Ring-Shaped Jets in Gamma-Ray Bursts

When the axis of a gamma-ray burst （GRB） does not coincide with the spin axis of its source, there may result a ring-shaped jet. Using some refined jet dynamics, we calculate multi-wavelength afterglow light curves for such ring-shaped jets. In the R-band we find an obvious break in the afterglow light curve due to the beaming effect and the break is affected by many parameters, such as the electron energy fraction ζe, the magnetic energy fraction ζB^2, the width of ring △θ and the medium number density n. The overall light curve can be divided into three power-law stages, i.e., an ultra-relativistic stage, an after-break stage and a deep Newtonian stage. For each stage the power-law index is larger in the ring-shaped jet than in the corresponding conical jet.

On the intrinsic shape of the gamma-ray spectrum for Fermi blazars

The curvature of the γ-ray spectrum in blazars may reflect the intrinsic distribution of emitting electrons, which will further give some information on the possible acceleration and cooling processes in the emitting region. The γ-ray spectra of Fermi blazars are normally fitted either by a single power-law（PL） or a log-normal（call Logarithmic Parabola, LP） form. The possible reason for this difference is not clear. We statistically explore this issue based on the different observational properties of 1419 Fermi blazars in the 3 LAC Clean Sample. We find that the γ-ray flux（100 Me V–100 Ge V） and variability index follow bimodal distributions for PL and LP blazars, where the γ-ray flux and variability index show a positive correlation. However, the distributions of γ-ray luminosity and redshift follow a unimodal distribution. Our results suggest that the bimodal distribution of γ-ray fluxes for LP and PL blazars may not be intrinsic and all blazars may have an intrinsically curved γ-ray spectrum, and the PL spectrum is just caused by the fitting effect due to less photons.

An algorithm to resolve γ-rays from charged cosmic rays with DAMPE

The DArk Matter Particle Explorer（DAMPE）,also known as Wukong in China,which was launched on 2015 December 17,is a new high energy cosmic ray and γ-ray satellite-borne observatory.One of the main scientific goals of DAMPE is to observe Ge V-Te V high energy γ-rays with accurate energy,angular and time resolution,to indirectly search for dark matter particles and for the study of high energy astrophysics. Due to the comparatively higher fluxes of charged cosmic rays with respect to γ-rays,it is challenging to identify γ-rays with sufficiently high efficiency,minimizing the amount of charged cosmic ray contamination. In this work we present a method to identify γ-rays in DAMPE data based on Monte Carlo simulations,using the powerful electromagnetic/hadronic shower discrimination provided by the calorimeter and the veto detection of charged particles provided by the plastic scintillation detector. Monte Carlo simulations show that after this selection the number of electrons and protons that contaminate the selected γ-ray events at～10 Ge V amounts to less than 1% of the selected sample.Finally,we use flight data to verify the effectiveness of the method by highlighting known γ-ray sources in the sky and by reconstructing preliminary light curves of the Geminga pulsar.

A statistical classification of the unassociated gamma-ray sources in the second Fermi Large Area Telescope Catalog

With the assistance of the identified/associated sources in the second Fermi Large Area Telescope （LAT） catalog, we analyze and resolve the spatial distribution and the distributions of the gamma-ray spectral and variability indices of the remaining 575 unassociated Fermi LAT sources. Consequently, it is suggested that the unassociated sources could statistically consist of Galactic supernova remnants/pulsar wind nebulae, BL Lacertae objects, fiat spectrum radio quasars and other types of active galaxies with fractions of 25%, 29%, 41% and 5%, respectively.

Effect of Conversion Efficiency on Gamma-Ray Burst Energy

Beaming effect makes it possible that gamma-ray bursts have a standardenergy, but the gamma-ray energy release is sensitive to some parameters. Our attention is focusedon the effect of the gamma ray conversion efficiency (η_γ), which may range between 0.01 and 0.9,and which probably has a random value for different GRBs under certain conditions. Making use of theafterglow data from the literature, we carried out a complete correction to the conical openingangle formula. Within the framework of the conical jet model, we ran a simple Monte Carlo simulationfor random values of η_γ, and found that the gamma-ray energy release is narrowly clustered,whether we use a constant value of η_γ or random values for different gamma-ray bursts.

On the Evolution of the Apparent Size of Gamma-Ray Burst Remnants

The remnants of two gamma-ray bursts, GRB 030329 and GRB 041227, have been resolved by Very Long Baseline Interferometry observations. The radio counterparts were observed to expand with time. These observations provide an important way to test the dynamics of the standard fireball model. We show that the observed size evolution of these two events cannot be explained by a simple jet model, rather, it can be satisfactorily explained by the two-component jet model. It strongly hints that gamma-ray burst ejecta may have complicated structures.

