Zhuo Li

Peking University, Peping, Beijing, China

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Publications (25)104.38 Total impact

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    ABSTRACT: The IceCube (IC) collaboration recently reported the detection of TeV-PeV extraterrestrial neutrinos whose origin is yet unknown. By the photon-neutrino connection in p-p and p-gamma interactions, we use the Fermi-LAT observations to constrain the origin of the IC detected neutrinos. We find that Galactic origins, i.e., the diffuse Galactic neutrinos due to cosmic ray (CR) propagation in the Milky Way, and the neutrinos from the Galactic point sources, may not produce the IC neutrino flux, thus these neutrinos should be of extragalactic origin. Moreover, the extragalactic gamma-ray bursts (GRBs) may not account for the IC neutrino flux, but the starburst galaxies (SBGs) may be promising sources. As suggested by the consistency between the IC detected neutrino flux and the Waxman-Bahcall bound, GRBs in SBGs may be the sources of both the ultrahigh energy, >1E19 eV, CRs and the 1-100 PeV CRs that produce the IC detected TeV-PeV neutrinos.
    07/2014;
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    He Gao, Zhuo Li, Bing Zhang
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    ABSTRACT: Recently, both theoretical arguments and observational evidence suggested that a small fraction of fast radio bursts (FRBs) could be associated with gamma-ray bursts (GRBs). If such FRB/GRB association systems are commonly detected in the future, the combination of dispersion measure (DM) derived from FRBs and redshifts derived from GRBs makes these systems a plausible tool to conduct cosmography. We quantify uncertainties in deriving redshift-dependent DM_{IGM}} as a function of z, and test how well dark energy models can be constrained with Monte Carlo simulations. We show that with potentially several 10s of FRB/GRB systems detected in a decade or so, one can reach reasonable constraints on wCDM models. When combined with SN Ia data, unprecedented constraints on dark energy equation of state can be achieved, thanks to the prospects of detecting FRB/GRB systems at relatively high redshifts. The ratio between the mean value < DM_{IGM} (z)> and luminosity distance (D_{L} (z)) is insensitive to dark energy models. This gives the prospects of applying SN Ia data to calibrate < DM_{IGM} (z)> using a relatively small sample of FRB/GRB systems, allowing a reliable constraint on the baryon inhomogeneity distribution as a function of redshift.
    The Astrophysical Journal 02/2014; 788(2). · 6.73 Impact Factor
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    ABSTRACT: The origin of magnetic fields that permeate the blast waves of gamma-ray bursts (GRBs) is a long-standing problem. The present paper argues that in four GRBs revealing extended emission at >100 MeV, with follow-up in the radio, optical and X-ray domains at later times, this magnetization can be described as the partial decay of the micro-turbulence that is generated in the shock precursor. Assuming that the bulk of the extended emission >100 MeV can be interpreted as synchrotron emission of shock accelerated electrons, we model the multi-wavelength light curves of GRB 090902B, GRB 090323, GRB 090328 and GRB 110731A, using a simplified then a full synchrotron calculation with power-law-decaying microturbulence \epsilon_B \propto t^{\alpha_t} (t denotes the time since injection through the shock, in the comoving blast frame). We find that these models point to a consistent value of the decay exponent -0.5 < \alpha_t < -0.4.
    Monthly Notices of the Royal Astronomical Society 05/2013; 435(4). · 5.52 Impact Factor
  • Biao Li, Zhuo Li
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    ABSTRACT: It is expected that there should be a spectral cutoff at the high energy end of emission from a prompt gamma-ray burst (GRB), due to, e.g. γγ absorption and/or a high energy cutoff in the electron distribution. We analyze the spectral data of Fermi-LAT detected GRBs 080916C and 090926A, aiming at locating the spectral cutoff. By assuming that the prompt GRB spectrum at the high energy end is a power law with an exponential cutoff, our analysis finds that the cutoff energy Ecutoff depends on the photon index β and the cutoff occurs at very high energy, Ecutoff = 161+533−95 GeV in GRB 080916C and Ecutoff 100 GeV (for β ≈ −2.3) in GRB 090926A. Such high energy photons, if they exist, may disfavor the synchrotron origin and need alternative generation mechanisms.
