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 t b ) and model parameters (energy injection index q, electron spectral index p, jet opening angle j q , radiative efficiency η γ , and so on) of the GRBs in the Gold sample, and derive constraints on the magnetization parameter ϵ 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.

In this paper we propose the model that the coalescence of primordial black holes (PBHs) binaries with equal mass M ∼ 10 28 g can emit luminous gigahertz (GHz) radio transient, which may be candidate sources for the observed fast radio bursts (FRBs), if at least one black hole holds appropriate amount of net electric charge Q. Using a dimensionless quantity for the charge q = Q/ √ GM , our analyses infer that q ∼ O(10 −4.5 ) can explain the FRBs with released energy of order O(10 40 )ergs. With the current sample of FRBs and assuming a distribution of charge φ(q) for all PBHs, we can deduce that its form is proportional to q −3.0±0.1 for q ≥ 7.2 × 10 −5 if PBHs are sources of the observed FRBs. Furthermore, with the proposed hypothetical scenario and by estimating the local event rate of FRBs ∼ 2.6 × 10 3 Gpc −3 yr −1 , one derives a lower bound for the fraction of PBHs (at the mass of 10 28 g) against that of matter fPBH(10 28 g) 10 −5 needed to explain the rate. With this inspiring estimate, we expect that future observations of FRBs can help to falsify their physical origins from the PBH binaries coalescences. In the future, the gravitational waves produced by mergers of small black holes can be detected by high frequency gravitational wave detectors. We believe that this work would be a useful addition to the current literature on multimessenger astronomy and cosmology.

Illite-smectite clay is a new mixed mineral of illite and montmorillonite. The ability of nano illite/smectite clay to remove Pb(II) from slightly polluted aqueous solutions has been investigated. The effects of pH, contact time, initial concentration of Pb(II), nano illite/smectite clay dosage, and temperature on the adsorption process were studied. The nano illite/smectite clay was characterized by X-Ray Diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and Scanning electron microscopy (SEM). The results showed that Pb(II) was adsorbed efficiently by nano illite/smectite clay in aqueous solution. The pseudo-second-order kinetic model best described the kinetic of the adsorption, and the adsorption capacity of nano illite/smectite (I-S m ) clay was found to be 256.41 µg·g −1 for Pb(II). The adsorption patterns followed the Langmuir isotherm model. Thermodynamic parameters, including the Gibbs free energy (∆G), enthalpy (∆H), and entropy (∆S) changes, indicated that the present adsorption process was feasible, spontaneous, and endothermic in the temperature range of 298-333 K.

Hi absorption studies yield information on both AGN feeding and feedback processes. This AGN activity interacts with the neutral gas in compact radio sources, which are believed to represent the young or recently re-triggered AGN population. We present the results of a survey for Hi absorption in a sample of 66 compact radio sources at 0.040 < z < 0.096 with the Australia Telescope Compact Array. In total, we obtained seven detections, five of which are new, with a large range of peak optical depths (3% to 87%). Of the detections, 71% exhibit asymmetric, broad (∆v FWHM > 100 km s −1 ) features, indicative of disturbed gas kinematics. Such broad, shallow and offset features are also found within low-excitation radio galaxies which is attributed to disturbed circumnuclear gas, consistent with early-type galaxies typically devoid of a gas-rich disk. Comparing mid-infrared colours of our galaxies with Hi detections indicates that narrow and deep absorption features are preferentially found in late-type and high-excitation radio galaxies in our sample. These features are attributed to gas in galactic disks. By combining XMM-Newton archival data with 21-cm data, we find support that absorbed X-ray sources may be good tracers of HI content within the host galaxy. This sample extends previous Hi surveys in compact radio galaxies to lower radio luminosities and provides a basis for future work exploring the higher redshift universe.