arXiv subjects

Xue-Feng Wu

Publications and source records attributed to Xue-Feng Wu.

At least 91 records · Page 5Linked to original sources

arXiv2020

The SiTian project

SiTian is an ambitious ground-based all-sky optical monitoring project, developed by the Chinese Academy of Sciences. The concept is an integrated network of dozens of 1-m-class telescopes deployed partly in China and partly at various other sites around the world. The main science goals are the detection, identification and monitoring of optical transients (such as gravitational wave events, fast radio bursts, supernovae) on the largely unknown timescales of less than 1 day; SiTian will also provide a treasure trove of data for studies of AGN, quasars, variable stars, planets, asteroids, and microlensing events. To achieve those goals, SiTian will scan at least 10,000 square deg of sky every 30 min, down to a detection limit of $V \approx 21$ mag. The scans will produce simultaneous light-curves in 3 optical bands. In addition, SiTian will include at least three 4-m telescopes specifically allocated for follow-up spectroscopy of the most interesting targets. We plan to complete the installation of 72 telescopes by 2030 and start full scientific operations in 2032.

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arXiv2020

Testing the Weak Equivalence Principle and Lorentz Invariance with Multiwavelength Polarization Observations of GRB Optical Afterglows

Violations of both the weak equivalence principle (WEP) and Lorentz invariance can produce vacuum birefringence, which leads to an energy-dependent rotation of the polarization vector of linearly polarized emission from a given astrophysical source. However, the search for the birefringent effect has been hindered by our ignorance concerning the intrinsic polarization angle in different energy bands. Considering the contributions to the observed linear polarization angle from both the intrinsic polarization angle and the rotation angles induced by violations of the WEP and Lorentz invariance, and assuming the intrinsic polarization angle is an unknown constant, we simultaneously obtain robust bounds on possible deviations from the WEP and Lorentz invariance, by directly fitting the multiwavelength polarimetric data of the optical afterglows of gamma-ray burst (GRB) 020813 and GRB 021004. Here we show that at the $3σ$ confidence level, the difference of the parameterized post-Newtonian parameter $γ$ values characterizing the departure from the WEP is constrained to be $Δγ=\left(-4.5^{+10.0}_{-16.0}\right)\times10^{-24}$ and the birefringent parameter $η$ quantifying the broken degree of Lorentz invariance is limited to be $η=\left(6.5^{+15.0}_{-14.0}\right)\times10^{-7}$. These are the first simultaneous verifications of the WEP and Lorentz invariance in the photon sector. More stringent limits can be expected as the analysis presented here is applied to future multiwavelength polarization observations in the prompt gamma-ray emission of GRB

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arXiv2020

Combined Limit on the Photon Mass with Nine Localized Fast Radio Bursts

A nonzero-mass hypothesis for the photon can produces a frequency-dependent dispersion of light, which results in arrival-time differences of photons with different frequencies originating from a given transient source. Extragalactic fast radio bursts (FRBs), with their low frequency emissions, short time durations, and long propagation distances, are excellent astrophysical probes to constrain the rest mass of the photon $m_γ$. However, the derivation of a limit on $m_γ$ is complicated by the similar frequency dependences of dispersion expected from the plasma and nonzero photon mass effects. If a handful measurements of redshift for FRBs are available, the different redshift dependences of the plasma and photon mass contributions to the dispersion measure (DM) might be able to break dispersion degeneracy in testing the photon mass. For now, nine FRBs with redshift measurements have been reported, which can turn this idea into reality. Taking into account the DM contributions from both the plasma and a possible photon mass, we use the data on the nine FRBs to derive a combined limit of $m_γ\leq7.1\times10^{-51}\;{\rm kg}$, or equivalently $m_γ\leq4.0\times10^{-15}\; {\rm eV}/c^{2}$ at 68\% confidence level, which is essentially as good as or represents a factor of 7 improvement over previous limits obtained by the single FRBs. Additionally, a reasonable estimation for the DM contribution from the host galaxy, $\rm DM_{host}$, can be simultaneously achieved in our analysis. The rapid progress in localizing FRBs will further tighten the constraints on both $m_γ$ and $\rm DM_{host}$.

