arXiv subjects

Z. G. Dai

Publications and source records attributed to Z. G. Dai.

At least 145 records · Page 8Linked to original sources

arXiv2013

PTF11agg as the First Evidence for Reverse Shock Emission from a Postmerger Millisecond Magnetar

Based on the stiff equations of state of neutron stars (NS) and the discovery of high-mass NSs, a NS-NS merger will leave behind, with high probabilities, a rapidly rotating massive magnetar. The central magnetar will dissipate its rotational energy to the outflow by injecting Poynting flux, which will become lepton-dominated so that a long-lasting reverse shock (RS) is developed. We calculate the emission of the RS as well as the emission of forward shock (FS) and find that in most cases the RS emission is stronger than FS emission. It is found that the recently discovered transient, PTF11agg, can be neatly accounted for by the RS emission powered by a millisecond magnetar. Other alternative models have been considered and cannot explain the observed light curves well. We therefore suggest that PTF11agg be the first evidence for RS emission from a postmerger millisecond magnetar.

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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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arXiv2013

GeV Emission during X-Ray Flares from Late Internal Shocks: Application to GRB 100728A

Recently, the GeV radiation during the X-ray flare activity in GRB 100728A was detected by Feimi/LAT. Here we study the dynamics and emission properties of collision between two homogeneous shells based on the late internal shock model. The GeV photons can be produced from X-ray flare photons up-scattered by relativistic electrons accelerated by forward-reverse shocks, where involved radiative processes include synchrotron self-Compton and crossing inverse-Compton scattering. By analytical and numerical calculations, the observed spectral properties in GRB 100728A can be well explained.

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arXiv2013

Testing the local-void alternative to dark energy using galaxy pairs

The possibility that we live in a special place in the universe, close to the center of a large, radially inhomogeneous void, has attracted attention recently as an alternative to dark energy or modified gravity to explain the accelerating universe. We show that the distribution of orientations of galaxy pairs can be used to test the Copernican principle that we are not in a central or special region of Universe. The popular void models can not fit both the latest type Ia supernova, cosmic microwave background data and the distribution of orientations of galaxy pairs simultaneously. Our results rule out the void models at the $4σ$ confidence level as the origin of cosmic acceleration and favor the Copernican principle.

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arXiv2012

Probing Pre-galactic Metal Enrichment with High-Redshift Gamma-Ray Bursts

We explore high-redshift gamma-ray bursts (GRBs) as promising tools to probe pre-galactic metal enrichment. We utilize the bright afterglow of a Pop III GRB exploding in a primordial dwarf galaxy as a luminous background source, and calculate the strength of metal absorption lines that are imprinted by the first heavy elements in the intergalactic medium (IGM). To derive the GRB absorption line diagnostics, we use an existing highly-resolved simulation of the formation of a first galaxy which is characterized by the onset of atomic hydrogen cooling in a halo with virial temperature >10^4 K. We explore the unusual circumburst environment inside the systems that hosted Pop III stars, modeling the density evolution with the self-similar solution for a champagne flow. For minihalos close to the cooling threshold, the circumburst density is roughly proportional to (1+z) with values of about a few cm^{-3}. In more massive halos, corresponding to the first galaxies, the density may be larger, n>100 cm^{-3}. The resulting afterglow fluxes may be detectable with the JWST and VLA in the near-IR and radio wavebands, respectively, out to redshift z>20. We predict that the maximum of the afterglow emission shifts from near-IR to millimeter bands with peak fluxes from mJy to Jy at different observed times. GRBs are ideal tools for probing the metal enrichment in the early IGM, due to their high luminosities and featureless power-law spectra. The metals in the first galaxies produced by the first supernova (SN) explosions are likely to reside in low-ionization stages. We show that if the afterglow can be observed sufficiently early, analysis of the metal lines can distinguish whether the first heavy elements were produced in a PISN, or a core-collapse (Type II) SN, thus constraining the initial mass function of the first stars.

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arXiv2012

Spin Evolution of Millisecond Magnetars with Hyperaccreting Fallback Disks: Implications for Early Afterglows of Gamma-Ray Bursts

The shallow decay phase or plateau phase of early afterglows of gamma-ray bursts (GRBs), discovered by Swift, is currently understood as being due to energy injection to a relativistic blast wave. One natural scenario for energy injection invokes a millisecond magnetar as the central engine of GRBs, because the conventional model of a pulsar predicts a nearly constant magnetic-dipole-radiation luminosity within the spin-down timescale. However, we note that significant brightening occurs in some early afterglows, which apparently conflicts with the above scenario. Here we propose a new model to explain this significant brightening phenomena by considering a hyperaccreting fallback disk around a newborn millisecond magnetar. We show that for typical values of the model parameters, sufficient angular momentum of the accreted matter is transferred to the magnetar and spins it up. It is this spin-up that leads to a dramatic increase of the magnetic dipole radiation luminosity with time and thus significant brightening of an early afterglow. Based on this model, we carry out numerical calculations and fit well early afterglows of 12 GRBs assuming sufficiently strong fallback accretion. If the accretion is very weak, our model turns out to be the conventional energy-injection scenario of a pulsar. Therefore, our model can provide a unified explanation for the shallow decay phase, plateaus, and significant brightening of early afterglows.

