5,912 research outputs found
Ergonomics of the Operative Field in Paediatric Minimal Access Surgery
Imperial Users onl
Time evolution of broadband non-thermal emission from supernova remnants in different circumstellar environments
Supernova remnants (SNRs) are thought to be one of the major acceleration
sites of galactic cosmic rays (CRs) and an important class of objects for
high-energy astrophysics. SNRs produce multi-wavelength, non-thermal emission
via accelerated particles at collisionless shocks generated by the interactions
between the SN ejecta and the circumstellar medium (CSM). Although it is
expected that the rich diversities observed in supernovae (SNe) and their CSM
can result in distinct very-high-energy (VHE) electromagnetic signals in the
SNR phase, there are only a handful of SNRs observed in both GeV and TeV
gamma-rays so far. A systematic understanding of particle acceleration at SNRs
in different ambient environments is therefore limited. Here, we explore
non-thermal emission from SNRs in various circumstellar environments up to 5000
yrs from explosion using hydrodynamical simulations coupled with efficient
particle acceleration. We find that time-evolution of emission characteristics
in the VHE regime is mainly dictated by two factors; the number density of the
target particles and the amplified magnetic field in the shocked medium. We
also predict that Cherenkov Telescope Array (CTA) will have a sufficient
sensitivity to detect VHE gamma-rays from most young SNRs at distances <~ 5.0
kpc. Future SNR observations with CTA will thus be promising for probing the
CSM environment of SNe and hence their progenitor properties, including the
mass loss history of massive stars.Comment: 16 pages, 13 figures, 3 tables, accepted for publication in the Ap
Shock acceleration of electrons and synchrotron emission from the dynamical ejecta of neutron star mergers
Neutron star mergers (NSMs) eject energetic sub-relativistic dynamical ejecta
into the circumbinary media. As analogous to supernovae and supernova remnants,
the NSM dynamical ejecta are expected to produce non-thermal emission by
electrons accelerated at a shock wave. In this paper, we present expected radio
and X-ray signals by this mechanism, taking into account non-linear diffusive
shock acceleration (DSA) and magnetic field amplification. We suggest that the
NSM has a unique nature as a DSA site, where the seed relativistic electrons
are abundantly provided by the decays of r-process elements. The signal is
predicted to peak at a few 100 - 1,000 days after the merger, determined by the
balance between the decrease of the number of seed electrons and the increase
of the dissipated kinetic energy due to the shock expansion. While the
resulting flux can ideally reach to the maximum flux expected from
near-equipartition, the available kinetic energy dissipation rate of the NSM
ejecta limits the detectability of such a signal. It is likely that the radio
and X-ray emission are overwhelmed by other mechanisms (e.g., an off-axis jet)
for an observer placed to a jet direction (i.e., for GW170817). On the other
hand, for an off-axis observer, to be discovered once a number of NSMs are
identified, the dynamical ejecta component is predicted to dominate the
non-thermal emission. While the detection of this signal is challenging even
with near-future facilities, this potentially provides a robust probe of the
creation of r-process elements in NSMs.Comment: 7 pages, 7 figures, accepted for publication in The Astrophysical
Journa
Competing roughening mechanisms in strained heteroepitaxy: a fast kinetic Monte Carlo study
We study the morphological evolution of strained heteroepitaxial films using
kinetic Monte Carlo simulations in two dimensions. A novel Green's function
approach, analogous to boundary integral methods, is used to calculate elastic
energies efficiently. We observe island formation at low lattice misfit and
high temperature that is consistent with the Asaro-Tiller-Grinfeld instability
theory. At high misfit and low temperature, islands or pits form according to
the nucleation theory of Tersoff and LeGoues.Comment: 4 pages, 4 figures, ReVTe
Smart Pacing for Effective Online Ad Campaign Optimization
In targeted online advertising, advertisers look for maximizing campaign
performance under delivery constraint within budget schedule. Most of the
advertisers typically prefer to impose the delivery constraint to spend budget
smoothly over the time in order to reach a wider range of audiences and have a
sustainable impact. Since lots of impressions are traded through public
auctions for online advertising today, the liquidity makes price elasticity and
bid landscape between demand and supply change quite dynamically. Therefore, it
is challenging to perform smooth pacing control and maximize campaign
performance simultaneously. In this paper, we propose a smart pacing approach
in which the delivery pace of each campaign is learned from both offline and
online data to achieve smooth delivery and optimal performance goals. The
implementation of the proposed approach in a real DSP system is also presented.
Experimental evaluations on both real online ad campaigns and offline
simulations show that our approach can effectively improve campaign performance
and achieve delivery goals.Comment: KDD'15, August 10-13, 2015, Sydney, NSW, Australi
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