Angular Signatures of Annihilating Dark Matter in the Cosmic Gamma-Ray Background

The extragalactic cosmic gamma-ray background (CGB) is an interesting channel to look for signatures of dark matter annihilation. In particular, besides the imprint in the energy spectrum, peculiar anisotropy patterns are expected compared to the case of a pure astrophysical origin of the CGB. We take into account the uncertainties in the dark matter clustering properties on sub-galactic scales, deriving two possible anisotropy scenarios. A clear dark matter angular signature is achieved when the annihilation signal receives only a moderate contribution from sub-galactic clumps and/or cuspy haloes. Experimentally, if galactic foregrounds systematics are efficiently kept under control, the angular differences are detectable with the forthcoming GLAST observatory, provided that the annihilation signal contributes to the CGB for a fraction >10-20%. If, instead, sub-galactic structures have a more prominent role, the astrophysical and dark matter anisotropies become degenerate, correspondingly diluting the DM signature. As complementary observables we also introduce the cross-correlation between surveys of galaxies and the CGB and the cross-correlation between different energy bands of the CGB and we find that they provide a further sensitive tool to detect the dark matter angular signatures.Comment: 13 pages, 8 figures; improved discussion; matches published versio

A simple and efficient solver for self-gravity in the DISPATCH astrophysical simulation framework

We describe a simple and effective algorithm for solving Poisson's equation in the context of self-gravity within the DISPATCH astrophysical fluid framework. The algorithm leverages the fact that DISPATCH stores multiple time slices and uses asynchronous time-stepping to produce a scheme that does not require any explicit global communication or sub-cycling, only the normal, local communication between patches and the iterative solution to Poisson's equation. We demonstrate that the implementation is suitable for both collections of patches of a single resolution and for hierarchies of adaptively resolved patches. Benchmarks are presented that demonstrate the accuracy, effectiveness and efficiency of the scheme.Comment: 10 pages, 2 figures, proceedings of the ASTRONUM 2017 conferenc

Clustering properties of ultrahigh energy cosmic rays and the search for their astrophysical sources

The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scales, from just above the experimental angular resolution up to medium scales and dipole anisotropies. We find that a global comparison of the two-point auto-correlation function of the data with the one of catalogues of potential sources is a powerful diagnostic tool. In particular, this method is far less sensitive to unknown deflections in magnetic fields than cross-correlation studies while keeping a strong discrimination power among source candidates. We illustrate these advantages by considering ordinary galaxies, gamma ray bursts and active galactic nuclei as possible sources. Already the sparse publicly available data suggest that the sources of UHECRs may be a strongly clustered sub-sample of galaxies or of active galactic nuclei. We present forecasts for various cases of source distributions which can be checked soon by the Pierre Auger Observatory.Comment: 11 pages, 8 figures, 4 tables; minor changes, matches published versio

From the CMF to the IMF: Beyond the Core-Collapse Model

Observations have indicated that the prestellar core mass function (CMF) is similar to the stellar initial mass function (IMF), except for an offset towards larger masses. This has led to the idea that there is a one-to-one relation between cores and stars, such that the whole stellar mass reservoir is contained in a gravitationally-bound prestellar core, as postulated by the core-collapse model, and assumed in recent theoretical models of the stellar IMF. We test the validity of this assumption by comparing the final mass of stars with the mass of their progenitor cores in a high-resolution star-formation simulation that generates a realistic IMF under physical conditions characteristic of observed molecular clouds. Using a definition of bound cores similar to previous works we obtain a CMF that converges with increasing numerical resolution. We find that the CMF and the IMF are closely related in a statistical sense only; for any individual star there is only a weak correlation between the progenitor core mass and the final stellar mass. In particular, for high mass stars only a small fraction of the final stellar mass comes from the progenitor core, and even for low mass stars the fraction is highly variable, with a median fraction of only about 50%. We conclude that the core-collapse scenario and related models for the origin of the IMF are incomplete. We also show that competitive accretion is not a viable alternative.Comment: 23 pages, 29 figures. Link to supplementary material and full Table 1: http://www.erda.dk/vgrid/core-mass-function/ . Submitted to MNRA

Kinetic modeling of particle acceleration in a solar null point reconnection region