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arXiv2011

Weak gravitational lensing effects on cosmological parameters and dark energy from gamma-ray bursts

Gamma-ray bursts (GRBs) are attractive objects for constraining the nature of dark energy in a way complementary to other cosmological probes, especially at high redshifts. However, the apparent magnitude of distant GRBs can be distorted by the gravitational lensing from the density fluctuations along the line of sight. We investigate the gravitational lensing effect on the cosmological parameters and dark energy equation of state from GRBs. We first calibrated the GRB luminosity relations without assuming any cosmological models. The luminosity distances of low-redshift GRBs were calibrated with the cosmography method using a latest type Ia supernova (SNe Ia) sample. We find that the gravitational lensing has non-negligible effects on the determination of cosmological parameters and dark energy. The gravitational lensing shifts the best-fit constraints on cosmological parameters and dark energy. Because high-redshift GRBs are more likely to be reduced, the most probable value of the observed matter density $Ω_M$ is slightly lower than its actual value. In the $Λ$CDM model, we find that the matter density parameter $Ω_M$ will shift from 0.30 to 0.26 after including the gravitational lensing effect. The gravitational lensing also affects the dark energy equation of state by shifting it to a more negative value. We constrain the dark energy equation of state out to redshift $z\sim 8$ using GRBs for the first time, and find that the equation of state deviates from $Λ$CDM at the $1σ$ confidence level, but agrees with $w=-1$ at the $2σ$ confidence level.

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arXiv2011

Multiband Fitting to Three Long GRBs with Fermi/LAT Data: Structured Ejecta Sweeping up a Density-Jump Medium

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 (GBM) 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 emission from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. If the ceasing time for majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with continuous energy injection to the forward shock can well explain the early rising feature of the GeV mission from these burst, and the density-jump medium can account for some certain plateaus or flares in the late afterglows. From our fits, we find that, on one hand, the external shock origin of the GeV photons will make the optical depth have not significant contribution to the early LAT rising part, which will loosen strong constraint of lower limits of Lorentz factor. On the other hand, these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff will significantly suppress the Self-Synchrotron Compton (SSC) radiation. Such an environment might result from superbubbles or low-metallicity progenitor stars (which have a low mass-loss rate at late times of stellar evolution) of type Ib/c supernovae.

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arXiv2011

The updated luminosity correlations of gamma-ray bursts and cosmological implications

Several interesting luminosity correlations among gamma-ray burst (GRB) variables have been recently discussed extensively. In this paper, we derive the six luminosity correlations ($\tlag-L$, $V-L$, $\epkk-L$, $\epkk-E_γ$, $\trt-L$, $\epkk-E_{γ, \mathrm{iso}}$) from the light curves and spectra of the latest 116 long GRBs, including the time lag ($\tlag$) between low and high photon energy light curves, the variability ($V$) of the light curve, the peak energy of the spectrum ($\epkk$), and the minimum rise time ($\trt$) of the peaks. We find that the intrinsic scatter of the $V-L$ correlation is too large and there seems no inherent correlation between the two parameters using the latest GRB data. The other five correlations indeed exist when the sample is enlarged. The $\epkk-E_γ$ correlation has a significantly lower intrinsic scatter compared to the other correlations. We divide the full data into four redshift bins when testing possible evolution of the correlations with redshift. We find no statistically significant evidence for the redshift evolution of the luminosity correlations. To avoid the circularity problem when constraining the cosmological parameters, we simultaneously minimize $χ^2$ with respect to both correlation parameters $a$, $b$ and the cosmological parameters using the maximum likelihood method. For the flat $Λ$CDM, the best fit is $\omm=0.31^{+0.13}_{-0.10}$. We also constrain the possible evolution of the equation of state (EOS) of the dark energy using the GRBs together with the Union2 compilation of SNe Ia and the $H(z)$ data. The result is consistent with the cosmological constant at $2 σ$ confidence level and mainly due to the GRB data, the dark energy EOS shows slight deviation from -1 at $z \geq0.5$ as was persistently presented with many previous data sets.

