分类: 医学、药学 >> 临床医学 提交时间: 2023-11-10 合作期刊: 《中国全科医学》
摘要: none
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Long gamma-ray bursts (GRBs) have been discussed as a potential tool to probe the cosmic star formation rate (SFR) for a long time. Some studies found an enhancement in the GRB rate relative to the galaxy-inferred SFR at high redshifts, which indicates that GRBs may not be good tracers of star formation. However, in these studies, the GRB rate measured at any redshift is an average over all galaxies at that epoch. A deep understanding of the connection between GRB production and environment also needs to characterize the population of GRB host galaxies directly. Based on a complete sample of GRB hosts, we constrain the stellar-mass function (SMF) of GRB hosts, and examine redshift evolution in the GRB host population. Our results confirm that a strong redshift evolution in energy (with an evolution index of $\delta=2.47^{+0.73}_{-0.89}$) or in density ($\delta=1.82^{+0.22}_{-0.59}$) is needed in order to account for the observations. The GRB host SMF can be well described by the Schechter function with a power-law index $\xi\approx-1.10$ and a break mass $M_{b,0}\approx4.9\times10^{10}$ ${\rm M}_\odot$, independent of the assumed evolutionary effects. This is the first formulation of the GRB host SMF. The observed discrepancy between the GRB rate and the galaxy-inferred SFR may also be explained by an evolving SMF.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Long gamma-ray bursts (GRBs) have been discussed as a potential tool to probe the cosmic star formation rate (SFR) for a long time. Some studies found an enhancement in the GRB rate relative to the galaxy-inferred SFR at high redshifts, which indicates that GRBs may not be good tracers of star formation. However, in these studies, the GRB rate measured at any redshift is an average over all galaxies at that epoch. A deep understanding of the connection between GRB production and environment also needs to characterize the population of GRB host galaxies directly. Based on a complete sample of GRB hosts, we constrain the stellar-mass function (SMF) of GRB hosts, and examine redshift evolution in the GRB host population. Our results confirm that a strong redshift evolution in energy (with an evolution index of $\delta=2.47^{+0.73}_{-0.89}$) or in density ($\delta=1.82^{+0.22}_{-0.59}$) is needed in order to account for the observations. The GRB host SMF can be well described by the Schechter function with a power-law index $\xi\approx-1.10$ and a break mass $M_{b,0}\approx4.9\times10^{10}$ ${\rm M}_\odot$, independent of the assumed evolutionary effects. This is the first formulation of the GRB host SMF. The observed discrepancy between the GRB rate and the galaxy-inferred SFR may also be explained by an evolving SMF.
分类: 生物学 >> 动物学 提交时间: 2017-11-07 合作期刊: 《古脊椎动物学报》
摘要: Our systematic revision of Anhuichelys Yeh, 1979, based on abundant new material from the Paleocene of Qianshan Basin, Anhui Province, China provides evidences for its assignment to the stem Testudinidae. Anhuichelys contains three species in the Paleocene of Qianshan Basin: A. siaoshihensis Yeh, 1979 from the Early Paleocene of Wanghudun Formation, A. tsienshanensis Yeh, 1979 from both Wanghudun and Doumu formations and A. doumuensis sp. nov. from the Middle Paleocene Doumu Formation. A. xinzhouensis Chen, 1983 from the Paleocene of Xinzhou, Hubei Province is synonymized with A. tsienshanensis. Associated with a terrestrial vertebrate fauna, Anhuichelys is likely a land turtle and also the first testudinoid to develop the hinge on the shell.
分类: 光学 >> 量子光学 提交时间: 2023-02-19
摘要: Optical clock networks play important roles in various fields, such as precise navigation, redefinition of "second" unit, and gravitational tests. To establish a global-scale optical clock network, it is essential to disseminate time and frequency with a stability of $10^{-19}$ over a long-distance free-space link. However, such attempts were limited to dozens of kilometers in mirror-folded configuration. Here, we take a crucial step toward future satellite-based time-frequency disseminations. By developing the key technologies, including high-power frequency combs, high-stability and high-efficiency optical transceiver systems, and efficient linear optical sampling, we demonstrate free-space time-frequency dissemination over two independent links with femtosecond time deviation, $3\times10^{-19}$ at 10,000 s residual instability and $1.6\times10^{-20}\pm 4.3\times10^{-19}$ offset. This level of the stability retains for an increased channel loss up to 89 dB. Our work can not only be directly used in ground-based application, but also firmly laid the groundwork for future satellite time-frequency dissemination.