    Research in Astronomy and Astrophysics 11/2012; 12(11):1519. · 1.35 Impact Factor
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    Zhuo Li
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    ABSTRACT: We discuss the implications of Fermi/LAT observations on several aspects of gamma-ray burst (GRB) physics, including the radiation process, the emission sites, the bulk Lorentz factor, and the pre-shock magnetic field: (1) MeV-range emission favors synchrotron process but the highest energy (>10GeV) emission may not be synchrotron origin, more likely inverse Compton origin; (2) GRB should have multi-zone emission region, with MeV emission produced at smaller radii while optical and >100MeV emission at larger radii; (3) the bulk Lorentz factor can be a few 100's, much lower than 10^3, in multi-zone model; (4) the upstream magnetic field of afterglow shock is strongly amplified to be at least in mG scale.
    12/2011;
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    ABSTRACT: Recently, Fermi-LAT detected GeV emission during the X-ray flaring activity in GRB 100728A. We study various scenarios for its origin. The hard spectrum of the GeV emission favors the external inverse-Compton origin in which X-ray flare photons are up-scattered by relativistic electrons in the external forward shock. This external IC scenario, with anisotropic scattering effect taken into account, can reproduce the temporal and spectral properties of the GeV emission in GRB 100728A.
    The Astrophysical Journal 12/2011; 753(2). · 6.73 Impact Factor
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    ABSTRACT: The Lorentz factor (LF) of gamma-ray burst (GRB) ejecta may be constrained by observations of high-energy (HE) spectral attenuation. The recent Fermi-LAT observations of prompt GeV emission from several bright GRBs have led to conclusions of unexpectedly large LFs, Γ>103. Here we revisit this problem with two main concerns. (1) With a one-zone assumption where all photons are assumed to be generated in the same region (radius) and time, we self-consistently calculate the γγ optical depth by adopting a target photon spectrum with an HE cutoff. We find that this might be important when the GRB LF is below a few hundreds. (2) Recent Fermi-LAT observations suggest that the bulk MeV-range and HE (100 MeV) emission may arise from different regions. We then consider a two-zone case where HE emission is generated in much larger radii than that of the MeV-range emission. We find that the HE emission may mainly be attenuated by the MeV-range emission and that the attenuated HE spectrum does not show an exponential spectral cutoff but a slight steepening. This suggests that there may be no abrupt cutoff due to γγ attenuation if relaxing the one-zone assumption. By studying the spectra of three bright Fermi-LAT GRBs, 080916C, 090510, and 090902B, we show that bulk LFs of Γ ~ 600 can be consistent with observations in the two-zone case. Even lower LFs can be obtained in the multi-zone case.
    The Astrophysical Journal 12/2010; 726(2):89. · 6.73 Impact Factor
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    Zhuo Li, Xiao-Hong Zhao
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    ABSTRACT: Long-lived >100 MeV emission has been a common feature of most Fermi-LAT detected gamma-ray bursts (GRBs), e.g., detected up to ~10^3s in long GRBs 080916C and 090902B and ~10^2s in short GRB 090510. This emission is consistent with being produced by synchrotron emission of electrons accelerated to high energy by the relativistic collisionless shock propagating into the weakly magnetized medium. Here we show that this high-energy afterglow emission constrains the preshock magnetic field to satisfy 1(n/1cc)^{9/8} mG<B<10^2(n/1cc)^{3/8}mG, where n is the preshock density, more stringent than the previous constraint by X-ray afterglow observations on day scale. This suggests that the preshock magnetic field is strongly amplified, most likely by the streaming of high energy shock accelerated particles.
