090618 and get a lower limit of the Lorentz factor to be 22. The method can be used to any relativistically moving object, such as gamma-ray bursts, blazars, and soft gamma-ray repeaters, providing the thermal component of the emission being observed.

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arXiv2014

Universal Behavior of X-ray Flares from Black Hole Systems

X-ray flares have been discovered in black hole systems, such as gamma-ray bursts, the tidal disruption event Swift J1644+57, the supermassive black hole Sagittarius A$^*$ at the center of our Galaxy, and some active galactic nuclei. Their occurrences are always companied by relativistic jets. However, it is still unknown whether there is a physical analogy among such X-ray flares produced in black hole systems spanning nine orders of magnitude in mass. Here we report the observed data of X-ray flares, and show that they have three statistical properties similar to solar flares, including power-law distributions of energies, durations, and waiting times, which both can be explained by a fractal-diffusive self-organized criticality model. These statistical similarities, together with the fact that solar flares are triggered by a magnetic reconnection process, suggest that all of the X-ray flares are consistent with magnetic reconnection events, implying that their concomitant relativistic jets may be magnetically dominated.

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arXiv2014

Reconciling the cosmic age problem in the R_h=ct Universe

Many dark energy models fail to pass the cosmic age test. In this paper, we investigate the cosmic age problem associated with nine extremely old Global Clusters (GCs) and the old quasar APM $08279+5255$ in the $R_h=ct$ Universe. The age data of these oldest GCs in M31 is acquired from the Beijing-Arizona-Taiwan-Connecticut system with up-to-date theoretical synthesis models. They have not been used to test the cosmic age problem in the $R_h=ct$ Universe in previous literature. By evaluating the age of the $R_h=ct$ Universe with the observational constraints from the type Ia supernovae and Hubble parameter, we find that the $R_h=ct$ Universe can accommodate five GCs and the quasar APM 08279+5255 at redshift $z=3.91$. But for other models, such as $Λ$CDM, interacting dark energy model, generalized Chaplygin gas model and holographic dark energy model, can not accommodate all GCs and the quasar APM 08279+5255. It is worthwhile to note that the age estimates of some GCs are controversial. So, unlike other cosmological models, the $R_h=ct$ Universe can marginally solve the cosmic age problem, especially at high redshift.

astro-ph.CO↗

arXiv2014

Probing the anisotropic expansion from supernovae and GRBs in a model-independent way

In this paper, we study the anisotropic expansion of the universe using type Ia supernovae Union 2.1 sample and 116 long gamma-ray bursts. The luminosity distance is expanded with model-independent cosmographic parameters as a function of $z/(1+z)$ directly. Thus the results are independent of cosmology model. We find a dipolar anisotropy in the direction ($l=309.2^\circ \pm 15.8^\circ$, $b=-8.6^\circ \pm 10.5^\circ$) in galactic coordinates with a significant evidence $97.29\%$ (more than $2~σ$). The magnitude is $(1.37\pm 0.57) \times 10^{-3}$ for the dipole, and $(2.6\pm 2.1)\times 10^{-4}$ for the monopole, respectively. This dipolar anisotropy is more significant at low redshift from the redshift tomography analysis. We also test whether this preferred direction is caused by bulk flow motion or dark energy dipolar scalar perturbation. We find that the direction and the amplitude of the bulk flow in our results are approximately consistent with the bulk flow surveys. Therefore, bulk flow motion may be the main reason for the anisotropic expansion at low redshift, but the effect of dipolar distribution dark energy can not be excluded, especially at high redshift.

astro-ph.CO↗

arXiv2014

Long GRBs are metallicity-biased tracers of star formation: evidence from host galaxies and redshift distribution

We investigate the mass distribution of long gamma-ray burst (GRB) host galaxies and the redshift distribution of long GRBs by considering that long GRBs occur in low-metallicity environments. We calculate the upper limit on the stellar mass of a galaxy which can produce long GRBs by utilizing the mass-metallicity (M-Z) relation of galaxies. After comparing with the observed GRB host galaxies masses, we find that the observed GRB host galaxy masses can fit the predicted masses well if GRBs occur in low-metallicity $12+\log\rm(O/H)_{\rm KK04}<8.7$. GRB host galaxies have low metallicity, low mass, and high star formation rate compared with galaxies of seventh data release of the Sloan Digital Sky Survey. We also study the cumulative redshift distribution of the latest \emph{Swift} long GRBs by adding dark GRBs and 10 new GRBs redshifts from TOUGH survey. The observed discrepancy between the GRB rate and the star formation history can be reconciled by considering that GRBs tend to occur in low-metallicity galaxies with $12+\log\rm(O/H)_{\rm KK04}<8.7$. We conclude that the metallicity cutoff that can produce long GRBs is about $12+\log\rm(O/H)_{\rm KK04}<8.7$ from the host mass distribution and redshift distribution.

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arXiv2014

Similar radiation mechanism in gamma-ray bursts and blazars: evidence from two luminosity correlations

Active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) are powerful astrophysical events with relativistic jets. In this Letter the broadband spectral properties are compared between GRBs and the well-observed blazars. The distribution of GRBs are consistent with the well-known blazar sequence including the $νL_ν(5\rm GHz)-α_{\rm RX}$ and $νL_ν(5\rm GHz)-ν_{\rm peak}$ correlations, where $α_{\rm RX}$ is defined as the broadband spectral slope in radio-to-X-ray bands, and $ν_{\rm peak}$ is defined as the spectral peak frequency. Moreover, GRBs occupy the low radio luminosity end of these sequences. These two correlations suggest that GRBs could have a similar radiation process with blazars both in the prompt emission and afterglow phases, i.e., synchrotron radiation.

