Zhuo Li

Peking University, Peping, Beijing, China

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Publications (31)129.72 Total impact

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    Bin Wang, Zhuo Li
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    ABSTRACT: IceCube has detected the diffuse TeV-PeV neutrino emission, for which the flat spectrum radio quasars (FSRQs) have been proposed to be the strong candidate sources. Here we assume that there is a correlation between the gamma-ray and neutrino fluxes from FSRQs, and use the \textit{Fermi}-LAT detected gamma-ray flux from FSRQs to constrain their neutrino flux, and then test if they can account for the diffuse neutrino emission. We first obtain the gamma-ray/neutrino flux ratio by the diffuse gamma-ray flux from \textit{Fermi}-LAT observations of FSRQs and the diffuse neutrino flux from IceCube detection, then apply this ratio to individual FSRQs, and predict their neutrino flux, to be compared with the upper limits that IceCube provides for individual FSRQs, especially those in the northern sky with more stringent constraint by IceCube. We find that a large fraction of candidate FSRQs from the northern sky in the IceCube point source search has predicted neutrino flux violating the IceCube limit, and in the stacking search the predicted neutrino flux of the FSRQs in the northern sky is close to the current IceCube limit. The sensitivity of IceCube after several years of running time may reach the level that can clearly examine the FSRQ origin of the diffuse neutrino emission. The caveat in the assumptions are also discussed.
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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.
    Journal of Cosmology and Astroparticle Physics 07/2014; 2014(11). DOI:10.1088/1475-7516/2014/11/028
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    Wei Wang, Zhuo Li
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    ABSTRACT: A deep hard X-ray survey of the INTEGRAL satellite first detected the non-thermal emission up to 90 keV in the Tycho supernova (SN) remnant. Its 3 -- 100 keV spectrum is fitted with a thermal bremsstrahlung of $kT\sim 0.81\pm 0.45$ keV plus a power-law model of $\Gamma \sim 3.01\pm 0.16$. Based on the diffusive shock acceleration theory, this non-thermal emission, together with radio measurements, implies that Tycho remnant may not accelerate protons up to $>$PeV but hundreds TeV. Only heavier nuclei may be accelerated to the cosmic ray spectral "knee". In addition, we search for soft gamma-ray lines at 67.9 and 78.4 keV coming from the decay of radioactive $^{44}$Ti in Tycho remnant by INTEGRAL. A bump feature in the 60-90 keV energy band, potentially associated with the $^{44}$Ti line emission, is found with a marginal significance level of $\sim$ 2.6 $\sigma$. The corresponding 3 $\sigma$ upper limit on the $^{44}$Ti line flux amounts to 1.5 $\times$ 10$^{-5}$ ph cm$^{-2}$ s$^{-1}$. Implications on the progenitor of Tycho SN, considered to be the prototype of type Ia SN, are discussed.
    The Astrophysical Journal 05/2014; 789(2). DOI:10.1088/0004-637X/789/2/123
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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). DOI:10.1088/0004-637X/788/2/189
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    ABSTRACT: In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However the magnetic field may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low energy slope $\nu F_\nu\propto\nu$, it is required that the postshock MF decay time is comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of $\sim10^5$ skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as $B\propto t^{-1.5}$. An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low energy slopes not harder than $\nu^{4/3}$.
    The Astrophysical Journal 10/2013; 780(1). DOI:10.1088/0004-637X/780/1/12
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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). DOI:10.1093/mnras/stt1494
  • 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. DOI:10.1088/1674-4527/12/11/006
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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; DOI:10.1142/S2010194513011343
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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). DOI:10.1088/0004-637X/753/2/178
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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. DOI:10.1088/0004-637X/726/2/89
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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; 2011(05). DOI:10.1088/1475-7516/2011/05/008
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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; DOI:10.1088/0004-637X/712/2/1232
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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). DOI:10.1088/0004-637X/698/2/L98
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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. DOI:10.1086/519228
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    Zhuo Li, L. M. Song
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    ABSTRACT: The central engine of gamma-ray bursts (GRBs) is believed to eject double-sided ultrarelativistic jets. For an observed GRB, one of the twin jets points toward us and is responsible for the prompt gamma-ray and subsequent afterglow emission. We consider in this Letter the other receding jet, which is expected to give rise to late-time radio rebrightening (RRB) when it becomes nonrelativistic (NR) and radiatively isotropic. The RRB peaks at a time 5tNR = 2(Ej, 51/n)1/3 yr after the GRB (where tNR is the observed NR timescale for the preceding jet, Ej is the jet energy, and n is the ambient medium density). The peak flux is comparable to the preceding jet emission at tNR. We expect the RRB of GRB 030329 1.7 yr after the burst, with a flux of ~0.6 mJy at 15 GHz. The cases of GRBs 970508 and 980703 have also been discussed. The detection of RRB, which requires a dense monitoring campaign as much as a few years after a GRB, will be the direct evidence for the existence of double-sided jets in GRBs and prove the black hole-disk system formation in the cores of progenitors.
    The Astrophysical Journal 12/2008; 614(1):L17. DOI:10.1086/425498
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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. DOI:10.1086/379231
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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. DOI:10.1086/529042
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    Zhuo Li, L. M. Song
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    ABSTRACT: We discuss the prompt emission of gamma-ray bursts (GRBs), allowing for γγ pair production and synchrotron self-absorption. The observed hard spectra suggest heavy pair loading in GRBs. The reemission of the generated pairs results in the energy transmission from high-energy gamma rays to long-wavelength radiation. Because of strong self-absorption, the synchrotron radiation by pairs is in an optically thick regime, showing a thermal-like spectral bump in the extreme-ultraviolet/soft X-ray band, other than the peak from the main burst. Recently, the prompt soft X-ray emission of GRB 031203 was detected thanks to the discovery of a delayed dust echo, and it seems to be consistent with the model prediction of a double-peak structure. The confirmation of the thermal-like feature and the double-peak structure by observation would indicate that the dominant radiation mechanism in GRBs is synchrotron rather than inverse Compton radiation.
    The Astrophysical Journal 12/2008; 608(1):L17. DOI:10.1086/422098
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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; DOI:10.1063/1.3027929
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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.

Publication Stats

379 Citations
129.72 Total Impact Points

Institutions

  • 2009–2014
    • Peking University
      • • Department of Astronomy
      • • Kavli Institute for Astronomy and Astrophysics
      Peping, Beijing, China
  • 2006–2008
    • Weizmann Institute of Science
      • Faculty of Physics
      Tel Aviv, Tel Aviv, Israel
  • 2003–2008
    • Chinese Academy of Sciences
      • • Institute of High Energy Physics
      • • Key Laboratory of Particle Astrophysics
      Peping, Beijing, China
  • 2001–2008
    • Nanjing University
      • Department of Astronomy
      Nanjing, Jiangsu Sheng, China
  • 2005
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China