Gamma-ray Bursts: a Probe of Black Holes

There is strong evidence for the existence of black holes (BHs) in some X-ray binaries and in most galactic nuclei based on different types of measurement, but black holes have not been definitely identified for the lack of very firm observational evidence up to now. Because direct evidence for BHs should come from determination of strong gravitational redshift, we expect an object can fall into the region near the BH horizon where radiation can be detected. Therefore the object must be a compact star such as a neutron star (NS), and intense astrophysical processes will release highly energetic radiation that is transient and fast-varying. These characteristics may point to the observed gamma-ray bursts (GRBs). Recent observations of iron lines suggest that afterglows of GRBs show properties similar to those observed in active galactic nuclei (AGNs), implying that the GRBs may originate from intense events related to black holes. A model for GRBs and after-glows is proposed here to obtain the range of gravitational redshifts (zg) of GRBs with known cosmological redshifts. Here, we provide a new method that, with a search for high-energy emission lines (X- or -γ-rays) in GRBs, one can determine the gravitational redshift. We expect zg to be 0.5 or even larger, so we can rule out the possibility of other compact objects such as NSs, and identify the central progenitors of GRBs as black holes.

Time-resolved spectral analysis of prompt emission from long gamma-ray bursts with GeV emission

We performed detailed time-resolved spectroscopy of bright tong gamma- ray bursts （GRBs） which show significant GeV emissions （GRB 080916C, GRB 090902B and GRB 090926A）. In addition to the standard Band model, we also use a model consisting of a black body and a power law to fit the spectra. We find that for the latter model there are indications of an additional soft component in the spectra. While previous studies have shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two blackbodies and a power law adequately fits the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several interesting features that appear in all three GRBs, like （a） temperatures of the blackbodies are strongly correlated with each other, （b） fluxes in the black body components are strongly correlated with each other, （c） the temperatures of the black body trace the profile of the individual pulses of the GRBs, and （d） the characteristics of power law components like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions. We discuss the implications of these results and the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.

Gamma-ray Emission from the γ-ray-loud BL Lac Objects

Using the HST observation data of BL Lac objects by Urry et al. and γ-ray observation data, we find that there is a correlation between Fγ and Fonuclei forγ-ray-loud BL Lac objects (correlation coefficients: 7γ=0.63,p = 4.0 × 10-2), but no correlation between Fγ and Fohost, where Fonuclei and Fohost are the fluxes of nuclei and host galaxy in V-band. For 19γ-ray-loud BL Lac objects with observed spectral index in multi-wavebands, the spectral index correlations between any two bands are as follow: (1) there is a strong correlation between aγ and aK for 15 BL Lac objects(γ = 0.84,p = 3.11 × 10-4); (2) the correlation between aγ and ao for 12 BL Lac objects isγ = 0.82, p = 1.5 × 10-3; (3) there is no correlation between a7 and ax for 16 BL Lac objects. The results, together with characteristic double-humped shape of their SEDs, show that the synchrotron self-Compton mechanism might be a main mechanism for theγ-ray emission of the BL Lac objects. The electrons emitting JR and optical radiation via synchrotron are also responsible for upscattering these photons toγ-rays, and a variability in IR-optical regime should be accompanied by a change in theγ-rays.

X-ray Emission Lines in GRB Afterglows: Evidence for a Two-component Jet Model

X-ray emission lines have been observed in X-ray afterglows of several T-ray bursts （GRBs）. It is a major breakthrough for understanding the nature of the progenitors. It has been proposed that the X-ray emission lines can be well explained by the Geometry-Dominated models, but in these models the illuminat- ing angle is much larger than that of the collimated jet of the GRB. For GRB 011211, we have obtained an illuminating angle of about 0 - 45°, while the angle of the GRB jet is only 3.6°. So we propose that the outflow of GRBs with emission lines should have two distinct components： a wide component that illuminates the reprocessing material and produces the emission lines and a narrow one that produces the GRB. Observations show the energy for producing the emission lines is higher than that of the GRB. In this case, when the wide component dominates the afterglows, a bump should appear in the GRB afterglow. For GRB 011211, the bump should occur within 0.05 days of the GRB, which is obviously too early for the observation to catch it. Alongside the X-ray emission lines there should also be a bright emission component between the UV and the soft X-rays. These features can be tested by the Swift satellite in the near future.

