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Multiband fitting to three long GRBs with Fermi/LAT data:structured ejecta sweeping up a density-jump medium

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Research in Astronomy and Astrophysics 2011年第9期11卷 1046-1066页

作者： Si-Yi Feng zi-gao dai Department of Astronomy Nanjing University Nanjing 210093 China Key Laboratory of Modern Astronomy and Astrophysics （Nanjing University） Ministry of Education Nanjing 210093 China

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 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 under- stood 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 a majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with con- tinuous energy injection towards the forward shock can well explain the early rising feature of the GeV mission from these bursts, and the density-jump medium can ac- count for some particular 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 not have a significant contribution to the early LAT rising part, which will loosen the strong constraint of lower limits of the 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 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.

关键词： gamma-rays: bursts -- gamma-rays: theory

Estimate of an environmental magnetic field of fast radio bursts

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Research in Astronomy and Astrophysics 2016年第3期16卷 11-16页

作者： Wei-Li Lin zi-gao dai School of Astronomy and Space Science Nanjing University Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) Ministry of Education

Fast radio bursts （FRBs） are a type of newly-discovered transient astronomical phenomenon. They have short durations, high dispersion measures and a high event rate. However, due to unknown dis- tances and undetected electromagnetic counterparts at other wavebands, it is difficult to further investigate FRBs. Here we propose a method to study their environmental magnetic field using an indirect method. Starting withdispersion measures and rotation measures （RMs）, we try to obtain the parallel magnetic field component ^-B ││ which is the average value along the line of sight in the host galaxy. Because both RMs and redshifls are now unavailable, we demonstrate the dependence of ^-B ││ on these two separate quantities. This result, if the RM and redshift of an FRB are measured, would be expected to provide a clue towards understanding an environmental magnetic field of an FRB.

关键词： radio continuum: general -- pulsars: general -- galaxies: magnetic fields -- Galaxy: structure

Time delay and extended halo for constraints on the intergalactic magnetic field

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Research in Astronomy and Astrophysics 2015年第12期15卷 2173-2186页

作者： Yuan-Pei Yang zi-gao dai School of Astronomy and Space Science Nanjing University Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) Ministry of Education

Primary gamma rays emitted from extragalactic very-high-energy （VHE） sources, such as blazars, will generate cascade radiation in intergalactic space with a scale of -- 100 Mpc, for z - 0.1 and Eγ -1 TeV. These cascades proceed through electron-positron pair production and inverse Compton （IC） scattering in the cosmic background radiation fields, mainly cosmic microwave background （CMB） radiation and extragalactic background light in the voids of the universe. The existence of an intergalactic magnetic field （IGMF） would deflect paths of electron-positron pairs that scatter CMB photons, causing some observable effects, such as time delay, an ex- tended halo, and a spectral change. Here we reanalyze the diffusion of an electron jet deflected by IGMF and propose a unified semi-analytical model. By using publicly available data from the Fermi/LAT detector and contemporaneous TeV observations, we find that the cascade photon flux is not significantly affected by the IGMF strength for non-variable blazars when the IGMF is weaker than ,-~ 10-16 G. This result is clearly different from previous works that analyzed the extended halo and time de- lay separately for non-variable blazars and flaring blazars. By applying our model to two extreme blazars （1ES 0229＋200 and 1ES 1218＋304）, we obtain the IGMF lower limit of order ≥10-13 --10-14 G in the non-variable case, which is a stronger constraint on the IGMF strength than previous works （≥10-16 ,-- 10-18 G）, and ≥10-18 -- 10-19 G in the case of flaring blazars. Furthermore, we study the light curves and extended halo of the cascade photons by considering the effects of the IGME

关键词： gamma rays: general -- galaxies: magnetic field -- galaxies: individual(1ES 0229+200 1ES 1218+304)

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

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Chinese Journal of Astronomy and Astrophysics 2006年第5期6卷 561-571页

作者： Fa-Yin Wang zi-gao dai Department of Astronomy Nanjing University Nanjing 210093