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arXiv2014

The high-redshift star formation rate derived from GRBs: possible origin and cosmic reionization

The collapsar model of long gamma-ray bursts (GRBs) indicates that they may trace the star formation history. So long GRBs may be a useful tool of measuring the high-redshift star formation rate (SFR). The collapsar model explains GRB formation via the collapse of a rapidly rotating massive star with $M> 30M_\odot$ into a black hole, which may imply a decrease of SFR at high redshift. However, we find that the \emph{Swift} GRBs during 2005-2012 are biased tracing the SFR, including a factor about $(1+z)^{0.5}$, which is in agreement with recent results. After taking this factor, the SFR derived from GRBs does not show steep drop up to $z\sim 9.4$. We consider the GRBs produced by rapidly rotating metal-poor stars with low masses to explain the high-redshift GRB rate excess. The chemically homogeneous evolution scenario (CHES) of rapidly rotating stars with mass larger than $12M_\odot$ is recognized as a promising path towards collapsars in connection with long GRBs. Our results indicate that the stars in the mass range $12M_\odot<M<30M_\odot$ for low enough metallicity $Z\leq 0.004$ with the GRB efficiency factor $10^{-5}$ can fit the derived SFR with good accuracy. Combining these two factors, we find that the conversion efficiency from massive stars to GRBs is enhanced by a factor of 10, which may be able to explain the excess of the high-redshift GRB rate. We also investigate the cosmic reionization history using the derived SFR. The GRB-inferred SFR would be sufficient to maintain cosmic reionization over $6<z<10$ and reproduce the observed optical depth of Thomson scattering to the cosmic microwave background.

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arXiv2014

X-Ray Afterglow Plateaus of Long Gamma-Ray Bursts: Further Evidence for Millisecond Magnetars

Many long-duration gamma-ray bursts (GRBs) were observed by {\it Swift}/XRT to have plateaus in their X-ray afterglow light curves. This plateau phase has been argued to be evidence for long-lasting activity of magnetar (ultra-strongly magnetized neutron stars) central engines. However, the emission efficiency of such magnetars in X-rays is still unknown. Here we collect 24 long GRB X-ray afterglows showing plateaus followed by steep decays. We extend the well-known relationship between the X-ray luminosity ${L_{\mathrm{X}}}$ and spin-down luminosity $L_{\mathrm{sd}}$ of pulsars to magnetar central engines, and find that the initial rotation period $P_{0}$ ranges from 1 ms to 10 ms and that the dipole magnetic field $B$ is centered around $10^{15}$ G. These constraints not only favor the suggestion that the central engines of some long GRBs are very likely to be rapidly rotating magnetars but also indicate that the magnetar plateau emission efficiency in X-rays is close to 100%.

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arXiv2013

Estimating the uncorrelated dark energy evolution in the Planck era

The equation of state (EOS), $w(z)$, is the most important parameter of dark energy. We reconstruct the evolution of this EOS in a model-independent way using the latest cosmic microwave background (CMB) data from Planck and other observations, such as type Ia supernovae (SNe Ia), the baryonic acoustic oscillation measurements (SDSS, 6dF, BOSS, and WiggleZ), and the Hubble parameter value $H(z)$. The results show that the EOS is consistent with the cosmological constant at the $2σ$ confidence level, not preferring a dynamical dark energy. The uncorrelated EOS of dark energy constraints from Planck CMB data are much tighter than those from the WMAP 9-year CMB data.

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arXiv2013

Searching for a preferred direction with Union2.1 data

A cosmological preferred direction was reported from the type Ia supernovae (SNe Ia) data in recent years. We use the Union2.1 data to give a simple classification of such studies for the first time. Because the maximum anisotropic direction is independent of isotropic dark energy models, we adopt two cosmological models ($Λ$CDM, $w$CDM) for the hemisphere comparison analysis and $Λ$CDM model for dipole fit approach. In hemisphere comparison method, the matter density and the equation of state of dark energy are adopted as the diagnostic qualities in the $Λ$CDM model and $w$CDM model, respectively. In dipole fit approach, we fit the fluctuation of distance modulus. We find that there is a null signal for the hemisphere comparison method, while a preferred direction ($b=-14.3^\circ \pm 10.1^\circ, l=307.1^\circ \pm 16.2^\circ$) for the dipole fit method. This result indicates that the dipole fit is more sensitive than the hemisphere comparison method.

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arXiv2013

Self-organized criticality in X-ray flares of gamma-ray burst afterglows

X-ray flares detected in nearly half of gamma-ray burst (GRB) afterglows are one of the most intriguing phenomena in high-energy astrophysics. All the observations indicate that the central engines of bursts, after the gamma-ray emission has ended, still have long periods of activity, during which energetic explosions eject relativistic materials, leading to late-time X-ray emission. It is thus expected that X-ray flares provide important clues to the nature of the central engines of GRBs, and more importantly, unveil the physical mechanism of the flares themselves, which has so far remained mysterious. Here we report statistical results of X-ray flares of GRBs with known redshifts, and show that X-ray flares and solar flares share three statistical properties: power-law frequency distributions for energies, durations, and waiting times. All of the distributions can be well understood within the physical framework of a self-organized criticality (SOC) system. The statistical properties of X-ray flares of GRBs are similar to solar flares, and thus can both be attributed to a SOC process. Both types of flares may be driven by a magnetic reconnection process, but X-ray flares of GRBs are produced in ultra-strongly magnetized millisecond pulsars or long-term hyperaccreting disks around stellar-mass black holes.

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