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 $^{56}$Ni is dominantly synthesized during the thermonuclear disruption of a CO white dwarf (WD). The measurement of $\gamma$-ray lines from the decay chain $^{56}$Ni$\rightarrow$$^{56}$Co$\rightarrow$$^{56}$Fe provides unique information about the explosion in supernovae. Canonical models assume $^{56}$Ni buried deeply in the supernova cloud, absorbing most of the early $\gamma$-rays, and only the consecutive decay of $^{56}$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 $^{56}$Ni $\gamma$-ray lines at 158 and 812 keV at early times with flux levels corresponding to roughly 10% of the total expected amount of $^{56}$Ni, and at relatively small velocities. This implies some mechanism to create a major amout of $^{56}$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 $^{56}$Co $\gamma$-ray lines at 847 and 1238 keV, and we determine for the first time a SN Ia $\gamma$-ray light curve. The irregular appearance of these $\gamma$-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 Z+γZ+\gamma + jets production with Sherpa

We present precise predictions for prompt photon production in association with a $Z$ boson and jets. They are obtained within the Sherpa framework as a consistently merged inclusive sample. Leptonic decays of the $Z$ 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 $Z\gamma$ production but also for the $Z\gamma$ + 1-jet process we find significantly reduced systematic uncertainties and very good agreement with experimental measurements at $\sqrt{s}=8$ TeV. Predictions at $\sqrt{s}=13$ 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 $Z$ + 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 $z=4$ is obtained.Comment: 12 pages, 4 figure