The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae （SNe Ia） and anisotropy of cosmic microwave background （CMB）. The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ＂Gold＂ sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a fiat universe with the cosmological constant, we measure ΩM=0.28-0.05^＋0.04 which is consistent with Riess et al. The transition redshift is zT=0.60-0.08^＋0.04. We also discuss several dark energy models that define w（z） of the parameterized equation of state of dark energy including one parameter and two parameters （w（z） being the ratio of the pressure to energy density）. Our calculations show that the accurately calculated transition redshift varies from zT =0.29-0.06^＋0.07 to zT =0.60-0.08^＋0.06 across these models. We also calculate the minimum redshift zc at which the current observations need the universe to accelerate.

关键词： cosmology observations -- distance scale -- supernovae general

Testing Lorentz violation using propagating UHECRs

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Chinese Journal of Astronomy and Astrophysics 2009年第4期9卷 409-421页

作者： Cong-Xin Qiu zi-gao dai Department of Astronomy Nanjing University Nanjing 210093 China

Lorentz invariant violation （LIV） test is important for studying modem physics. All the known astrophysical constraints either have a very small examinable parameter space or are only suitable for some special theoretical models. Here, we suggest that it is possible to directly detect the time-delay of ultra-high-energy cosmic-rays （UHECRs）. We discuss some difficulties in our method, including the intergalactic magnetic fields. It seems that none of them are crucial, hence this method could give a larger examinable parameter space and a stronger constraint on LIV.

关键词： cosmic rays-gamma rays bursts-ISM magnetic fields-relativity

Tail Emission from a Ring-like Jet: Its Application to Shallow Decays of Early Afterglows and GRB 050709

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Chinese Journal of Astronomy and Astrophysics 2006年第5期6卷 551-554页

作者： Yuan-Chuan Zou zi-gao dai Department of Astronomy Nanjing University Nanjing 210093

Similar to the case of pulsars the magnetic axis and the spin axis of gamma-ray burst sources may not lie on the same line. This may cause the formation of a ring-like jet due to collimation of the precessing magnetic axis. We analyze the tail emission from such a jet, and find that it has a shallow decay phase with a temporal index of - 1/2 if the Lorentz factor of the ejecta is not very high, which is consistent with the shallow decay phase of some early X-ray afterglow detected by Swift. The ring-like jet has a tail cusp with sharp rising and very sharp decay. This effect can provide an explanation for the re-brightening and sharp decay of the X-ray afterglow＇of GRB 050709.

关键词： gamma rays bursts - X-rays general

Constraints on generalized Chaplygin gas model including gamma-ray bursts

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Chinese Journal of Astronomy and Astrophysics 2009年第5期9卷 547-557页

作者： Fa-Yin Wang zi-gao dai Shi Qi Department of Astronomy Nanjing University Nanjing 210093 China Purple Mountain Observatory Chinese Academy of Sciences Nanjing 210008 China Joint Center for Particle Nuclear Physics and Cosmology Nanjing University- Purple Mountain Observatory Nanjing 210093 China

Generalized Chaplygin gas （whose equation of state is PGCG = -A/ρGCG^α） was proposed as a candidate for unification of dark energy and dark matter. We investigate constraints on this model with the latest observed data. We test the model with type-Ia supernovae （SNe Ia）, cosmic microwave background （CMB） anisotropy, X-ray gas mass fractions in clusters, and gamma-ray bursts （GRBs）. We calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. We show that GRBs can extend the Hubble diagram to higher redshifts （z 〉 6）. The GRB Hubble diagram is well behaved and delineates the shape of the Hubble diagram well. We measure As≡A/ρGCG,0^α＋1 =0.68-0.08^＋0.04（where PGCG,0 is the energy density today） and α=-0.22 -0.13^＋0.15 at the 1σ confidence level using all the datasets. Our results rule out the standard Chaplygin gas model （α = 1） at the 3σ confidence level. The ACDM is allowed at the 2σ confidence level. We find that acceleration could have started at a redshift of z - 0.70. The concordance of the generalized Chaplygin gas model with the age estimate of an old high redshift quasar is found. In addition, we show that GRBs can break the degeneracy between the generalized Chaplygin gas model and the XCDM model.