Magnetic reconnection acceleration of astrophysical jets for different jet geometries

The acceleration mechanisms of relativistic jets are of great importance for understanding various astrophysical phenomena such as gamma-ray bursts, active galactic nuclei and microquasars. One of the most popular scenarios is that the jets are initially Poynting-flux dominated and succumb to magnetohydrodynamic instability leading to magnetic reconnections. We suggest that the reconnection timescale and efficiency could strongly depend on the geometry of the jet, which determines the length scale on which the orientations of the field lines change. In contrast to a usually- assumed conical jet, the acceleration of a collimated jet can be found to be more rapid and efficient （i.e. a much more highly saturated Lorentz factor can be reached） while the jets with lateral expansion show the opposite behavior. The shape of the jet could be formed due to the lateral squeezing on the jet by the stellar envelope of a collapsing massive star or the interaction of the jet with stellar winds.

Highly variable γ-ray emission of CTD 135 and implications for its compact symmetric structure

Theγ-ray emission properties of CTD 135,a typical compact symmetric object(CSO),are investigated with-11-year Fermi/LAT observations.We show that it has bright and significantly variable GeV emission,with theγ-ray luminosity of L_(γ)-10^(47) erg s^(-1) and a variation index of TS_(var)=1002.A quasi-periodic oscillation(QPO)with a periodicity of-460 days is detected in the global 95%false-alarm level.Theseγ-ray emission features are similar to that of blazars.Its broadband spectral energy distribution(SED)can be attributed to the radiations of the relativistic electrons accelerated in the core region and the extended region.The SED modeling shows that theγ-rays are from the core region,which has a Doppler boosting factor ofδ-10.8 and relativistically moves with a small viewing angle,being similar to blazar jets.On the base of the analysis results,we propose that the episodic activity of the central engine in CTD135 results in a blazar-like jet and the bubble-like lobes as the Fermi bubbles in the Galaxy.The strong 7-ray emission with obvious variability is from the jet radiations and the symmetric radio structure is attributed to the bubbles.The jet radiation power and disk luminosity in units of Eddington luminosity of CTD 135 follow the same relation as other young radio sources,indicating that its jet radiation may also be driven by the Eddington ratio.

Possible modulated γ-ray emission from the transitional millisecond pulsar binary PSR J1023+0038

We report the results from our analysis of Fermi Large Area Telescope （LAT） data for the transitional millisecond pulsar binary PSR J1023＋0038. The time period of the data is nearly 9 yr, and that after the source＇s transition, in June 2013 from the disk-free state to the active state of having an accretion disk, is approximately 4 yr. We identify a high-energy 〉5.5 GeV component in the source＇s spectrum in the active state, and find this component is only significantly detected in half of the orbital phase centered at the descending node （when the pulsar is moving towards the Earth）. Considering the pulsar scenario proposed for multi-frequency emission from the source, in which the pulsar is still active and a cold-relativistic pulsar wind inverse-Compton scatters the photons from the accretion disk, we discuss the origin of the high-energy component. In order to explain the observed spectrum, a power-law distribution of particles, with an index of ~3, in the pulsar wind is required, while the orbital variations are possibly due to changes in power-law index as a function of orbital phase.

The connection between radio and γ-ray emission in Fermi/LAT blazars

We collect the second Large Area Telescope AGN catalog （2LAC） and Monitor of Jets in AGN with VLBA Equipment （MOJAVE） quasi-simultaneous data to investigate the radio-γ connection of blazars. The cross sample contains 166 sources. The statistical analysis based on this sample confirms positive correlations between these two bands, but the correlations become weaker as the γ-ray energy increases. The statistical results between various parameters show negative correla- tions of γ-ray photon spectral index with γ-ray loudness for both Flat Spectrum Radio Quasars （FSRQs） and BL Lacertae objects, positive correlations of γ-ray variability index with the γ-ray loudness for FSRQs, a negative correlation of the γ-ray variabil- ity index with the γ-ray photon spectral index for FSRQs, and negative correlations of γ-ray photon spectral index with γ-ray luminosity for FSRQs. These results suggest that the γ-ray variability may be due to changes inside the γ-ray emission region like the injected power, rather than changes in the photon density of the external radiation fields, and the variability amplitude tends to be larger as the γ-rays are closer to the high energy peak of the spectral energy distribution （SED）. No correlation of variabil- ity index found for BL Lacertae objects implies that variability behavior may differ below and above the peak energy.