    Journal of Cosmology and Astroparticle Physics 04/2010; · 6.04 Impact Factor
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    ABSTRACT: Extended high-energy(>100MeV) gamma-ray emission that lasts much longer than the prompt sub-MeV emission has been detected from quite a few gamma-ray bursts (GRBs) by Fermi Large Area Telescope (LAT) recently. A plausible scenario is that this emission is the afterglow synchrotron emission produced by electrons accelerated in the forward shocks. In this scenario, the electrons that produce synchrotron high-energy emission also undergo inverse-Compton (IC) loss and the IC scattering with the synchrotron photons should be in the Klein-Nishina regime. Here we study effects of the Klein-Nishina scattering on the high-energy synchrotron afterglow emission. We find that, at early times the Klein-Nishina suppression effect on those electrons that produce the high-energy emission is usually strong and therefore their inverse-Compton loss is small with a Compton parameter Y < a few for a wide range of parameter space. This leads to a relatively bright synchrotron afterglow at high energies that can be detected by Fermi LAT. As the Klein-Nishina suppression effect weakens with time, the inverse-Compton loss increases and could dominate over the synchrotron loss in some parameter space. This will lead to a faster temporal decay of the high-energy synchrotron emission than what is predicted by the standard synchrotron model, which may explain the observed rapid decay of the early high-energy gamma-ray emission in GRB090510 and GRB090902B. Comment: 8 page (emulateapj style), 8 figures, submitted to ApJ
    The Astrophysical Journal 11/2009; · 6.73 Impact Factor
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    ABSTRACT: Fermi observations of high-energy gamma-ray emission from GRB 080916C shows that its spectrum is consistent with the Band function from MeV to tens of GeV. Assuming one single emission mechanism dominates in the whole energy range, we show that this spectrum is consistent with synchrotron origin by shock-accelerated electrons. The simple electron inverse-Compton model and the hadronic model are found to be less viable. In the synchrotron scenario, the synchrotron self-Compton scattering is likely to be in the Klein-Nishina regime and therefore the resulting high-energy emission is subdominant, even though the magnetic field energy density is lower than that in relativistic electrons. The Klein-Nishina inverse-Compton cooling may also affect the low-energy electron number distribution and hence results in a low-energy synchrotron photon spectrum $n(\nu)\propto\nu^{-1}$ below the peak energy. Under the framework of the electron synchrotron interpretation, we constrain the shock microphysical parameters and derive a lower limit of the upstream magnetic fields. The detection of synchrotron emission extending to about 70 GeV in the source frame in GRB 080916C favors the Bohm diffusive shock acceleration if the bulk Lorentz factor of the relativistic outflow is not significantly greater than thousands.
    The Astrophysical Journal 03/2009; 698(2). · 6.73 Impact Factor
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    ABSTRACT: Thermal X-ray emission that is simultaneous with prompt gamma-ray emission has been detected for the first time from a supernova associated with a gamma-ray burst (GRB), namely GRB 060218/SN 2006aj. It has been interpreted as arising from the breakout of a mildly relativistic, radiation-dominated shock from a dense stellar wind surrounding the progenitor star. There is also evidence for the presence of a mildly relativistic ejecta in GRB 980425/SN 1998bw, based on its X-ray and radio afterglow. Here we study the process of repeated bulk Compton scatterings of shock breakout thermal photons by the mildly relativistic ejecta. During the shock breakout process, a fraction of the thermal photons would be repeatedly scattered between the preshock material and the shocked material, as well as the mildly relativistic ejecta, and as a result, the thermal photons get boosted to increasingly higher energies. This bulk motion Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes, which could account for the prompt gamma-ray burst emission in low-luminosity supernova-connected GRBs, such as GRB 060218. A Monte Carlo code has been developed to simulate this repeated scattering process, which confirms that a significant fraction of the thermal photons get "accelerated" to form a nonthermal component, with a dominant luminosity. This interpretation for the prompt nonthermal emission of GRB 060218 may imply that either the usual internal shock emission from highly relativistic jets in these low-luminosity GRBs is weak, or alternatively, that there are no highly relativistic jets in this particular class of bursts.
    The Astrophysical Journal 12/2008; 664(2):1026. · 6.73 Impact Factor
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    Zhuo Li, Eli Waxman
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    ABSTRACT: The prompt γ-ray emission in γ-ray bursts is believed to be produced by internal shocks within a relativistic unsteady outflow. The recent detection of prompt optical emission accompanying the prompt γ-ray emission appears to be inconsistent with this model, because the outflowing plasma is expected to be highly optically thick to optical photons. We show here that fluctuations in flow properties on short, ~1 ms, timescales, which drive the γ-ray-producing collisions at small radii, are expected to lead to "residual" collisions at much larger radii, where the optical depth to optical photons is low. The late residual collisions naturally account for the relatively bright optical emission. The apparent simultaneity of γ-ray and optical emission is due to the highly relativistic speed with which the plasma expands. Residual collisions may also account for the X-ray emission during the early "steep decline" phase, where the radius is inferred to be larger than the γ-ray emission radius. Finally, we point out that inverse Compton emission from residual collisions at large radii is expected to contribute significantly to the emission at high energy and may therefore "smear" the pair-production spectral cutoff.