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arXiv2020

The Bright Reverse Shock Emission in the Optical Afterglows of Gamma-ray Bursts in a Stratified Medium

The reverse shock (RS) model is generally introduced to interpret the optical afterglows with the rapid rising and decaying, such as the early optical afterglow of GRB 990123 (which is also called optical flash). In this paper, we collected 11 gamma-ray bursts (GRBs) early optical afterglows, which have such signatures of dominant RS emission. Since the temporal slopes of the optical flashes are determined by both the medium density distribution index $k$ and the electron spectral index $p$, we apply the RS model of the thin shell case to the optical flashes and determine the ambient medium of the progenitors. We find that the $k$ value is in the range of 0 - 1.5. The $k$ value in this paper is consistent with the result in Yi et al. (2013), where the forward shock (FS) model was applied to some onset bumps. However, the method adopted in this paper is only applicable to GRB afterglows with significant sharp rising and decaying RS emission. Our results indicate that the RS model can also be applied to confirm the circumburst medium, further implying that GRBs may have diverse circumburst media.

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arXiv2019

"Double-tracking" Characteristic of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index $α$ and the peak energy $E_{\rm p}$ exhibit the "flux-tracking" pattern ("double-tracking" characteristics). (ii) The parameter relations, i.e., $F$ (the energy flux)-$α$, $F$-$E_{\rm p}$, and $E_{\rm p}$-$α$, along with the analogous Yonetoku $E_{\rm p}$-$L_{γ,\rm iso}$ relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of $α$ do not exceed the synchrotron limit ($α$= -2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.

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arXiv2019

The time-resolved spectra of photospheric emission from a structured jet for gamma-ray bursts

The quasi-thermal components found in many Fermi gamma-ray bursts (GRBs) imply that the photosphere emission indeed contributes to the prompt emission of many GRBs. But whether the observed spectra empirically fitted by the Band function or cutoff power law, especially the spectral and peak energy ($E_{p}$) evolutions can be explained by the photosphere emission model alone needs further discussion. In this work, we investigate in detail the time-resolved spectra and $E_{p}$ evolutions of photospheric emission from a structured jet, with an inner-constant and outer-decreasing angular Lorentz factor profile. Also, a continuous wind with a time-dependent wind luminosity has been considered. We show that the photosphere spectrum near the peak luminosity is similar to the cutoff power-law spectrum. The spectrum can have the observed average low-energy spectral index $α$ $ \sim -1$, and the distribution of the low-energy spectral index in our photosphere model is similar to that observed ($-2\lesssim $ $α\lesssim 0$). Furthermore, the two kinds of spectral evolutions during the decay phase, separated by the width of the core ($θ_{c}$), are consistent with the time-resolved spectral analysis results of several Fermi multi-pulse GRBs and single-pulse GRBs, respectively. Also, for this photosphere model we can reproduce the two kinds of observed $E_{p}$ evolution patterns rather well. Thus, by considering the photospheric emission from a structured jet, we reproduce the observations well for the GRBs best fitted by the cutoff power-law model for the peak-flux spectrum or the time-integrated spectrum.

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arXiv2019

Constraining the Evolution of the Baryon Fraction in the IGM with FRB and H(z) data

Extragalactic dispersion measures (DMs) obtained from observations of fast radio bursts (FRBs) are an excellent tool for probing intergalactic medium (IGM) and for conducting cosmography. However, the DM contribution from the IGM (${\rm DM_{IGM}}$) depends on the fraction of baryon mass in the IGM, $f_{\rm IGM}$, which is not properly constrained. As $f_{\rm IGM}(z)$ is geometrically related to the Hubble parameter $H(z)$ and ${\rm DM_{IGM}}(z)$, here we propose that combining two independent measurements of FRBs and $H(z)$ in similar redshift ranges provides a novel and cosmology-free method to constrain the evolution of $f_{\rm IGM}(z)$. Under the assumption that $f_{\rm IGM}$ is evolving with redshift in a functional form, we forecast that the evolution of $f_{\rm IGM}(z)$ can be well inferred in a combined analysis of $\sim3000$ ${\rm DM_{IGM}}(z)$ derived from FRBs and $\sim50$ $H(z)$ derived from the Hubble parameter data. Though the efficiency of our method is not as good as that of the other model-independent method involving the joint measurements of DM and luminosity distance of FRBs, our method offers a new model-independent way to constrain $f_{\rm IGM}(z)$.