Study of temporal and spectral characteristics of the X-ray emission from solar flares

Temporal and spectral characteristics of X-ray emission from 60 flares of intensity ≥C class observed by the Solar X-ray Spectrometer （SOXS） during 2003-2011 are presented. We analyze the X-ray emission observed in four and three energy bands by the Si and Cadmium-Zinc-Telluride （CZT） detectors, respectively. The number of peaks in the intensity profile of the flares varies between 1 and 3. We find moderate correlation （R ~=0.2） between the rise time and the peak flux of the first peak of the flare irrespective of energy band, which is indicative of its energy-independent nature. Moreover, the magnetic field complexity of the flaring region is found to be highly anti-correlated （R = 0.61） with the rise time of the flares while positively correlated （R = 0.28） with the peak flux of the flare. The time delay between the peak of the X-ray emission in a given energy band and that in 25-30keV decreases with increasing energy, suggesting conduction cooling is dominant in the lower energies. Analysis of 340 spectra from 14 flares reveals that the peak of differential emission measure （DEM） evolution is delayed by 60-360 s relative to that of the temperature, and this time delay is inversely proportional to the peak flux of the flare. We conclude that temporal and intensity characteristics of flares are dependent on energy as well as the magnetic field configuration of the active region.

Searching forγ-ray counterparts to very faint X-ray transient neutron star binaries

Very faint X-ray transients （VFXTs） are a group of X-ray binaries with low luminosities, displaying peak X-ray luminosities during their outbursts of only 1034-1036 erg s^-1. Using γ-ray data obtained with the Large Area Telescope （LAT） onboard the Fermi Gamma-Ray Space Telescope （Fermi）, we investigate their possible nature of containing rotation-powered pulsars, or more specifically being transitional millisecond pulsars （MSPs）. Among more than 40 known VFXTs, we select 12 neutron star systems. We analyze the LAT data for the fields of 12 VFXTs in the energy range 0.2-300GeV, but do not find any counterparts likely detected by Fermi. We obtain luminosity upper limits for the 12 sources. While the distances to the sources are largely uncertain, the upper limits are comparable to the luminosities of two transitional systems, PSR J1023-0038 and XSS J12270-4859. From our study, we conclude that no evidence is found at γ-rays for the suggestion that some VFXTs could contain rotation-powered MSPs （or be transitional MSP systems）.

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

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.

A two-component jet model based on the Blandford-Znajek and Blandford-Payne processes

We propose a two-component jet model consistent with the observations of several gamma ray bursts （GRBs） and active galactic nuclei （AGNs）. The jet consists of inner and outer components, which are supposed to be driven by the Blandford- Znajek （BZ） and Blandford-Payne （BP） processes, respectively. The baryons in the BP jet are accelerated centrifugally via the magnetic field anchored in the accretion disk. The BZ jet is assumed to be entrained in a fraction of accreting matter leaving the inner edge of the accretion disk, and the baryons are accelerated in the conversion from electromagnetic energy to kinetic energy. By fitting the Lorentz factors of some GRBs （GRB 030329, GRB 051221A and GRB 080413B） and AGNs （Cen A, Mkn 501 and Mkn 421） with this model, we constrain the physical parameters related to the accretion and outflow of these two kinds of objects. We conclude that the spine/sheath structure of the jet from these sources can be interpreted naturally by the BZ and BP processes.

The Relationship between the Rise Width and the Full Width of γ-ray Burst Pulses and Its Implications

We investigate the relationship between the rise width and the full width of gamma-ray burst pulses. Theoretical analysis shows that either width is proportional to Г^-2△τθ,FWHMRc/c（Г the Lorentz factor of the bulk motion, △τθ,FWHM a local pulse＇s width, Rc the radius of fireballs and c the velocity of light）. We study the relationship for four samples of observed pulses. We find： （1） merely the curvature effect could reproduce the relationship between the rise and full widths with the same slope as derived from the model of Qin et al.; （2） gamma-ray burst pulses, selected from both the short and long GRBs, follow the same sequence in the rise width vs. full width diagram, with the shorter pulses at one end; （3） all GRBs may intrinsically result from local Gaussian pulses. These features place constraints on the physical mechanism（s） for producing long and short GRBs.

Constraint on the counter-jet emission in gamma-ray burst afterglows from GRB 980703

We present a numerical investigation of emission from the receding jet of gamma-ray bursts.It is found that the peak time of the receding jet emission is significantly affected by synchrotron self-absorption in radio wavelengths.However,the receding jet component is generally very weak.It is observable mainly for those nearby events in a dense environment.Although GRB 980703 has been observed in radio wavelengths for more than 1000 days,we argue that the receding jet emission still has not been detected for this event.Actually,it is completely submerged by the host galaxy.