    The Astrophysical Journal 12/2008; 674(2):L65. · 6.73 Impact Factor
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    ABSTRACT: Gamma-ray bursts (GRBs) are believed to originate from ultrarelativistic winds/fireballs to avoid the "compactness problem." However, the most energetic photons in GRBs may still suffer from γ-γ absorption leading to e± pair production in the winds/fireballs. We show here that in a wide range of model parameters, the resulting pairs may dominate those electrons associated with baryons. Later on, the pairs would be carried into a reverse shock so that a shocked pair-rich fireball may produce a strong flash at lower frequencies, i.e., in the IR band, in contrast to optical/UV emission from a pair-poor fireball. The IR emission would show a 5/2 spectral index due to strong self-absorption. Rapid responses to GRB triggers in the IR band would detect such strong flashes. The future detections of many IR flashes will infer that the rarity of prompt optical/UV emissions is in fact due to dust obscuration in the star formation regions.
    The Astrophysical Journal 12/2008; 599(1):380. · 6.73 Impact Factor
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    ABSTRACT: We suggest that non-thermal emission can be produced by multiple scatterings of the photons between the supernova ejecta and pre-shock material in supernova shock breakout. Such bulk-Comptonization process may significantly change the original thermal photon spectrum, forming a power-law non-thermal component at higher energies. We then show that the luminous X-ray outburst XRO081009 associated with SN2008D is likely to be such shock breakout emission from an ordinary type Ib/c supernova. Comment: 6 pages, 1 figure, based on the talk given by Xiang-Yu Wang at the "2008 Nanjing GRB Conference" (evening focused discussion), To appear in the "2008 Nanjing GRB Conference" conference proceedings
    08/2008;
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    Zhuo Li, Eli Waxman
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    ABSTRACT: Electromagnetic energy losses of charged pions and muons suppress the expected high energy, >1E18 eV, neutrino emission from sources of ultrahigh energy, >1E19 eV, cosmic-rays. We show here that >1E19 eV photons produced in such sources by neutral pion decay may escape the sources, thanks to the Klein-Nishina suppression of the pair production cross section, and produce muon pairs in interactions with the cosmic microwave background. The flux of muon decay neutrinos, which are expected to be associated in time and direction with the electromagnetic emission from the sources, may reach a few percent of the Waxman-Bahcall bound. Their detection may allow one to directly identify the sources of >1E19 eV cosmic-rays, and will provide the most stringent constraints on quantum-gravity-induced Lorentz violation.
    12/2007;
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    ABSTRACT: The predicted thermal flash from SN shock breakout might have been detected for the first time by Swift in GRB 060218/SN 2006aj. The detected thermal X-ray emission in this event implies emergence of a trans-relativistic (TR) SN shock with kinetic energy of E_k>1E49 erg. During TRSN shock breakout, the thermal photons could be "accelerated" by the shock through repeated bulk Compton scattering, forming a nonthermal gamma/X-ray component with dominant energy over thermal one. This mechanism of "photon acceleration" at TRSN shock breakout might also account for gamma-rays in the other similar low-luminosity GRBs, implying that they are atypical GRBs with only TR outflows. TRSNe form a peculiar type of SNe with large kinetic energy, >1E49 erg, in TR ejecta, \Gamma\beta ~2.
    06/2007;
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    ABSTRACT: The recent detection of X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the forward shock. There must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons when they are passing through the forward shock. We find that this IC emission produces high energy gamma-ray flares, which may be detected by AGILE, GLAST and ground-based TeV telescopes. The anisotropic IC scattering between flare photons and forward shock electrons does not affect the total IC component intensity, but cause a time delay of the IC component peak relative to the flare peak. The anisotropic scattering effect may also weaken, to some extent, the suppression effect of the afterglow intensity induced by the enhanced electron cooling due to flare photons. We speculate that this IC component may already have been detected by EGRET from a very strong burst--GRB940217. Future observations by GLAST may help to distinguish whether X-ray flares originate from late central engine activity or from external shocks.
