A kilonova associated with GRB 070809

For on-axis typical short gamma-ray bursts (sGRBs), the forward shock emission is usually so bright that renders the identification of kilonovae (also known as macronovae) in the early afterglow ($t<0.5$ d) phase rather challenging. This is why previously no thermal-like kilonova component has been identified at such early time except in the off-axis dim GRB 170817A associated with GW170817. Here we report the identification of an unusual optical radiation component in GRB 070809 at $t\sim 0.47$ d, thanks plausibly to the very-weak/subdominant forward shock emission. The optical emission with a very red spectrum is well in excess of the extrapolation of the X-ray emission that is distinguished by an unusually hard spectrum, which is at odds with the forward shock afterglow prediction but can be naturally interpreted as a kilonova. Our finding supports the speculation that kilonovae are ubiquitous , and demonstrates the possibility of revealing the neutron star merger origin with the early afterglow data of some typical sGRBs that take place well beyond the sensitive radius of the advanced gravitational wave detectors and hence the opportunity of organizing dedicated follow-up observations for events of interest.Comment: 20 pages, 5 figures, published in Nature Astronom

Changes of Metal Element Contents in Base Liquor for Maotai-Flavor Baijiu during Storage and Their Effects on Flavor Compounds

Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the contents of 25 metal elements in base liquor of different ages for Maotai-flavor Baijiu from three companies. A discriminant model was constructed using chemometrics to identify key differential metal elements. The results showed that the contents of metal elements in base liquor were positively correlated with age for each brand. Principal component analysis (PCA), hierarchical cluster analysis (HCA) and partial least squares discriminant analysis (PLS-DA) could effectively distinguish among six age groups; among them, PLS-DA had the best degree of discrimination, and the results of the PLS-DA model were consistent with those of the PCA model. Five key differential metal elements, namely Al, Ca, K, Na and Fe, were determined in the base liquor stored for 0–10 years by PLS-DA and one-way analysis of variance (ANOVA). The differential metal elements were found to play a positive role in the storage of base liquor through gas chromatography triple quadrupole mass spectrometry (GC-TQ-MS) detection. The discriminant model could discriminate base liquor of different ages from other companies. In summary, ICP-MS combined with chemometric analysis can effectively classify and identify base liquor of different ages for Maotai-flavor Baijiu, and determine the changes of metal element contents in base liquor for Maotai-flavor Baijiu during storage more accurately and efficiently

Generation of bottle beam using low-density channel in air

Cylindrical density depressions generated by femtosecond laser pulses filamenting in air for different energy depositions is investigated numerically, by using a set of hydrodynamic equations. The evolution of density profile is calculated for different temperature elevations, the results indicate that the gas density hole is getting shallower and wider with the increasing temperature elevations. A simulation of the propagation inside low-density channel implies a new way to generate a type of bottle beam.Comment: 4 pages,5 figure