Rui-Jing Lu

A COMPREHENSIVE ANALYSIS OF FERMI GAMMA-RAY BURST DATA. II. E-p EVOLUTION PATTERNS AND IMPLICATIONS FOR THE OBSERVED SPECTRUM-LUMINOSITY RELATIONS

We present a time-resolved spectral analysis of 51 long and 11 short bright gamma-ray bursts (GRBs) observed with the Fermi/Gamma-Ray Burst Monitor, paying special attention to E p evolution within each burst. Among eight single-pulse long GRBs, five show an evolution from hard to soft, while three show intensity tracking. The multi-pulse long GRBs have more complicated patterns. Statistically, the hard-to-soft evolution pulses tend to be more asymmetric than the intensity-tracking ones, with a steeper rising wing than the falling wing. Short GRBs have E p tracking intensity exclusively with the 16 ms time-resolution analysis. We performed a simulation analysis and suggest that for at least some bursts, the late intensity-tracking pulses could be a consequence of overlapping hard-to-soft pulses. However, the fact that the intensity-tracking pattern exists in the first pulse of the multi-pulse long GRBs and some single-pulse GRBs, suggests that intensity tracking is an independent component, which may operate in some late pulses as well. For the GRBs with measured redshifts, we present a time-resolved E p – L γ, iso correlation analysis and show that the scatter of the correlation is comparable to that of the global Amati/Yonetoku relation. We discuss the predictions of various radiation models regarding E p evolution, as well as the possibility of a precessing jet in GRBs. The data pose a great challenge to each of these models, and hold the key to unveiling the physics behind GRB prompt emission.

A COMPREHENSIVE STUDY OF GAMMA-RAY BURST OPTICAL EMISSION. II. AFTERGLOW ONSET AND LATE RE-BRIGHTENING COMPONENTS

We continue our systematic statistical study of various components of gamma-ray burst (GRB) optical light curves. We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical light curves, the onset and re-brightening bumps are observed in 38 and 26 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are similar to 1.5 and similar to-1.15, respectively. No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time L-p proportional to t(p)(-1.81 +/- 0.32) for the onset bumps and L-p proportional to t(p)(-0.83 +/- 0.17) for the re-brightening bumps. Both L-p and the isotropic energy release of the onset bumps are correlated with E-gamma,E- iso, whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as n proportional to r(-1) within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between the initial Lorentz factor (Gamma(0)) and E-iso,E-gamma in our previous work. The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component. Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component, but with a lower Gamma(0), typically several tens.

A COMPREHENSIVE ANALYSIS OF FERMI GAMMA-RAY BURST DATA. III. ENERGY-DEPENDENT T 90 DISTRIBUTIONS OF GBM GRBs AND INSTRUMENTAL SELECTION EFFECT ON DURATION CLASSIFICATION

The durations (T90) of 315 gamma-ray bursts (GRBs) detected with Fermi/GBM (8‐1000 keV) up to 2011 September are calculated using the Bayesian Block method. We compare the T90 distributions between this sample and those derived from previous/current GRB missions. We show that the T90 distribution of this GRB sample is bimodal, with a statistical significance level comparable to those derived from the BeppoSAX/GRBM sample and theSwift/BAT sample, but lower than that derived from theCGRO/BATSE sample. The short-to-long GRB number ratio is also much lower than that derived from the BATSE sample, i.e., 1:6.5 versus 1:3. We measure T90 in several bands, i.e., 8‐15, 15‐25, 25‐50, 50‐100, 100‐350, and 350‐1000 keV, to investigate the energy-dependence effect of the bimodal T90 distribution. It is found that the bimodal feature is well observed in the 50‐100 and 100‐350 keV bands, but is only marginally acceptable in the 25‐50 keV and 350‐1000 keV bands. The hypothesis of bimodality is confidently rejected in the 8‐15 and 15‐25 keV bands. The T90 distributions in these bands are roughly consistent with those observed by missions with similar energy bands. The parameter T90 as a function of energy follows ¯ T90 ∝ E −0.20±0.02 for long GRBs. Considering the erratic X-ray and optical flares, the duration of a burst would be even longer for most GRBs. Our results, together with the observed extended emission of some short GRBs, indicate that the central engine activity timescale would be much longer than T90 for both long and short GRBs and the observed bimodal T90 distribution may be due to an instrumental selection effect.

