16,801 research outputs found
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
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