    03/2007;
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    ABSTRACT: The recent detection of delayed X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the deceleration radius characterizing the beginning of the forward shock afterglow emission. Given the observed temporal overlapping between the flares and afterglows, there must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons. We find that this IC emission produces GeV-TeV flares, which may be detected by GLAST and ground-based TeV telescopes. We speculate that this kind of emission may already have been detected by EGRET from a very strong burst--GRB940217. The enhanced cooling of the forward shock electrons by the X-ray flare photons may suppress the synchrotron emission of the afterglows during the flare period. The detection of GeV-TeV flares combined with low energy observations may help to constrain the poorly known magnetic field in afterglow shocks. We also consider the self-IC emission in the context of internal-shock and external-shock models for X-ray flares. The emission above GeV from internal shocks is low, while the external shock model can also produce GeV-TeV flares, but with a different temporal behavior from that caused by IC scattering of flare photons by afterglow electrons. This suggests a useful approach for distinguishing whether X-ray flares originate from late central engine activity or from external shocks. Comment: slightly shortened version, accepted for publication in ApJ Letters, 4 emulateapj pages, no figures
    The Astrophysical Journal 01/2006; · 6.73 Impact Factor
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    Zhuo Li, Eli Waxman
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    ABSTRACT: Gamma-ray burst (GRB) afterglow emission is believed to be produced by synchrotron emission of electrons accelerated to high energy by a relativistic collisionless shock propagating into a weakly magnetized plasma. Afterglow observations have been used to constrain the postshock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day timescales constrain the preshock magnetic field to satisfy B>0.2(n/1 cm-3)5/8 mG, where n is the preshock density. This suggests that either the shock propagates into a highly magnetized fast, v~103 km s-1, wind, or the preshock magnetic field is strongly amplified, most likely by the streaming of high-energy shock-accelerated particles. More stringent constraints may be obtained by afterglow observations at high photon energy at late, >1 day, times.
    The Astrophysical Journal 01/2006; 651(1):328-332. · 6.73 Impact Factor
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    ABSTRACT: We compute the temporal profiles of the gamma-ray burst pulse in the four Burst and Transient Source Experiment (BATSE) Large Area Detector (LAD) discriminator energy channels, with the relativistic curvature effect of an expanding fireball being explicitly investigated. Assuming an intrinsic ‘Band’ shape spectrum and an intrinsic energy-independent emission profile, we show that merely the curvature effect can produce detectable spectral lags if the intrinsic pulse profile has a gradually decaying phase. We examine the spectral lag's dependences on some physical parameters, such as the Lorentz factor Γ, the low-energy spectral index, α, of the intrinsic spectrum, the duration of the intrinsic radiation t′d and the fireball radius R. It is shown that approximately the lag ∝Γ−1 and ∝t′d, and a spectrum with a more extruded shape (a larger α) causes a larger lag. We find no dependence of the lag on R. Quantitatively, the lags produced from the curvature effect are marginally close to the observed ones, while larger lags require extreme physical parameter values, e.g. Γ < 50, or α > −0.5. The curvature effect causes an energy-dependent pulse width distribution but the energy dependence of the pulse width we obtained is much weaker than the observed W∝E−0.4 one. This indicates that some intrinsic mechanism(s), other than the curvature effect, dominates the pulse narrowing of gamma-ray bursts.
    Monthly Notices of the Royal Astronomical Society 08/2005; 362(1):59 - 65. · 5.52 Impact Factor

Publication Stats

268 Citations
104.38 Total Impact Points

Institutions

  • 2009–2014
    • Peking University
      • • Department of Astronomy
      • • Kavli Institute for Astronomy and Astrophysics
      Peping, Beijing, China
  • 2011
    • University of Nevada, Las Vegas
      • Department of Physics and Astronomy
      Las Vegas, Nevada, United States
  • 2006–2008
    • Weizmann Institute of Science
      • Faculty of Physics
      Tel Aviv, Tel Aviv, Israel
  • 2003–2008
    • Nanjing University
      • Department of Astronomy
      Nanjing, Jiangsu Sheng, China
  • 2005
    • Northeast Institute of Geography and Agroecology
      • Institute of High Energy Physics
      Beijing, Beijing Shi, China
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China