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arXiv2019

The Luminosity Function and Formation Rate of A Complete Sample of Long Gamma-Ray Bursts

We study the luminosity function and formation rate of long gamma-ray bursts (GRBs) by using a maximum likelihood method. This is the first time this method is applied to a well-defined sample of GRBs that is complete in redshift. The sample is composed of 99 bursts detected by the $Swift$ satellite, 81 of them with measured redshift and luminosity for a completeness level of $82\%$. We confirm that a strong redshift evolution in luminosity (with an evolution index of $δ=2.22^{+0.32}_{-0.31}$) or in density ($δ=1.92^{+0.20}_{-0.21}$) is needed in order to reproduce the observations well. But since the predicted redshift and luminosity distributions in the two scenarios are very similar, it is difficult to distinguish between these two kinds of evolutions only on the basis of the current sample. Furthermore, we also consider an empirical density case in which the GRB rate density is directly described as a broken power-law function and the luminosity function is taken to be non-evolving. In this case, we find that the GRB formation rate rises like $(1+z)^{3.85^{+0.48}_{-0.45}}$ for $z\leq2$ and is proportional to $(1+z)^{-1.07^{+0.98}_{-1.12}}$ for $z\geq2$. The local GRB rate is $1.49^{+0.63}_{-0.64}$ Gpc$^{-3}$ yr$^{-1}$. The GRB rate may be consistent with the cosmic star formation rate (SFR) at $z\leq2$, but shows an enhancement compared to the SFR at $z\geq2$.

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arXiv2019

Limits on the Weak Equivalence Principle and Photon Mass with FRB 121102 Sub-pulses

Fast radio bursts (FRBs) are short duration ($\sim$millisecond) radio transients with cosmological origin. The simple sharp features of the FRB signal have been utilized to probe two fundamental laws of physics, namey, testing Einstein's weak equivalence principle and constraining the rest mass of the photon. Recently, \cite{hessels18} found that after correcting for dispersive delay, some of the bursts in FRB 121102 have complex time-frequency structures that include sub-pulses with a time-frequency downward drifting property. Using the delay time between sub-pulses in FRB 121102, here we show that the parameterized post-Newtonian parameter $γ$ is the same for photons with different energies to the level of $\left|γ_1-γ_2\right|<2.5\times 10^{-16}$, which is 1000 times better than previous constraints from FRBs using similar methods. We also obtain a stringent constraint on the photon mass, $m_γ < 5.1\times10^{-48}$ g, which is 10 times smaller than previous best limits on the photon mass derived through the velocity dispersion method.

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arXiv2019

The Energy Function and Cosmic Formation Rate of Fast Radio Bursts

Fast radio bursts (FRBs) are intense radio transients whose physical origin remains unknown. Therefore, it is of crucial importance to use a model-independent method to obtain the energy function and cosmic formation rate directly from the observational data. Based on current samples from the Parkes and ASKAP telecsopes, we determine, for the first time, the energy function and formation rate of FRBs by using the Lynden-Bell $\rm C^{-}$ method. The energy function derived from the Parkes sample is a broken power law, however it is a simple power law for the ASKAP sample. For Parkes sample, we derive the formation rate which is roughly consistent with the star formation rate up to $z\sim1.7$, with a local formation rate of $\dot ρ(0) \simeq (3.2\pm 0.3)\; \times {10^4}\;{\rm{Gp}}{\rm{c}^{ - 3}}{\rm{y}}{\rm{r}^{ - 1}}$ above a detection threshold of $2\,{\rm{Jyms}}$. For ASKAP sample, we find that the formation rate evolves much faster than the star formation rate up to $z\sim0.7$, namely $\dot ρ(z) \propto {(1 + z)^{6.9 \pm 1.9}}$, with a local formation rate of $\dot ρ(0) \simeq (4.6\pm 0.8)\; \times {10^3}\;{\rm{Gp}}{\rm{c}^{ - 3}}{\rm{y}}{\rm{r}^{ - 1}}$ above a detection threshold of $51\,{\rm{Jyms}}$. This might be a important clue for the physical origin of FRBs.

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arXiv2019

Precision Test of the Weak Equivalence Principle from Gamma-Ray Burst Polarization

If the weak equivalence principle (WEP) is broken, the measured values of the parametrized post-Newtonian parameter $γ$ from photons with left- and right-handed circular polarizations should differ slightly, leading to the arrival-time difference of these two circular components. Thus, the polarization vector of a linearly polarized light may rotate during the propagation. The rotation angle of the polarization vector depends on both the photon energy and the distance of the source. It is believed that if the rotation angle differs by more than $π/2$ over an energy range, then the net polarization of the signal would be significantly suppressed and could not be as high as the observed level. Thus, the detection of highly polarized photons implies that the relative rotation angle ($ΔΘ$) should not be too large. In this paper, we give a detailed calculation on the evolution of gamma-ray burst (GRB) polarization arising from a possible violation of the WEP, and we find that more than $60\%$ of the initial polarization degree can be conserved even if $ΔΘ$ is larger than $π/2$. In addition, to tightly constrain the WEP violation, GRBs with harder spectra and polarization observations in a wider energy range seem to be favored. Applying our formulas to the measurements of linear polarization from GRB 110721A and GRB 061122, we obtain strict limits on the differences of the $γ$ values as low as $Δγ<1.3\times10^{-33}$ and $Δγ<0.8\times10^{-33}$. These provide the most stringent limits to date on a deviation from the WEP, improving at least 6 orders of magnitude over previous bounds.

