X-Ray Flares from Postmerger Millisecond Pulsars

Recent observations support the suggestion that short-duration gamma-ray bursts are produced by compact star mergers. The X-ray flares discovered in two short gamma-ray bursts last much longer than the previously proposed postmerger energy release time scales. Here we show that they can be produced by differentially rotating, millisecond pulsars after the mergers of binary neutron stars. The differential rotation leads to windup of interior poloidal magnetic fields and the resulting toroidal fields are strong enough to float up and break through the stellar surface. Magnetic reconnection--driven explosive events then occur, leading to multiple X-ray flares minutes after the original gamma-ray burst.Comment: 10 pages, published in Scienc

Broadband lightcurve characteristics of GRBs 980425 and 060218 and comparison with long-lag, wide-pulse GRBs

It has been recently argued that low-luminosity gamma-ray bursts (LL-GRBs) are likely a unique GRB population. Here, we present systematic analysis of the lightcurve characteristics from X-ray to gamma-ray energy bands for the two prototypical LL-GRBs 980425 and 060218. It is found that both the pulse width ($w$) and the ratio of the rising width to the decaying width ($r/d$) of theses two bursts are energy-dependent over a broad energy band. There exists a significant trend that the pulses tend to be narrower and more symmetry with respect to the higher energy bands for the two events. Both the X-rays and the gamma-rays follow the same $w - E$ and $r/d - E$ relations. These facts may indicate that the X-ray emission tracks the gamma-ray emission and both are likely to be originated from the same physical mechanism. Their light curves show significant spectral lags. We calculate the three types of lags with the pulse peaking time ($t_{peak}$), the pulse centroid time ($t_{cen}$), and the cross-correlation function (CCF). The derived $t_{peak}$ and $t_{cen}$ are a power-law function of energy. The lag calculated by CCF is strongly correlated with that derived from $t_{peak}$. But the lag derived from $t_{cen}$ is less correlated with that derived from $t_{peak}$ and CCF. The energy dependence of the lags is shallower at higher energy bands. These characteristics are well consistent with that observed in typical long-lag, wide-pulse GRBs, suggesting that GRBs 980425 and 060218 may share the similar radiation physics with them.Comment: 26 pages, 10 figures, 3 tables, accepted for publication in Ap

Soil, grain and water chemistry and human selenium imbalances in Enshi district, Hubei Province, China

Many elements which are essential to human and other animal health in small doses can be toxic if ingested in excess. Selenium (Se), a naturally occurring metalloid element is found in all natural materials on earth including rocks, soils, waters, air, plant and animal tissues. Since the early 1930’s, it has been recognised that Se toxicity causes hoof disorders and hair loss in livestock. Se was also identified as an essential trace element to humans and other animals in the late 1950’s. It forms a vital constituent of the biologically important enzyme glutathione peroxidase which acts as an anti-oxidant preventing cell degeneration. Se deficiency has been implicated in the aetiology of several diseases including cancer, muscular dystrophy, muscular sclerosis and cystic fibrosis. Se can be assimilated in humans through several pathways including food, drinking water and inhalation of Se-bearing particles from the atmosphere. In the majority of situations, food is the most important source of Se, as levels in water are very low. The narrow range between deficiency levels (<40 pg per day) and toxic levels in susceptible people (> 900 pg per day) makes it necessary to carefully control the amount of Se in the diet. In China, Se deficiency has been linked to an endemic degenerative heart disease known as Keshan Disease (KD) and an endemic osteoarthropathy which causes deformity of affected joints, known as Kaschin-Beck Disease. These diseases occur in a geographic belt stretching from Heilongjiang Province in north-east China to Yunnan Province in the south-west. In the period between 1959 and 1970, peak KD incidence rates exceeded 40 per 100 000 (approximately 8500 cases per annum) with 1400 - 3000 deaths recorded each year. Incidence rates have since fallen to less than 5 per 100 000 with approximately 1000 new cases reported annually (Levander, 1986). Se toxicity (selenosis) resulting in hair and nail loss and disorders of the nervous system in the human population, has also been recorded in Enshi District, Hubei Province and in Ziyang County, Shanxi Province. China possesses one of the best epidemiological databases in the world on Se-related diseases which has been used in conjunction with geochemical data to demonstrate a significant geochemical control on human Se exposure. However, the precise geographical areas at risk and the geochemical controls on selenium availability have yet to be established