5,383 research outputs found
Breit-Wigner Enhancement of Dark Matter Annihilation
We point out that annihilation of dark matter in the galactic halo can be
enhanced relative to that in the early universe due to a Breit-Wigner tail, if
the dark matter annihilates through a pole just below the threshold. This
provides a new explanation to the "boost factor" which is suggested by the
recent data of the PAMELA, ATIC and PPB-BETS cosmic-ray experiments.Comment: 4 pages, 4 figure
Numerical Study on GRB-Jet Formation in Collapsars
Two-dimensional magnetohydrodynamic simulations are performed using the
ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB
jet, taking account of realistic equation of state, neutrino cooling and
heating processes, magnetic fields, and gravitational force from the central
black hole and self gravity. It is found that neutrino heating processes are
not so efficient to launch a jet in this study. It is also found that a jet is
launched mainly by B_\phi fields that are amplified by the winding-up effect.
However, since the ratio of total energy relative to the rest mass energy in
the jet is not so high as several hundred, we conclude that the jets seen in
this study are not be a GRB jet. This result suggests that general relativistic
effects, which are not included in this study, will be important to generate a
GRB jet. Also, the accretion disk with magnetic fields may still play an
important role to launch a GRB jet, although a simulation for much longer
physical time (\sim 10-100 s) is required to confirm this effect. It is shown
that considerable amount of 56Ni is synthesized in the accretion disk. Thus
there will be a possibility for the accretion disk to supply sufficient amount
of 56Ni required to explain the luminosity of a hypernova. Also, it is shown
that neutron-rich matter due to electron captures with high entropy per baryon
is ejected along the polar axis. Moreover, it is found that the electron
fraction becomes larger than 0.5 around the polar axis near the black hole by
\nu_e capture at the region. Thus there will be a possibility that r-process
and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons
will be ejected from the jet, which suggests that signals from the neutron
decays may be observed as the delayed bump of afterglow or gamma-rays.Comment: 54 pages with 19 postscript figures. Accepted for publication in ApJ.
High resolution version is available at
http://www2.yukawa.kyoto-u.ac.jp/~nagataki/collapsar.pd
Explosive Nucleosynthesis in GRB Jets Accompanied by Hypernovae
Two-dimensional hydrodynamic simulations are performed to investigate
explosive nucleosynthesis in a collapsar using the model of MacFadyen and
Woosley (1999). It is shown that 56Ni is not produced in the jet of the
collapsar sufficiently to explain the observed amount of a hypernova when the
duration of the explosion is \sim 10 sec, which is considered to be the typical
timescale of explosion in the collapsar model. Even though a considerable
amount of 56Ni is synthesized if all explosion energy is deposited initially,
the opening angles of the jets become too wide to realize highly relativistic
outflows and gamma-ray bursts in such a case. From these results, it is
concluded that the origin of 56Ni in hypernovae associated with GRBs is not the
explosive nucleosynthesis in the jet. We consider that the idea that the origin
is the explosive nucleosynthesis in the accretion disk is more promising. We
also show that the explosion becomes bi-polar naturally due to the effect of
the deformed progenitor. This fact suggests that the 56Ni synthesized in the
accretion disk and conveyed as outflows are blown along to the rotation axis,
which will explain the line features of SN 1998bw and double peaked line
features of SN 2003jd. Some fraction of the gamma-ray lines from 56Ni decays in
the jet will appear without losing their energies because the jet becomes
optically thin before a considerable amount of 56Ni decays as long as the jet
is a relativistic flow. We show that abundance of nuclei whose mass number \sim
40 in the ejecta depends sensitively on the energy deposition rate. So it may
be determined by observations of chemical composition in metal poor stars which
model is the proper one as a model of a gamma-ray burst accompanied by a
hypernova.Comment: 29 pages with 16 figures. ApJ, accepte
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