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.

We propose a method to search for stellar-mass black hole (BH) candidates with giant companions from spectroscopic observations. Based on the stellar spectra of LAMOST Data Release 6, we obtain a sample of seven giants in binaries with large radial velocity variation ∆V R > 80 km s −1 . With the effective temperature, surface gravity, and metallicity provided by LAMOST, and the parallax given by Gaia, we can estimate the mass and radius of the giant, and therefore evaluate the possible mass of the optically invisible star in the binary. We show that the sources in our sample are potential BH candidates, and are worthy of dynamical measurement by further spectroscopic observations. Our method may be particularly valid for the selection of BH candidates in binaries with unknown orbital periods.

Most dynamically confirmed stellar-mass black holes and the candidates were originally selected from X-ray outbursts. In the present work, we search for black hole candidates in the LAMOST survey by using the spectra along with photometry from the ASAS-SN survey, where the orbital period of the binary may be revealed by the periodic light curve, such as the ellipsoidal modulation type. Our sample consists of 9 binaries, where each source contains a giant star with large radial velocity variation (∆V R 70 km s −1 ) and periods known from light curves. We focus on the 9 sources with long periods (T ph > 5 days) and evaluate the mass M 2 of the optically invisible companion. Since the observed ∆V R from only a few repeating spectroscopic observations is a lower limit of the real amplitude, the real mass M 2 can be significantly higher than the current evaluation. It is likely an efficient method to place constraints on M 2 by combining ∆V R from LAMOST and T ph from ASAS-SN, particularly by the ongoing LAMOST Medium Resolution Survey.

We present time-resolved spectral analysis of the steep decay segments of 29 bright X-ray flares of gamma-ray bursts (GRBs) observed with the Swift/X-ray telescope, and model their lightcurves and spectral index evolution behaviors with the curvature effect model. Our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fit with this model, and the derived characteristic timescales (t c ) are in the range of 33 ∼ 264 seconds. Using an empirical relation between the peak luminosity and the Lorentz factor derived from the prompt gammarays, we estimate the Lorentz factors of the flares (Γ X ). We obtain Γ X = 17 ∼ 87 with a median value of 52, which is smaller than the initial Lorentz factors of prompt gamma-ray fireballs. With the derived t c and Γ X , we constrain the radiating regions of 13 X-ray flares, yielding R X = (0.2 ∼ 1.1) × 10 16 cm, which are smaller than the radii of the afterglow fireballs at the peak times of the flares. A long evolution feature from prompt gamma-ray phase to the X-ray epoch is found by incorporating our results with a sample of GRBs whose initial Lorentz factors are available in literatures, i.e., Γ ∝ [t p /(1 + z)] −0.69±0.06 . These results may shed lights on the long term evolution of GRB central engines.