Nucleosynthesis in the Outflow from Gamma Ray Burst Accretion Disks

We examine the nucleosynthesis products that are produced in the outflow from rapidly accreting disks. We find that the type of element synthesis varies dramatically with the degree of neutrino trapping in the disk and therefore the accretion rate of the disk. Disks with relatively high accretion rates such as 10 M_solar/s can produce very neutron rich nuclei that are found in the r process. Disks with more moderate accretion rates can produce copious amounts of Nickel as well as the light elements such as Lithium and Boron. Disks with lower accretion rates such as 0.1 M_solar/s produce large amounts of Nickel as well as some unusual nuclei such as Ti-49, Sc-45, Zn-64, and Mo-92. This wide array of potential nucleosynthesis products is due to the varying influence of electron neutrinos and antineutrinos emitted from the disk on the neutron-to-proton ratio in the outflow. We use a parameterization for the outflow and discuss our results in terms of entropy and outflow acceleration.Comment: 12 pages, 12 figures; submitted to Ap

Neutrino Interactions in the Outflow from Gamma-Ray Burst Accretion Disks

We examine the composition of matter as it flows away from gamma ray burst accretion disks, in order to determine what sort of nucleosynthesis may occur. Since there is a large flux of neutrinos leaving the surface of the disk, the electron fraction of the outflowing material will change due to charged current neutrino interactions. We calculate the electron fraction in the wind using detailed neutrino fluxes from every point on the disk and study a range of trajectories and outflow conditions for several different accretion disk models. We find that low electron fractions, conducive to making r-process elements, only appear in outflows from disks with high accretion rates that have a significant region both of trapped neutrinos and antineutrinos. Disks with lower accretion rates that have only a significant region of trapped neutrinos can have outflows with very high electron fractions, whereas the lowest accretion rate disks with little trapping have outflow electrons fractions of closer to one half.Comment: 11 pages, 10 figure

A bright millisecond radio burst of extragalactic origin

Pulsar surveys offer one of the few opportunities to monitor even a small fraction (~0.00001) of the radio sky for impulsive burst-like events with millisecond durations. In analysis of archival survey data, we have discovered a 30-Jy dispersed burst of duration <5 ms located three degrees from the Small Magellanic Cloud. The burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. Current models for the free electron content in the Universe imply a distance to the burst of <1 Gpc No further bursts are seen in 90-hr of additional observations, implying that it was a singular event such as a supernova or coalescence of relativistic objects. Hundreds of similar events could occur every day and act as insightful cosmological probes.Comment: 18 pages, 4 figures. Accepted by Science. Published electronically via Science Express on September 27, 200