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arXiv2011

An Evolving Stellar Initial Mass Function and the Gamma-Ray Burst Redshift Distribution

Recent studies suggest that Swift gamma-ray bursts (GRBs) may not trace an ordinary star formation history. Here we show that the GRB rate turns out to be consistent with the star formation history with an evolving stellar initial mass function (IMF). We first show that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the ordinary star formation rate at high redshifts. We then assume only massive stars with masses greater than the critical value to produce GRBs, and use an evolving stellar IMF suggested by Davé (2010) to fit the latest GRB redshift distribution. This evolving IMF would increase the relative number of massive stars, which could lead to more GRB explosions at high redshifts. We find that the evolving IMF can well reproduce the observed redshift distribution of Swift GRBs.

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arXiv2010

Hyperaccreting Disks around Neutrons Stars and Magnetars for GRBs: Neutrino Annihilation and Strong Magnetic Fields

Hyperaccreting disks around neutron stars or magnetars cooled via neutrino emission can be the potential central engine of GRBs. The neutron-star disk can cool more efficiently, produce much higher neutrino luminosity and neutrino annihilation luminosity than its black hole counterpart with the same accretion rate. The neutron star surface boundary layer could increase the annihilation luminosity as well. An ultra relativistic jet via neutrino annihilation can be produced along the stellar poles. Moreover, we investigate the effects of strong fields on the disks around magnetars. In general, stronger fields give higher disk densities, pressures, temperatures and neutrino luminosity; the neutrino annihilation mechanism and the magnetically-driven pulsar wind which extracts the stellar rotational energy can work together to generate and feed an even stronger ultra-relativistic jet along the stellar magnetic poles.

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arXiv2010

The termination shock of a magnetar wind: a possible origin of gamma-ray burst X-ray afterglow emission

Context: Swift observations suggest that the X-ray afterglow emission of some gamma-ray bursts (GRB) may have internal origins, and the conventional external shock (ES) cannot be the exclusive source of the afterglow emission. Aims: If the central compact objects of some GRBs are millisecond magentars, the magnetar winds could play an important role in the (internal) X-ray afterglow emission, which is our focus here. Methods: The dynamics and the synchrotron radiation of the termination shock (TS) of the magmnetar winds, as well as the simultaneous GRB ES, are investigated by considering the magnetization of the winds. Results: As a result of the competition between the emission of the wind TS and the GRB ES, two basic types of X-ray afterglows are predicted, i.e., the TS-dominated and the ES-dominated types. Moreover, our results also show that both of the two types of afterglows have a shallow-decay phase and a normal-decay one, as observed by the \textit{Swift} satellite. This indicates that some observed X-ray afterglows could be (internally) produced by the magnetar winds, but not necessarily GRB ESs.

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arXiv2010

Hyperaccreting Disks around Magnetars for Gamma-Ray Bursts: Effects of Strong Magnetic Fields

(Abridged) The hyperaccreting neutron star or magnetar disks cooled via neutrino emission can be a candidate of gamma-ray burst (GRB) central engines. The strong field $\geq10^{15}-10^{16}$ G of the magnetar can play a significant role in affecting the disk properties and even lead to the funnel accretion process. We investigate the effects of strong fields on the disks around magnetars, and discuss implications of such accreting magnetar systems for GRB and GRB-like events. We discuss quantum effects of the strong fields on the disk, and use the MHD conservation equations to describe the behavior of the disk flow coupled with a large scale field, which is generated by the star-disk interaction. In general, stronger fields give higher disk densities, pressures, temperatures and neutrino luminosity, and change the electron fraction and degeneracy state significantly. A magnetized disk is always viscously stable outside the Alfvén radius, but will be thermally unstable near the Alfvén radius where the magnetic field plays a more important role in transferring the angular momentum and heating the disk than the viscous stress. The funnel accretion process will be only important for an extremely strong field, which creates a magnetosphere inside the Alfvén radius and truncates the plane disk. Because of higher temperature and more concentrated neutrino emission of the magnetar surface ring-like belt region covered by funnel accretion, the neutrino annihilation rate from the accreting magnetars can be much higher than that from accreting neutron stars without fields. Furthermore, the neutrino annihilation mechanism and the magnetically-driven pulsar wind from the magnetar surface can work together to generate and feed an ultra-relativistic jet along the stellar magnetic poles.