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arXiv2019

Polarization of Astrophysical Events with Precessing Jets

A central compact object (CCO, e.g. a black hole) with an accretion disk has been suggested as the common central engine of various astrophysical phenomena, such as gamma-ray bursts (GRBs), tidal disruption events (TDEs) and active galactic nuclei (AGNs). A jet powered by such a system might precess due to the misalignment of the angular momenta of the CCO and accretion disk. Some quasi-periodic behaviors observed in the light curves of these phenomena can be well interpreted within the framework of a precessing jet model. In this paper, we study the emission polarization of precessing jets in the three kinds of phenomena. The polarization angle also shows a gradual change for the synchrotron emission in both the random and toroidal magnetic field configurations with the precessing jet, while it can only change abruptly by $90^\circ$ for the non-precessing top-hat jet. Polarization properties are periodic due to the assumptions made in our model. The polarization observations are crucial to confirm the precession nature of jets in GRBs, TDEs and AGNs.

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arXiv2019

Multimessenger Tests of Einstein's Weak Equivalence Principle and Lorentz Invariance with a High-energy Neutrino from a Flaring Blazar

The detection of the high-energy ($\sim290$ TeV) neutrino coincident with the flaring blazar TXS 0506+056, the first and only $3σ$ neutrino-source association to date, provides new, multimessenger tests of the weak equivalence principle (WEP) and Lorentz invariance. Assuming that the flight time difference between the TeV neutrino and gamma-ray photons from the blazar flare is mainly caused by the gravitational potential of the Laniakea supercluster of galaxies, we show that the deviation from the WEP for neutrinos and photons is conservatively constrained to have an accuracy of $10^{-6}-10^{-7}$, which is 3--4 orders of magnitude better than previous results placed by MeV neutrinos from supernova 1987A. In addition, we demonstrate that the association of the TeV neutrino with the blazar flare sets limits on the energy scales of quantum gravity for both linear and quadratic violations of Lorentz invariance (LIV) to $E_{\rm QG, 1}>3.2\times10^{15}-3.7\times10^{16}$ GeV and $E_{\rm QG, 2}>4.0\times10^{10}-1.4\times10^{11}$ GeV. These improve previous limits on both linear and quadratic LIV energy scales in neutrino propagation by 5--7 orders of magnitude.

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arXiv2019

Detailed polarization measurements of the prompt emission of five Gamma-Ray Bursts

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.

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arXiv2018

Fast Radio Bursts From Primordial Black Hole Binaries Coalescence

In this paper we propose the model that the coalescence of primordial black holes (PBHs) binaries with equal mass $M \sim 10^{28}$g can emit luminous gigahertz (GHz) radio transient, which may be candidate sources for the observed fast radio bursts (FRBs), if at least one black hole holds appropriate amount of net electric charge $Q$. Using a dimensionless quantity for the charge $q = Q/\sqrt{G}M$, our analyses infer that $q\sim O(10^{-4.5})$ can explain the FRBs with released energy of order $O(10^{40}) {\rm ergs}$. With the current sample of FRBs and assuming a distribution of charge $ϕ(q)$ for all PBHs, we can deduce that its form is proportional to $q^{-3.0\pm0.1}$ for $q\geq 7.2\times10^{-5}$ if PBHs are sources of the observed FRBs. Furthermore, with the proposed hypothetical scenario and by estimating the local event rate of FRBs $\sim 2.6 \times 10^3 {\rm Gpc}^{-3} {\rm yr}^{-1}$, one derives a lower bound for the fraction of PBHs (at the mass of $10^{28}$g) against that of matter $f_{\rm PBH}(10^{28}{\rm g})$ $\gtrsim 10^{-5}$ needed to explain the rate. With this inspiring estimate, we expect that future observations of FRBs can help to falsify their physical origins from the PBH binaries coalescences. In the future, the gravitational waves produced by mergers of small black holes can be detected by high frequency gravitational wave detectors. We believe that this work would be a useful addition to the current literature on multimessenger astronomy and cosmology.

