We compile a complete collection of reliable Hubble parameter H(z) data to redshift z ≤ 2.36 and use them with the Gaussian Process method to determine continuous H(z) functions for various data subsets. From these continuous H(z)'s, summarizing across the data subsets considered, we find H 0 ∼ 67 ± 4 km/s/Mpc, more consistent with the recent lower values determined using a variety of techniques. In most data subsets, we see a cosmological decelerationacceleration transition at 2σ significance, with the data subsets transition redshifts varying over 0.33 < z da < 1.0 at 1σ significance. We find that the flat-ΛCDM model is consistent with the H(z) data to a z of 1.5 to 2.0, depending on data subset considered, with 2σ deviations from flat-ΛCDM above this redshift range. Using the continuous H(z) with baryon acoustic oscillation distance-redshift observations, we constrain the current spatial curvature density parameter to be Ω K0 = −0.03 ± 0.21, consistent with a flat universe, but the large error bar does not rule out small values of spatial curvature that are now under debate.

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 calculate the dispersion measures (DMs) contributed by host galaxies of fast radio bursts (FRBs). Based on a few host galaxy observations, a large sample of galaxies with similar properties to observed ones has been selected from the IllustrisTNG simulation. They are used to compute the distributions of host galaxy DMs for repeating and nonrepeating FRBs. For repeating FRBs, we infer the DMhost for FRBs like FRB 121102 and FRB 180916 by assuming that the burst sites are tracing the star formation rates in host galaxies. The median DMshost are 35 (1 + z)1.08 and 96(1 + z)0.83 pc cm−3 for FRBs like FRB 121102 and FRB 180916, respectively. In another case, the median of DMhost is about 30–70 pc cm−3 for nonrepeating FRBs in the redshift range z = 0.1–1.5, assuming that the burst sites are the locations of binary neutron star mergers. In this case, the evolution of the median DMhost can be calculated by 33(1 + z)0.84 pc cm−3. The distributions of DMhost of repeating and nonrepeating FRBs can be well fitted with the log-normal function. Our results can be used to infer redshifts of nonlocalized FRBs.

Gamma-ray bursts (GRBs) are the most violent explosions in the universe and can be used to explore the properties of the high-redshift universe. It is believed that long GRBs are associated with the deaths of massive stars. Therefore, it is possible to use GRBs to investigate the star formation rate (SFR). In this paper, we use Lynden-Bell's − c method to study the luminosity function and rate of Swift long GRBs without any assumptions. We find that the luminosities of GRBs evolve with redshift as . We also find that the formation rate of GRBs is almost constant at < z 1.0 for the first time, which is remarkably different from the SFR. At > z 1.0, the formation rate of GRBs is consistent with the SFR. Our results are dramatically different from previous studies. We discuss a few possible reasons for this low-redshift excess. We also test the robustness of our results using Monte Carlo simulations. The distributions of mock data (i.e., luminosity-redshift distribution, luminosity function, cumulative distribution, and log N-log S distribution) are in good agreement with observations. Also, we find that there are remarkable differences between the mock data and the observations if long GRBs are unbiased tracers of SFR at < z 1.0.

We propose a new model-independent method to test the cosmic curvature by comparing the proper distance and transverse comoving distance. Using the measurements of Hubble parameter H(z) and angular diameter distance d A , the cosmic curvature parameter Ω K is constrained to be −0.09±0.19, which is consistent with a flat universe. We also use Monte Carlo simulation to test the validity and efficiency, and find that our method can give a reliable and efficient constraint on cosmic curvature. Compared with other model-independent methods testing the cosmic curvature, our method can avoid some drawbacks and give a better constraint.

Fast radio bursts (FRBs) are millisecond-duration radio signals occurring at cosmological distances. However the physical model of FRBs is mystery, many models have been proposed. Here we study the frequency distributions of peak flux, fluence, duration and waiting time for the repeating FRB 121102. The cumulative distributions of peak flux, fluence and duration show power-law forms. The waiting time distribution also shows power-law distribution, and is consistent with a non-stationary Poisson process. These distributions are similar as those of soft gamma repeaters (SGRs). We also use the statistical results to test the proposed models for FRBs. These distributions are consistent with the predictions from avalanche models of slowly driven nonlinear dissipative systems.

Cosmic rays interact with the Earth’s atmosphere to produce 14C, which can be absorbed by trees. Therefore, rapid increases of 14C in tree rings can be used to probe previous cosmic-ray events. By this method, three 14C rapidly increasing events have been found. Plausible causes of these events include large solar proton events, supernovae, or short gamma-ray bursts. However, due to the lack of measurements of 14C by year, the occurrence frequency of such 14C rapidly increasing events is poorly known. In addition, rapid increases may be hidden in the IntCal13 data with five-year resolution. Here we report the result of 14C measurements using an ancient buried tree during the period between bc 3388 and 3358. We found a rapid increase of about 9‰ in the 14C content from bc 3372 to bc 3371. We suggest that this event could originate from a large solar proton event.

Broad-lined type Ic supernovae (SNe Ic-BL) are peculiar stellar explosions that distinguish themselves from ordinary SNe. Some SNe Ic-BL are associated with long-duration ( 2 s) gamma-ray bursts (GRBs). Black holes and magnetars are two types of compact objects that are hypothesized to be central engines of GRBs. In spite of decades of investigations, no direct evidence for the formation of black holes or magnetars has been found for GRBs so far. Here we report the finding that the early peak (t 50 days) and late-time (t 300 days) slow decay displayed in the light curves of both SNe 1998bw (associated with GRB 980425) and 2002ap (not GRB-associated) can be attributed to magnetar spin-down with initial rotation period P 0 ∼ 20 ms, while the intermediate-time (50 t 300 days) exponential decline is caused by radioactive decay of 56 Ni. The connection between the early peak and late-time slow decline in the light curves is unexpected in alternative models. We thus suggest that GRB 980425 and SN 2002ap were powered by magnetars.