10,082 research outputs found
Penetration of Antarctic subglacial lakes by VHF electromagnetic pulses: Information on the depth and electrical conductivity of basal water bodies
Comment on "A numerical model for an alternative origin of lake vostok and its exobiological implications for Mars'' by N. S. Duxbury, I. A. Zotikov, K. H. Nealson, V. E. Romanovsky, and F. D. Carsey
Positron Annihilation in the Milky Way and beyond
The electron-positron annihilation gamma-ray signal at 511 keV in the Milky
Way is investigated towards a possible dark matter interpretation. If all bulge
positrons were created by dark matter particle annihilation, the satellite
galaxies of the Milky Way, apparently being dominated by dark matter, should
also show measurable 511 keV signals. Using INTEGRAL/SPI, we test for emission
in 39 neighbouring dwarf satellite galaxies, and found a consistent trend
against a dark matter scenario. One galaxy, Reticulum II, shows up as a strong
source of annihilation emission, which we interpret as the presence of a
microquasar, ejecting pair-plasma into the galaxy's interstellar medium.Comment: 9 pages, 3 figures, accepted contribution to the 11th INTEGRAL
Conference, "Gamma-Ray Astrophysics in Multi-Wavelength Perspective", 10-14
October 2016, Amsterdam, The Netherland
Gamma-ray lines from SN2014J
On 21 January 2014, SN2014J was discovered in M82 and found to be the closest
type Ia supernova (SN Ia) in the last four decades. INTEGRAL observed SN2014J
from the end of January until late June for a total exposure time of about 7
Ms. SNe Ia light curves are understood to be powered by the radioactive decay
of iron peak elements of which Ni is dominantly synthesized during the
thermonuclear disruption of a CO white dwarf (WD). The measurement of
-ray lines from the decay chain
NiCoFe provides unique
information about the explosion in supernovae. Canonical models assume
Ni buried deeply in the supernova cloud, absorbing most of the early
-rays, and only the consecutive decay of Co should become
directly observable through the overlaying material several weeks after the
explosion when the supernova envelope dilutes as it expands. Surprisingly, with
the spectrometer on INTEGRAL, SPI, we detected Ni -ray lines at
158 and 812 keV at early times with flux levels corresponding to roughly 10% of
the total expected amount of Ni, and at relatively small velocities.
This implies some mechanism to create a major amout of Ni at the
outskirts, and at the same time to break the spherical symmetry of the
supernova. One plausible explanation would be a belt accreted from a He
companion star, exploding, and triggering the explosion of the white dwarf. The
full set of observations of SN2014J show Co -ray lines at 847
and 1238 keV, and we determine for the first time a SN Ia -ray light
curve. The irregular appearance of these -ray lines allows deeper
insights about the explosion morphology from its temporal evolution and
provides additional evidence for an asymmetric explosion, from our
high-resolution spectroscopy and comparisons with recent models.Comment: 12 pages, 15 figures, 10th INTEGRAL Workshop: "A Synergistic View of
the High Energy Sky" - Integral2014, 15-19 September 2014, Annapolis, MD, US
NLO QCD predictions for + jets production with Sherpa
We present precise predictions for prompt photon production in association
with a boson and jets. They are obtained within the Sherpa framework as a
consistently merged inclusive sample. Leptonic decays of the boson are
fully included in the calculation with all offshell effects. Virtual matrix
elements are provided by OpenLoops and parton shower effects are simulated with
a dipole parton shower. Thanks to the NLO QCD corrections included not only for
inclusive production but also for the + 1-jet process we
find significantly reduced systematic uncertainties and very good agreement
with experimental measurements at TeV. Predictions at
TeV are displayed including a study of theoretical uncertainties.
In view of an application of these simulations within LHC experiments, we
discuss in detail the necessary combination with a simulation of the + jets
final state. In addition to a corresponding prescription we introduce
recommended cross checks to avoid common pitfalls during the overlap removal
between the two samples.Comment: 20 pages, 15 Figure
Digital image processing of optical density wave propagation in Dictyostelium discoideum and analysis of the effects of caffeine and ammonia
Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum were analyzed by digital image processing in a manner that immediately shows the following parameters: wave propagation velocity, period length, wave amplitude und wave shape. We have characterized the aggregation of AX-2 and the streamer F mutant NP 377 in terms of these parameters and investigated the influence of caffeine and ammonia. It was found that during normal aggregation oscillation frequency increases while at the same time wave propagation velocity decreases. Caffeine, a known inhibitor of cyclic AMP relay, reduces oscillation frequency and wave propagation velocity in a dose-dependent manner but most notably leads to the appearance of bimodal (harmonic) oscillations. These bimodal waves are also found in streamer F mutants without caffeine during early aggregation. The effect of caffeine is interpreted as an increase in the average chemotactic deadaptation time due to elevated cyclic GMP levels after a cyclic AMP stimulus. This increased deadaptation time results in some cells responding to every chemotactic signal, while others respond only to every second signal, leading to mixed population behavior and hence biphasic optical density waves. Ammonia has no significant influence on oscillation frequency and wave propagation velocity but shows a clear increase in the amplitude of the optical density waves. This may indicate a more vigorous chemotactic response by individual cells or a better synchronization of the responding cell populations due to shortened chemotactic deadaptation times
Unstable Growth and Coarsening in Molecular-Beam Epitaxy
The coarsening dynamics of three-dimensional islands on a growing film is
discussed. It is assumed that the origin of the initial instability of a planar
surface is the Ehrlich-Schwoebel step-edge barrier for adatom diffusion. Two
mechanisms of coarsening are identified: (i) surface diffusion driven by an
uneven distribution of bonding energies, and (ii) mound coalescence driven by
random deposition. Semiquantitative estimates of the coarsening time are given
in each case. When the surface slope saturates, an asymptotic dynamical
exponent is obtained.Comment: 12 pages, 4 figure
- …