SELECTION EFFECTS ON THE OBSERVED REDSHIFT DEPENDENCE OF GAMMA-RAY BURST JET OPENING ANGLES

An apparent redshift dependence of the jet opening angles (θj) of gamma-ray bursts (GRBs) is observed from the current GRB sample. We investigate whether this dependence can be explained with instrumental selection effects and observational biases by a bootstrapping method. Assuming that (1) the GRB rate follows the star formation history and the cosmic metallicity history and (2) the intrinsic distributions of the jet-corrected luminosity (L γ) and θj are a Gaussian or a power-law function, we generate a mock Swift/Burst Alert Telescope (BAT) sample by considering various instrumental selection effects, including the flux threshold and the trigger probability of BAT, the probabilities of a GRB jet pointing to the instrument solid angle, and the probability of redshift measurement. Our results reproduce the observed θj – z dependence well. We find that in the case of L γ∝θ2 j good consistency between the mock and observed samples can be obtained, indicating that both L γ and θj are degenerate for a flux-limited sample. The parameter set (L γ, θj) = (4.9 × 1049 erg s–1, 0.054 rad) gives the best consistency for the current Swift GRB sample. Considering the beaming effect, the derived intrinsic local GRB rate is accordingly 2.85 × 102 Gpc–3 yr–1, inferring that ~0.59% of Type Ib/c supernovae may be accompanied by a GRB.

RELATIVISTIC JET PROPERTIES OF GeV-TeV BLAZARS AND POSSIBLE IMPLICATIONS FOR THE JET FORMATION, COMPOSITION, AND CAVITY KINEMATICS

We fit the spectral energy distributions of a GeV-TeV flat spectrum radio quasar (FSRQ) sample with the leptonic model. Their gamma min of the relativistic electron distributions, which significantly affect the estimate of the jet properties, are constrained, with a typical value of similar to 48. Their jet power, magnetized parameter, radiation efficiency, and jet production and radiation rates per central black hole (BH) mass are derived and compared with those of BL Lacertae (BL Lac) objects. We show that the FSRQ jets may be dominated by the Poynting flux and have a high radiation efficiency, whereas the BL Lac object jets are likely dominated by particles and have a lower radiation efficiency than FSRQs. Being different from BL Lac objects, the jet powers of FSRQs are proportional to their central BH masses. The jet production and radiation rates of the FSRQs distribute in narrow ranges and are correlated with each other, whereas no similar feature is found for the BL Lac objects. We also show that the jet power is correlated with the cavity kinetic power: the magnetic field energy in the jets may provide the cavity kinetic energy of FSRQs, and the kinetic energy of cold protons in the jets may be crucial for the cavity kinetic energy of BL Lac objects. We suggest that the dominating formation mechanism of FSRQ jets may be the Blandford-Znajek process, but BL Lac object jets may be produced via the Blandford-Payne and/ or Blandford-Znajek processes, depending on the structures and accretion rates of accretion disks.

Photosphere emission in the X-ray flares of swift gamma-ray bursts and implications for the fireball properties

X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft X-ray range and the spectral coverage is limited. In this paper, we present an analysis of 32 GRB X-ray flares that are simultaneously observed by both Burst Alert Telescope and X-Ray Telescope on board the Swift mission, so that a joint spectral analysis with a wider spectral coverage is possible. Our results show that the joint spectra of 19 flares are fitted with the absorbed single power law or the Band function models. More interestingly, the joint spectra of the other 13 X-ray flares are fitted with the absorbed single power-law model plus a blackbody component. Phenomenally, the observed spectra of these 13 flares are analogous to several GRBs with a thermal component, but only with a much lower temperature of kT = 1 similar to 3 keV. Assuming that the thermal emission is the photosphere emission of the GRB fireball, we derive the fireball properties of the 13 flares that have redshift measurements, such as the bulk Lorentz factor Gamma(ph) of the outflow. The derived Gamma(ph) range from 50 to 150 and a relation of Gamma(ph) to the thermal emission luminosity is found. It is consistent with the Gamma(0) - L-iso relations that are derived for the prompt gamma-ray emission. We discuss the physical implications of these results within the content of jet composition and the radiation mechanism of GRBs and X-ray flares.