关键词： gamma-rays bursts -- cosmology theory

Polarization of GRB prompt emission and its application to POLAR’s data

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Research in Astronomy and Astrophysics 2021年第3期21卷 43-56页

作者： Mi-Xiang Lan Xue-Feng Wu zi-gao dai Center for Theoretical Physics and College of Physics Jilin UniversityChangchun 130012China Purple Mountain Observatory Chinese Academy of SciencesNanjing 210008China School of Astronomy and Space Science Nanjing UniversityNanjing 210093China Key Laboratory of Modern Astronomy and Astrophysics(Nanjing University) Ministry of EducationNanjing 210093China

Synchrotron emission polarization is very sensitive to the magnetic field ***,polarization of synchrotron emission with a mixed(SM)magnetic field in the gamma-ray burst(GRB)afterglow phase has been ***,we apply these SM models to the GRB prompt phase and compare their polarization properties with that of synchrotron emission in purely ordered(SO)magnetic *** find that the polarization properties in an SM model are very similar to these in a corresponding SO model(e.g.,synchrotron emission in a mixed magnetic field with an aligned ordered part(SMA)and synchrotron emission with a purely ordered aligned magnetic field(SOA)),only with a lower polarization degree(PD).We also discuss the statistical properties of the *** find PDs of the simulated bursts are concentrated around 25%for both SOA and synchrotron emission in a purely ordered toroidal magnetic field(SOT),while they can range from 0%to 25%for SMA and synchrotron emission in a mixed magnetic field with a toroidal ordered part(SMT),depending onξ_B value,i.e.,the ratio of magnetic reduction of the ordered magnetic field over that of random magnetic *** statistics,if PDs of majority GRBs are non-zero,then it favors SO and SM ***,if there are some bright GRBs with prominently lower PDs than that of the majority GRBs,it favors SOT(SMT)models;if all the bright GRBs have comparable PDs with the majority ones,it favors SOA(SMA)***,we apply our results to POLAR’s data and find that~10%time-integrated PDs of the observed bursts favor SMA and SMT models,and theξ_B parameter of these bursts is constrained to be around 1.135.

关键词： gamma-ray burst:general magnetic fields polarization radiation mechanisms:nonthermal

The Energy Sources of Superluminous Supernovae

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Research in Astronomy and Astrophysics 2019年第5期19卷 1-14页

作者： Shan-Qin Wang Ling-Jun Wang zi-gao dai School of Astronomy and Space Science Nanjing University Guangxi Key Laboratory for Relativistic Astrophysics School of Physical Science and Technology Guangxi University Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) Ministry of Education Astroparticle Physics Institute of High Energy Physics Chinese Academy of Sciences

Supernovae(SNe) are the most brilliant optical stellar-class explosions. Over the past two decades, several optical transient survey projects discovered more than-100 so-called superluminous supernovae(SLSNe) whose peak luminosities and radiated energy are ■ 7 × 10^(43) erg s^(-1) and > 10^(51) erg respectively, at least an order of magnitude larger than those of normal SNe. According to their optical spectra features, SLSNe have been split into two broad categories of type Ⅰ that are hydrogen-deficient and type Ⅱ that are hydrogen-rich. Investigating and determining the energy sources of SLSNe would be of outstanding importance for understanding their stellar evolution and explosion mechanisms. The energy sources of SLSNe can be determined by analyzing their light curves(LCs) and spectra. The most prevailing models accounting for the SLSN LCs are the ^(56)Ni cascade decay model, the magnetar spin-down model,the ejecta-circumstellar medium interaction model and the jet-ejecta interaction model. In this review, we present several energy-source models and their different combinations.

关键词： stars magnetars-supernovae general