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Gao H.,Beijing Normal University | Zhang B.,University of Nevada, Las Vegas | Zhang B.,Peking University | Lu H.-J.,Guangxi University | Lu H.-J.,Guangxi Key Laboratory for the Relativistic Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2016

Binary neutron star (NS) mergers are strong gravitational-wave (GW) sources and the leading candidates to interpret short-duration gamma-ray bursts (SGRBs). Under the assumptions that SGRBs are produced by double neutron star mergers and that the x-ray plateau followed by a steep decay as observed in SGRB x-ray light curves marks the collapse of a supramassive neutron star to a black hole (BH), we use the statistical observational properties of Swift SGRBs and the mass distribution of Galactic double neutron star systems to place constraints on the neutron star equation of state (EoS) and the properties of the post-merger product. We show that current observations already impose the following interesting constraints. (1) A neutron star EoS with a maximum mass close to a parametrization of Mmax=2.37M⊙(1+1.58×10-10P-2.84) is favored. (2) The fractions for the several outcomes of NS-NS mergers are as follows: ∼40% prompt BHs, ∼30% supramassive NSs that collapse to BHs in a range of delay time scales, and ∼30% stable NSs that never collapse. (3) The initial spin of the newly born supramassive NSs should be near the breakup limit (Pi∼1 ms), which is consistent with the merger scenario. (4) The surface magnetic field of the merger products is typically ∼1015 G. (5) The ellipticity of the supramassive NSs is ε∼(0.004-0.007), so that strong GW radiation is released after the merger. (6) Even though the initial spin energy of the merger product is similar, the final energy output of the merger product that goes into the electromagnetic channel varies in a wide range from several 1049 to several 1052 erg, since a good fraction of the spin energy is either released in the form of GWs or falls into the black hole as the supramassive NS collapses. © 2016 American Physical Society. Source

Yi S.-X.,Qufu Normal University | Yi S.-X.,Nanjing University | Yu H.,Nanjing University | Wang F.Y.,Nanjing University | And 5 more authors.
Astrophysical Journal, Supplement Series | Year: 2016

X-ray flares are generally supposed to be produced by later activities of the central engine, and may share a similar physical origin with the prompt emission of gamma-ray bursts (GRBs). In this paper, we have analyzed all significant X-ray flares from the GRBs observed by Swift from 2005 April to 2015 March. The catalog contains 468 bright X-ray flares, including 200 flares with redshifts. We obtain the fitting results of X-ray flares, such as start time, peak time, duration, peak flux, fluence, peak luminosity, and mean luminosity. The peak luminosity decreases with peak time, following a power-law behavior . The flare duration increases with peak time. The 0.3-10 keV isotropic energy of the distribution of X-ray flares is a log-normal peaked at erg. We also study the frequency distributions of flare parameters, including energies, durations, peak fluxes, rise times, decay times, and waiting times. Power-law distributions of energies, durations, peak fluxes, and waiting times are found in GRB X-ray flares and solar flares. These distributions could be well explained by a fractal-diffusive, self-organized criticality model. Some theoretical models based on magnetic reconnection have been proposed to explain X-ray flares. Our result shows that the relativistic jets of GRBs may be dominated by Poynting flux. © 2016. The American Astronomical Society. All rights reserved. Source

Wang X.-G.,Guangxi University | Wang X.-G.,University of Nevada, Las Vegas | Wang X.-G.,Guangxi Key Laboratory for the Relativistic Astrophysics | Zhang B.,University of Nevada, Las Vegas | And 12 more authors.
Astrophysical Journal, Supplement Series | Year: 2015

The external forward shock models have been the standard paradigm to interpret the broadband afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic; that is, the break times should be the same in different frequencies. Multiwavelength observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the external forward shock origin of GRB afterglows. In this paper, using a large sample of GRBs with both X-ray and optical afterglow data, we perform a systematic study to address the question: how bad or good are the external forward shock models? Our sample includes 85 GRBs up to 2014 March with well-monitored X-ray and optical light curves. Based on how well the data abide by the external forward shock models, we categorize them into five grades and three samples. The first two grades (Grade I and II) include 45 of 85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal and spectral behaviors in each afterglow segment are consistent with the predictions (the "closure relations") of the forward shock models. These GRBs are included in the Gold sample. The next two grades (Grade III and IV) include 37 of 85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. These GRBs are included in the Silver sample. Finally, Grade V (3/85) shows direct evidence of chromatic behaviors, suggesting that the external shock models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties (temporal index α, spectral index β, break time tb) and model parameters (energy injection index q, electron spectral index p, jet opening angle, radiative efficiency ηγ, and so on) of the GRBs in the Gold sample, and derive constraints on the magnetization parameter B in the forward shock. Overall, we conclude that the simplest external forward shock models can account for the multiwavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets, arbitrary circumburst medium density profile) is invoked, up to >90% of the afterglows may be interpreted within the framework of the external shock models. © 2015. The American Astronomical Society. All rights reserved. Source

Xin L.-P.,CAS National Astronomical Observatories | Xin L.-P.,Guangxi University | Xin L.-P.,Guangxi Key Laboratory for the Relativistic Astrophysics | Wang Y.-Z.,Guangxi University | And 17 more authors.
Astrophysical Journal | Year: 2016

Observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (GRBs). We report our optical observations of GRB 111228A from 95 s to about 50 hr after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data from the Swift and Fermi missions. Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features a low dust-to-gas ratio. Incorporating the energy injection effect, our best fit to the afterglow light curves with the standard afterglow model via the Markov Chain Monte Carlo technique shows that , , cm-3. The low medium density likely implies that the afterglow jet may be in a halo or in a hot ISM. A chromatic shallow decay segment observed in the optical and X-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. The Ep of its time-integrated prompt gamma-ray spectrum is ∼26 KeV. Using the initial Lorentz factor () derived from our afterglow model fit, it is found that GRB 111228A satisfies the relation and bridges the typical GRBs and low luminosity GRBs in this relation. © 2016. The American Astronomical Society. All rights reserved.. Source

Jiang S.-Z.,Guangxi University | Jiang S.-Z.,Guangxi Key Laboratory for the Relativistic Astrophysics | Wei Z.-L.,Guangxi University | Chen Q.-S.,Guangxi University | Wang Q.,Tsinghua University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

We update our original low-energy constant computations to the O(p6) order, including two and three flavors, the normal and anomalous ones. Following a comparative analysis, the O(p4) order results are considered better. In the O(p6) order, most of our results are consistent with or better than those we have found in the literature, although several are worse. © 2015 American Physical Society. Source

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