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arXiv2010

Probing the cosmographic parameters to distinguish between dark energy and modified gravity models

In this paper we investigate the deceleration, jerk and snap parameters to distinguish between the dark energy and modified gravity models by using high redshift gamma-ray bursts (GRBs) and supernovae (SNe). We first derive the expressions of deceleration, jerk and snap parameters in dark energy and modified gravity models. In order to constrain the cosmographic parameters, we calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. Then we constrain the models (including dark energy and modified gravity models) parameters using type Ia supernovae and gamma-ray bursts. Finally we calculate the cosmographic parameters. GRBs can extend the redshift - distance relation up to high redshifts, because they can be detected to high redshifts.We find that the statefinder pair (r,s) could not be used to distinguish between some dark energy and modified gravity models, but these models could be differentiated by the snap parameter. Using the model-independent constraints on cosmographic parameters, we conclude that the \LambdaCDM model is consistent with the current data.

astro-ph.CO↗

arXiv2010

High-redshift star formation rate up to z~8.3 derived from gamma-ray bursts and influence of background cosmology

The high-redshift star formation rate (SFR) is difficult to measure directly even by modern approaches. Long-duration gamma-ray bursts (GRBs) can be detected to the edge of the visible universe because of their high luminorsities. The collapsar model of long gamma-ray bursts indicates that they may trace the star formation history. So long gamma-ray bursts may be a useful tool of measuring the high-redshift SFR. Observations show that long gamma-ray bursts prefer to form in a low-metallicity environment. We study the high-redshift SFR up to z~8.3 considering the Swift GRBs tracing the star formation history and the cosmic metallicity evolution in different background cosmological models including $Λ$CDM, quintessence, quintessence with a time-varying equation of state, and brane-world model. We use latest Swift GRBs including two highest-$z$ GRBs, GRB 080913 at $z=6.7$ and GRB 090423 at $z=8.3$. We find that the SFR at $z>4$ shows a steep decay with a slope of $\sim -5.0$ in $Λ$CDM. In the other three models, the high-redshift SFR is slightly different from $Λ$CDM model, and also shows a steep decay.

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arXiv2009

GRB Jet Beaming Angle Statistics

Existing theory and models suggest that a Type I (merger) GRB should have a larger jet beaming angle than a Type II (collapsar) GRB, but so far no statistical evidence is available to support this suggestion. In this paper, we obtain a sample of 37 beaming angles and calculate the probability that this is true. A correction is also devised to account for the scarcity of Type I GRBs in our sample. The probability is calculated to be 83% without the correction and 71% with it.

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arXiv2009

Hyperaccreting Neutron-Star Disks and Neutrino Annihilation

Newborn neutron stars surrounded by hyperaccreting and neutrino-cooled disks may exist in some gamma-ray bursts (GRBs) and/or supernovae (SNe). In this paper we further study the structure of such a neutron-star disk based on the two-region (i.e., inner & outer) disk scenario following our previous work, and calculate the neutrino annihilation luminosity from the disk in various cases. We investigate the effects of the viscosity parameter, energy parameter (measuring the neutrino cooling efficiency of the inner disk) and outflow strength on the structure of the entire disk as well as the effect of emission from the neutron star surface boundary emission on the total neutrino annihilation rate. The inner disk satisfies the entropy-conservation or the advection-dominated self-similar structure depending on the energy parameter. An outflow from the disk decreases the density and pressure but increases the thickness of the disk. Moreover, compared with the black-hole disk, the neutrino annihilation luminosity above the neutron-star disk is higher, and the neutrino emission from the boundary layer could increase the neutrino annihilation luminosity by about one order of magnitude higher than the disk without boundary emission. The neutron-star disk with the advection-dominated inner disk could produce the highest neutrino luminosity while the disk with an outflow has the lowest. Although a heavily mass-loaded outflow from the neutron star surface at early times of neutron star formation prevents the outflow material from being accelerated to a high bulk Lorentz factor, an energetic ultrarelativistic jet via neutrino annihilation can be produced above the stellar polar region at late times if the disk accretion rate and the neutrino emission luminosity from the surface boundary layer are sufficiently high.

astro-ph.GA↗

arXiv2008

X-Ray and high energy flares from late internal shocks of gamma-ray bursts

We study afterglow flares of gamma-ray bursts (GRBs) in the framework of the late internal shock (LIS) model based on a careful description for the dynamics of a pair of shocks generated by a collision between two homogeneous shells,. First, by confronting the model with some fundamental observational features of X-ray flares, we find some constraints on the properties of the pre-collision shells that are directly determined by the central engine of GRBs. Second, high energy emission associated with X-ray flares, which arises from synchrotron self-Compton (SSC) emission of LISs, is investigated in a wide parameter space. The predicted flux of high energy flares may reach as high as $\sim 10^{-8}\rm erg cm^{-2}s^{-1}$, which is likely to be detectable with the Large Area Telescope (LAT) aboard \textit{the Fermi Space Telescope}

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