INTERNAL ENERGY DISSIPATION OF GAMMA-RAY BURSTS OBSERVED WITH SWIFT: PRECURSORS, PROMPT GAMMA-RAYS, EXTENDED EMISSION, AND LATE X-RAY FLARES

We jointly analyze the gamma-ray burst (GRB) data observed with Burst Alert Telescope (BAT) and X-ray Telescope on board the Swift mission to present a global view on the internal energy dissipation processes in GRBs, including precursors, prompt gamma-ray emission, extended soft gamma-ray emission, and late X-ray flares. The Bayesian block method is utilized to analyze the BAT light curves to identify various emission episodes. Our results suggest that these emission components likely share the same physical origin, which is the repeated activation of the GRB central engine. What we observe in the gamma-ray band may be a small part of more extended underlying activities. The precursor emission, which is detected in about 10% of Swift GRBs, is preferably detected in those GRBs that have a massive star core-collapse origin. The soft extended emission tail, on the other hand, is preferably detected in those GRBs that have a compact star merger origin. Bright X-ray emission is detected during the BAT quiescent phases prior to subsequent gamma-ray peaks, implying that X-ray emission may be detectable prior the BAT trigger time. Future GRB alert instruments with soft X-ray capability are essential for revealing the early stages of GRB central engine activities, and shedding light on jet composition and the jet launching mechanism in GRBs.

DISTRIBUTIONS OF GAMMA-RAY BURSTS AND BLAZARS IN THE L-p-E-p-PLANE AND POSSIBLE IMPLICATIONS FOR THEIR RADIATION PHYSICS

We present a spectral analysis for a sample of redshift-known gamma-ray bursts (GRBs) observed with Fermi/GBM. Together with the results derived from our systematical spectral energy distribution modeling with the leptonic models for a Fermi/LAT blazar sample, we compare the distributions of the GRBs and the blazars by plotting the synchrotron peak luminosity (L-s) and the corresponding peak photon energy E-s of blazars in the L-p-E-p-plane of GRBs, where L-p and E-p are the peak luminosity and peak photon energy of the GRB time-integrated nu f(nu) spectrum, respectively. The GRBs are in the high-L-p, high-E-p corner of the plane and a tight L-p-E-p relation is found, i.e., L-p alpha E-p(2.13-0.46) (+0.54). Both flat spectrum radio quasars (FSRQs) and low-synchrotron peaking BL Lac objects (LBLs) are clustered in the low-Ep, low-Lp corner. Intermediate-and high-synchrotron peaking BL Lac objects (IBLs and HBLs) have E-s similar to 2 x 10(-3)-10(2) keV and L-s similar to 10(44)-10(47) erg s(-1), but no dependence of L-s on E-s is found. We show that the tight L-p-E-p relation of GRBs is potentially explained with the synchrotron radiation of fast-cooling electrons in a highly magnetized ejecta, and the weak anti-correlation of L-s-E-s for FSRQs and LBLs may be attributed to synchrotron radiation of slow-cooling electrons in a moderately magnetized ejecta. The distributions of IBLs and HBLs in the L-p-E-p-plane may be interpreted with synchrotron radiation of fast-cooling electrons in a matter-dominated ejecta. These results may present a unified picture for the radiation physics of relativistic jets in GRBs and blazars within the framework of the leptonic synchrotron radiation models.

COSMIC EVOLUTION OF LONG GAMMA-RAY BURST LUMINOSITY

The cosmic evolution of gamma-ray burst (GRB) luminosity is essential for revealing the GRB physics and for using GRBs as cosmological probes. We investigate the luminosity evolution of long GRBs with a large sample of 258 {\em Swift}/BAT GRBs. Parameterized the peak luminosity of individual GRBs evolves as

Spectral hardness evolution of gamma-ray bursts due to the Doppler effect of fireballs

L_{\rm p}\propto{\rm }(1+z)^{k}