130 research outputs found
The Production of Ultra High Energy Cosmic Rays during the Early Epochs of Radio-loud AGN
Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets
have been theoretically motivated as emitters of high-energy cosmic rays.
Recent radio observations have established a populous class of young radio-loud
galaxies with compact ( kpc) symmetric jets that are morphologically
similar to large-scale AGNs. We show that these compact AGNs, so-called compact
symmetric objects (CSOs), can accelerate protons up to eV at their
hot spots via a Fermi type mechanism on the assumption of efficient
acceleration. The required magnetic field strengths are comparable to those
derived from the minimum energy condition. We further show that the accelerated
protons can escape through the photon fields of the cocoon without significant
energy loss. However, the local number density of powerful CSOs is insufficient
for CSOs to power the entire observed flux of ultra-high-energy cosmic rays,
providing maximally only a few percent. A heavy composition of UHECRs allows
more CSOs to accelerate particles to UHECR energies, but escaping the cocoon is
difficult. We comment on a method that may test CSOs as UHECR sources.Comment: 7 pages, 3 figure
High-energy neutrinos from reverse shocks in choked and successful relativistic jets
Highly relativistic jets are a key element of current gamma-ray burst models,
where the jet kinetic energy is converted to radiation energy at optically thin
shocks. High-energy neutrinos are also expected, from interactions of protons
accelerated in the same shocks. Here we revisit the early evolution of a
relativistic jet, while the jet is still inside the star, and investigate its
neutrino emission. In particular we study propagation of mildly relativistic
and ultrarelativistic jets through a type Ib progenitor, and follow reverse
shocks as the jets cross the star. We show that protons can be accelerated to
10^4-10^5 GeV at reverse shocks, and efficiently produce mesons. The mesons
experience significant cooling, suppressing subsequent neutrino emission. We
show, however, that the neutrino yield from the reverse shock is still
reasonably large, especially for low-luminosity and long-duration jets, where
meson cooling is less severe. We discuss implications of our results in the
context of neutrinos from choked jets, which are completely shock heated and do
not break out of the star. From a choked jet with isotropic equivalent energy
of 10^{53} erg at 10 Mpc, we expect ~20 neutrino events at IceCube.Comment: 11 pages, 7 figures, 2 tables; accepted for publication in Physical
Review
Investigating the Uniformity of the Excess Gamma rays towards the Galactic Center Region
We perform a composite likelihood analysis of subdivided regions within the
central of the Milky Way, with the aim of
characterizing the spectrum of the gamma-ray galactic center excess in regions
of varying galactocentric distance. Outside of the innermost few degrees, we
find that the radial profile of the excess is background-model dependent and
poorly constrained. The spectrum of the excess emission is observed to extend
upwards of 10 GeV outside in radius, but cuts off steeply between
10--20 GeV only in the innermost few degrees. If interpreted as a real feature
of the excess, this radial variation in the spectrum has important implications
for both astrophysical and dark matter interpretations of the galactic center
excess. Single-component dark matter annihilation models face challenges in
reproducing this variation; on the other hand, a population of unresolved
millisecond pulsars contributing both prompt and secondary inverse Compton
emission may be able to explain the spectrum as well as its spatial dependency.
We show that the expected differences in the photon-count distributions of a
smooth dark matter annihilation signal and an unresolved point source
population are an order of magnitude smaller than the fluctuations in residuals
after fitting the data, which implies that mismodeling is an important
systematic effect in point source analyses aimed at resolving the gamma-ray
excess.Comment: 27 pages, 9 figures. Matches accepted version: references added, typo
corrected in Sec. 4.2, some additional discussion added (results unchanged
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