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arXiv2018

Constraining the Type of Central Engine of GRBs with Swift Data

The central engine of gamma-ray bursts (GRBs) is poorly constrained. There exist two main candidates: a fast-rotating black hole and a rapidly spinning magnetar. Furthermore, the X-ray plateaus are widely accepted by the energy injection into the external shock. In this paper, we systematically analyze the \emph{Swift}/XRT light curves of 101 GRBs having plateau phases and known redshifts (before May 2017). Since a maximum energy budget ($\sim2\times10^{52}$ erg) exists for magnetars but not for a black hole, this provides a good clue to identify the type of GRB central engine. We calculate the isotropic kinetic energy $E_{\rm K,iso}$ and the isotropic X-ray energy release $E_{\rm X,iso}$ for individual GRB. We identify three categories based on how likely a black hole harbor at central engine: 'Gold' (9 out of 101, both $E_{\rm X,iso}$ and $E_{\rm K,iso}$ exceed the energy budget), 'Silver' (69 out of 101, $E_{\rm X,iso}$ less than the limit but $E_{\rm K,iso}$ is greater than the limit), and 'Bronze' (23 out of 101, the energies are not above the limit). We then derive and test the black hole parameters with the Blandford Znajek mechanism, and find that the observations of the black hole candidate ('Gold'+'Silver') samples are consistent with the expectations of the black hole model. Furthermore, we also test the magnetar candidate ('Bronze') sample with the magnetar model, and find that the magnetar surface magnetic led ($B_{p}$) and initial spin period ($P_{0}$) are consistent with the expectations of the magnetar model. Our analysis indicates that, if the magnetar wind is isotropic, a magnetar central engine is possible for 20\% of the analyzed GRBs. For most GRBs a black hole is most likely operating.

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arXiv2018

A Large Catalogue of Multi-wavelength GRB Afterglows I: Color Evolution And Its Physical Implication

The spectrum of gamma-ray burst (GRB) afterglows can be studied with color indices. Here we present a large comprehensive catalogue of 70 GRBs with multi-wavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. We categorize them into two samples based on how well the color indices are evaluated. The Golden sample includes 25 bursts mostly observed by GROND, and the Silver sample includes 45 bursts observed by other telescopes. For the Golden sample, we find that 95\% of the color indices do not vary over time. However, the color indices do vary during short periods in most bursts. The observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43\%) and in a constant density medium (30\%), (ii) early dust extinction (12\%), (iii) transition from reverse shock to forward shock emission (5\%), or (iv) an emergent supernova emission (10\%). We also study the evolutionary properties of the mean color indices for different emission episodes. We find that 86\% of the color indices in the 70 bursts show constancy between consecutive ones. The color index variations occur mainly during the late GRB-SN bump, the flare and early reversed-shock emission components. We further perform a statistical analysis of various observational properties and model parameters (spectral index $β_{o}^{\rm CI}$, electron spectral indices $p^{\rm CI}$, etc.) using color indices. Overall, we conclude that $\sim$ 90\% of colors are constant in time and can be accounted for by the simplest external forward shock model, while the varying color indices call for more detailed modeling.

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arXiv2018

Polarization with a 3-dimensional Mixed Magnetic Field and Its Application to GRB 170817A

A large-scale ordered magnetic field plays a very important role in the formation and acceleration of a gamma-ray burst (GRB) jet. During the GRB prompt phase, some dissipation processes may happen and disturb the magnetic field, making the field become random to some extent. Therefore, a mixed magnetic field consisting of an ordered component and a random component is plausible for the early afterglow phase. Here we investigate the polarization evolution and light curve of an afterglow under such a 3-dimensional mixed magnetic field. Three kinds of ordered component (i.e., aligned, toroidal and radial) are discussed. We find that the three cases are distinguishable through jet polarization evolution. The polarization angle for a 3D mixed magnetic field with an aligned ordered component can evolve gradually but only changes abruptly by $90^\circ$ in the toroidal and radial cases. Furthermore, during the reverse shock crossing time, the polarization degree (PD) can be non-zero for the toroidal case but roughly zero for the radial case. Since an aligned component in a jet corresponds to a magnetar central engine and a toroidal component corresponds to a black hole, GRB central engines are distinguishable through polarization observations even if the magnetic field is mixed in a jet. In addition, our polarization calculation can be applied to GRB 170817A associated with GW170817. Using the recently-observed PD upper limit $12\%$ of GRB 170817A at $t=244$ days, the magnetic field strength ratio of the ordered to random components in this afterglow is constrained to be $\lesssim0.9$.

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