The primary focus of this paper is on the particle acceleration mechanism in solar coronal three-dimensional reconnection null-point regions. Starting from a potential field extrapolation of a Solar and Heliospheric Observatory (SOHO) magnetogram taken on 2002 November 16, we first performed magnetohydrodynamics (MHD) simulations with horizontal motions observed by SOHO applied to the photospheric boundary of the computational box. After a build-up of electric current in the fan-plane of the null-point, a sub-section of the evolved MHD data was used as initial and boundary conditions for a kinetic particle-in-cell model of the plasma. We find that sub-relativistic electron acceleration is mainly driven by a systematic electric field in the current sheet. A non-thermal population of electrons with a power-law distribution in energy forms in the simulated pre-flare phase, featuring a power-law index of about -1.78. This work provides a first step towards bridging the gap between macroscopic scales on the order of hundreds of Mm and kinetic scales on the order of cm in the solar corona, and explains how to achieve such a cross-scale coupling by utilizing either physical modifications or (equivalent) modifications of the constants of nature. With their exceptionally high resolution - up to 135 billion particles and 3.5 billion grid cells of size 17.5 km - these simulations offer a new opportunity to study particle acceleration in solar-like settings.Comment: 18 pages, 12 figure

Modeling chemistry during star formation: Water deuteration in dynamic star-forming regions

Recent observations of the HDO/H$_2$O ratio toward protostars in isolated and clustered environments show an apparent dichotomy, where isolated sources show higher D/H ratios than clustered counterparts. Establishing which physical and chemical processes create this differentiation can provide insights into the chemical evolution of water during star formation and the chemical diversity during the star formation process and in young planetary systems. Methods: The evolution of water is modeled using 3D physicochemical models of a dynamic star-forming environment. The physical evolution during the protostellar collapse is described by tracer particles from a 3D MHD simulation of a molecular cloud region. Each particle trajectory is post-processed using RADMC-3D to calculate the temperature and radiation field. The chemical evolution is simulated using a three-phase grain-surface chemistry model and the results are compared with interferometric observations of H$_2$O, HDO, and D$_2$O in hot corinos toward low-mass protostars. Results: The physicochemical model reproduces the observed HDO/H$_2$O and D$_2$O/HDO ratios in hot corinos, but shows no correlation with cloud environment for similar identical conditions. The observed dichotomy in water D/H ratios requires variation in the initial conditions (e.g., the duration and temperature of the prestellar phase). Reproducing the observed D/H ratios in hot corinos requires a prestellar phase duration $t\sim$1-3 Myr and temperatures in the range $T \sim$ 10-20 K prior to collapse. This work demonstrates that the observed differentiation between clustered and isolated protostars stems from differences in the molecular cloud or prestellar core conditions and does not arise during the protostellar collapse itself.Comment: Accepted for publication in A&

Gravitational Instabilities in a proto-solar like disc I.: Dynamics and Chemistry

To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs

Looking the void in the eyes - the kSZ effect in LTB models

As an alternative explanation of the dimming of distant supernovae it has recently been advocated that we live in a special place in the Universe near the centre of a large void described by a Lemaitre-Tolman-Bondi (LTB) metric. The Universe is no longer homogeneous and isotropic and the apparent late time acceleration is actually a consequence of spatial gradients in the metric. If we did not live close to the centre of the void, we would have observed a Cosmic Microwave Background (CMB) dipole much larger than that allowed by observations. Hence, until now it has been argued, for the model to be consistent with observations, that by coincidence we happen to live very close to the centre of the void or we are moving towards it. However, even if we are at the centre of the void, we can observe distant galaxy clusters, which are off-centre. In their frame of reference there should be a large CMB dipole, which manifests itself observationally for us as a kinematic Sunyaev-Zeldovich (kSZ) effect. kSZ observations give far stronger constraints on the LTB model compared to other observational probes such as Type Ia Supernovae, the CMB, and baryon acoustic oscillations. We show that current observations of only 9 clusters with large error bars already rule out LTB models with void sizes greater than approximately 1.5 Gpc and a significant underdensity, and that near future kSZ surveys like the Atacama Cosmology Telescope, South Pole Telescope, APEX telescope, or the Planck satellite will be able to strongly rule out or confirm LTB models with giga parsec sized voids. On the other hand, if the LTB model is confirmed by observations, a kSZ survey gives a unique possibility of directly reconstructing the expansion rate and underdensity profile of the void.Comment: 20 pages, 9 figures, submitted to JCA

Probing Dark Energy Inhomogeneities with Supernovae

We discuss the possibility to identify anisotropic and/or inhomogeneous cosmological models using type Ia supernova data. A search for correlations in current type Ia peak magnitudes over a large range of angular scales yields a null result. However, the same analysis limited to supernovae at low redshift, shows a feeble anticorrelation at the two sigma level at angular scales of about 40 degrees. Upcoming data from, e.g., the SNLS (Supernova Legacy Survey) and the SDSS-II (SDSS: Sloan Digital Sky Survey) supernova searches will improve our limits on the size of - or possibly detect - possible correlations also at high redshift at the per cent level in the near future. With data from the proposed SNAP (SuperNova Acceleration Probe) satellite, we will be able to detect the induced correlations from gravitational lensing on type Ia peak magnitudes on scales less than a degree.Comment: 16 pages, 6 figures, matches the published version JCAP06(2008)02