428 research outputs found
The Surprising Crab Nebula
We will present our study of the flux and spectral variability of the Crab
above 100 MeV on different timescales ranging from days to weeks. In addition
to the four main intense and day-long flares detected by AGILE and Fermi-LAT
between Sept. 2007 and Sept. 2012, we find evidence for week-long and less
intense episodes of enhanced gamma-ray emission that we call "waves".
Statistically significant "waves" show timescales of 1-2 weeks, and can occur
by themselves or in association with shorter flares. The Sept. - Oct. 2007
gamma-ray enhancement episode detected by AGILE shows both "wave" and flaring
behavior. We extend our analysis to the publicly available Fermi-LAT dataset
and show that several additional "wave" episodes can be identified. We discuss
the spectral properties of the September 2007 "wave"/flare event and show that
the physical properties of the "waves" are intermediate between steady and
flaring states. Plasma instabilities inducing "waves" appear to involve spatial
distances cm and enhanced magnetic fields }mG. Day-long flares are characterized by smaller distances and larger
local magnetic fields. Typically, the deduced total energy associated with the
"wave" phenomenon (, where is the kinetic
energy of the emitting particles) is comparable with that associated to the
flares, and can reach a few percent of the total available pulsar spindown
energy. Most likely, flares and waves are the product of the same class of
plasma instabilities that we show acting on different timescales and radiation
intensities.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Quasar Evolution Driven by Galaxy Encounters in Hierarchical Structures
We link the evolution of the galaxies in the hierarchical clustering scenario
with the changing accretion rates of cold gas onto the central massive black
holes that power the quasars. We base on galaxy interactions as main triggers
of accretion; the related scaling laws are taken up from Cavaliere & Vittorini
(2000), and grafted to a semi-analytic code for galaxy formation. As a result,
at high the protogalaxies grow rapidly by hierarchical merging; meanwhile,
much fresh gas is imported and also destabilized, so the holes are fueled at
their full Eddington rates. At lower the galactic dynamical events are
mostly encounters in hierarchically growing groups; now the refueling peters
out, as the residual gas is exhausted while the destabilizing encounters
dwindle. So, with no parameter tuning other than needed for stellar
observables, our model uniquely produces at a rise, and at a decline of the bright quasar population as steep as observed. In addition,
our results closely fit the observed luminosity functions of quasars, their
space density at different magnitudes from to , and
the local relation.Comment: 5 pages. Accepted for publication in ApJ Letter
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