Can life survive Gamma-Ray Bursts in the high-redshift universe?

Nearby Gamma-Ray Bursts (GRBs) have been proposed as a possible cause of mass extinctions on Earth. Due to the higher event rate of GRBs at higher redshifts, it has been speculated that life as we know it may not survive above a certain redshift (e.g. $z>0.5$). We examine the duty cycle of lethal (life-threatening) GRBs in the solar neighborhood, in the Sloan Digital Sky Survey (SDSS) galaxies and GRB host galaxies, with the dependence of the long GRB rate on star formation and metallicity properly taken into account. We find that the number of lethal GRBs attacking Earth within the past 500 Myr ($\sim$ epoch of the Ordovician mass extinction) is $0.93$. The number of lethal GRBs hitting a certain planet increases with redshift, thanks to the increasing star formation rate and decreasing metallicity in high-$z$ galaxies. Taking 1 per 500 Myr as a conservative duty cycle for life to survive, as evidenced by our existence, we find that there are still a good fraction of SDSS galaxies beyond $z=0.5$ where the GRB rate at half-mass radius is lower than this value. We derive the fraction of such benign galaxies as a function of redshift through Monte Carlo simulations, and find that the fraction is $\sim 50\%$ at $z\sim 1.5$ and $\sim 10\%$ even at $z \sim 3$. The mass distribution of benign galaxies is dominated by Milky-Way-like ones, thanks to their commonness, relatively large mass, and low star formation rate. GRB host galaxies are among the most dangerous ones.Comment: Accepted for publication in The Astrophysical Journal, 7 pages, 4 figures, 1 tabl

PieceTimer: A Holistic Timing Analysis Framework Considering Setup/Hold Time Interdependency Using A Piecewise Model

In static timing analysis, clock-to-q delays of flip-flops are considered as constants. Setup times and hold times are characterized separately and also used as constants. The characterized delays, setup times and hold times, are ap- plied in timing analysis independently to verify the perfor- mance of circuits. In reality, however, clock-to-q delays of flip-flops depend on both setup and hold times. Instead of being constants, these delays change with respect to different setup/hold time combinations. Consequently, the simple ab- straction of setup/hold times and constant clock-to-q delays introduces inaccuracy in timing analysis. In this paper, we propose a holistic method to consider the relation between clock-to-q delays and setup/hold time combinations with a piecewise linear model. The result is more accurate than that of traditional timing analysis, and the incorporation of the interdependency between clock-to-q delays, setup times and hold times may also improve circuit performance.Comment: IEEE/ACM International Conference on Computer-Aided Design (ICCAD), November 201

Sampling-based Buffer Insertion for Post-Silicon Yield Improvement under Process Variability

At submicron manufacturing technology nodes process variations affect circuit performance significantly. This trend leads to a large timing margin and thus overdesign to maintain yield. To combat this pessimism, post-silicon clock tuning buffers can be inserted into circuits to balance timing budgets of critical paths with their neighbors. After manufacturing, these clock buffers can be configured for each chip individually so that chips with timing failures may be rescued to improve yield. In this paper, we propose a sampling-based method to determine the proper locations of these buffers. The goal of this buffer insertion is to reduce the number of buffers and their ranges, while still maintaining a good yield improvement. Experimental results demonstrate that our algorithm can achieve a significant yield improvement (up to 35%) with only a small number of buffers.Comment: Design, Automation and Test in Europe (DATE), 201

Fast Radio Burst/Gamma-Ray Burst Cosmography

Recently, both theoretical arguments and observational evidence suggested that a small fraction of fast radio bursts (FRBs) could be associated with gamma-ray bursts (GRBs). If such FRB/GRB association systems are commonly detected in the future, the combination of dispersion measure (DM) derived from FRBs and redshifts derived from GRBs makes these systems a plausible tool to conduct cosmography. We quantify uncertainties in deriving the redshift-dependent DM_{IGM} as a function of z, and test how well dark energy models can be constrained with Monte Carlo simulations. We show that with potentially several 10s of FRB/GRB systems detected in a decade or so, one may reach reasonable constraints on wCDM models. When combined with SN Ia data, unprecedented constraints on dark energy equation of state may be achieved, thanks to the prospects of detecting FRB/GRB systems at relatively high redshifts. The ratio between the mean value and luminosity distance (D_{L} (z)) is insensitive to dark energy models. This gives the prospects of applying SN Ia data to calibrate using a relatively small sample of FRB/GRB systems, allowing a reliable constraint on the baryon inhomogeneity distribution as a function of redshift. The methodology developed in this paper can also be applied, if the FRB redshifts can be measured by other means. Some caveats of putting this method into practice are also discussed.Comment: Accepted for publication in Ap

The deepest splits in Chloranthaceae as resolved by chloroplast sequences

Evidence from the fossil record, comparative morphology, and molecular phylogenetic analyses indicates that Chloranthaceae are among the oldest lineages of flowering plants alive today. Their four genera (ca. 65 species) today are disjunctly distributed in the Neotropics, China, tropical Asia, and Australasia, with a single species in Madagascar but none in mainland Africa. In the Cretaceous, Chloranthaceae occurred in much of Laurasia as well as Africa, Australia, and southern South America. We used DNA sequence data from the plastid rbcL gene, the rpl20-rps12 spacer, the trnL intron, and the trnL-F spacer to evaluate intra-Chloranthaceae relationships and geographic disjunctions. In agreement with earlier analyses, Hedyosmum was found to be sister to the remaining genera, followed by Ascarina and Chloranthus + Sarcandra. Bayesian and parsimony analyses of the combined data yielded resolved and well-supported trees except for polytomies among Andean Hedyosmum and Madagascan-Australasian-Polynesian Ascarina. The sole Asiatic species of Hedyosmum, Hedyosmum orientale from Hainan, China, was sister to Caribbean and Neotropical species. Likelihood ratio tests on the rbcL data set did not reject the assumption of a clock as long as the long-branched outgroup Canella was excluded. Two alternative fossil calibrations were used to convert genetic distances into absolute ages. Calibrations with Hedyosmum-like flowers from the Barremian-Aptian or Chloranthus-like androecia from the Turonian yielded substitution rates that differed by a factor of two, illustrating a perhaps unsolvable problem in molecular clock–based studies that use several calibration fossils. The alternative rates place the onset of divergence among crown group (extant) species of Hedyosmum at 60 or 29 Ma, between the Paleocene and the Oligocene; that among extant Chloranthus at 22 or 11 Ma; and that among extant Ascarina at 18 or 9 Ma, implying long-distance dispersal between Madagascar and Australasia-Polynesia