Abraham Achterberg

A description of the Galactic Center excess in the Minimal Supersymmetric Standard Model

Observations with the Fermi Large Area Telescope (LAT) indicate an excess in gamma rays originating from the center of our Galaxy. A possible explanation for this excess is the annihilation of Dark Matter particles. We have investigated the annihilation of neutralinos as Dark Matter candidates within the phenomenological Minimal Supersymmetric Standard Model (pMSSM). An iterative particle filter approach was used to search for solutions within the pMSSM. We found solutions that are consistent with astroparticle physics and collider experiments, and provide a reasonable fit to the energy spectrum of the excess. The neutralino is a Bino/Higgsino mixture and a mass in the range

Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

84-92

Relativistic AGN jets I. The delicate interplay between jet structure, cocoon morphology and jet-head propagation

~GeV yielding a Dark Matter relic density

Filaments in the southern giant lobe of Centaurus A: constraints on nature and origin from modelling and GMRT observations

0.06 < Omega h^2 <0.13

Simulations of Relativistic Astrophysical Flows

. These pMSSM solutions make clear forecasts for LHC, direct and indirect DM detection experiments. If the pMSSM explanation of the excess seen by Fermi-LAT is correct, a DM signal might be discovered soon.

Comparing Galactic Center MSSM dark matter solutions to the Reticulum II gamma-ray data

Observations of the FR I radio galaxy Centaurus A in radio, X-ray and gamma-ray bands provide evidence for lepton acceleration up to several TeV and clues about hadron acceleration to tens of EeV. Synthesising the available observational constraints on the physical conditions and particle content in the jets, inner lobes and giant lobes of Centaurus A, we aim to evaluate its feasibility as an ultra-high-energy cosmic-ray source. We apply several methods of determining jet power and affi rm the consistency of various power estimates of∼ 1× 10 43 erg s −1 . Employing scaling relations based on previous results for 3C 31, we estimate particle number densities in the jets, encompassing available radio throug h X-ray observations. Our model is compatible with the jets ingesting ∼ 3× 10 21 g s −1 of matter via external entrainment from hot gas and∼ 7× 10 22 g s −1 via internal entrainment from jet-contained stars. This leads to an imbalance between the internal lobe pressure available from radiating particles and magnetic fie ld, and our derived external pressure. Based on knowledge of the external environments of other FR I sources, we estimate the thermal pressure in the giant lobes as 1.5× 10 −12 dyn cm −2 , from which we deduce a lower limit to the temperature of∼ 1.6× 10 8 K. Using dynamical and buoyancy arguments, we infer∼ 440− 645 Myr and∼ 560 Myr as the sound-crossing and buoyancy ages of the giant lobes respectively, inconsistent with their spectral ages. We re -investigate the feasibility of particle acceleration via stochastic processes in the lobes, placing new constraints on the energetics and on turbulent input to the lobes. The same ‘very hot’ temperature s that allow self-consistency between the entrainment calculations and the missing pressure also allow stochastic UHECR acceleration models to work.

Relativistic AGN jets – II. Jet properties and mixing effects for episodic jet activity

Astrophysical jets reveal strong signs of radial structure. They suggest that the inner region of the jet, the jet spine, consists of a low-density, fast-moving gas, while the outer region of the jet consists of a more dense and slower moving gas, called the jet sheath. Moreover, if jets carry angular momentum, the resultant centrifugal forces lead to a radial stratification. Current observations are not able to fully resolve the radial structure, so little is known about its actual profile. We present three active galactic nuclei jet models in 2.5D of which two have been given a radial structure. The first model is a homogeneous jet, the only model that does not carry angular momentum; the second model is a spine-sheath jet with an isothermal equation of state; and the third jet model is a (piecewise) isochoric spine-sheath jet, with constant but different densities for jet spine and jet sheath. In this paper, we look at the effects of radial stratification on jet integrity, mixing between the different jet components and global morphology of the jet-head and surrounding cocoon. We consider steady jets that have been active for 23u2009Myr. All jets have developed the same number of strong internal shocks along their jet axis at the final time of simulation. These shocks arise when vortices are being shed by the jet-head. We find that all three jets maintain their stability all the way up to the jet-head. The isothermal jet maintains part of its structural integrity at the jet-head where the distinction between jet spine and jet sheath material can still be made. In this case, mixing between jet spine and jet sheath within the jet is fairly inefficient. The isochoric jet, on the other hand, loses its structural jet integrity fairly quickly after the jet is injected. At its jet-head, little structure is maintained and the central part of the jet predominantly consists of jet sheath material. In this case, jet spine and jet sheath material mix efficiently within the jet. We find that the propagation speed for all three models is less than expected from simple theoretical predictions. We propose this is due to an enlarged cross-section of the jet which impacts with the ambient medium. We show that in these models, the effective surface area is 16 times as large in the case of the homogeneous jet, 30 times as large in the case of the isochoric jet and can be up to 40 times as large in the case of the isothermal jet.

Implications of the Fermi-LAT Pass 8 Galactic Center Excess on Supersymmetric Dark Matter

We present results from imaging of the radio filaments in the s outhern giant lobe of Centaurus A using data from GMRT observations at 325 and 235 MHz, andoutcomes from filament modelling. The observations reveal a rich filamentary struc ture, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to th e jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressured with respect to the global lobe plasma showing no evidence of large-scale efficient Fermi I-type particle acceleration, and persist for � 2 3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318‐444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabil ities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic MHD turbulen ce or from a moderately recent jet activity of the parent AGN, or an interplay of both.

Conservative Formulation of the Fluid Equations

The multidimensional grid-adaptive magnetofluid dynamics code AMRVAC [1, 2] has been extended with special relativistic hydrodynamics and magnetohydrodynamics modules to simulate the complex and ultra-relativistic flow dynamics associated with astrophysical objects like Gamma Ray Bursts and Active Galactic Nuclei. The shock capturing numerical scheme and mesh refinement algorithm are crucial to resolve the strong shocks and wide range of length scales present in these flows. The first results look promising, we can simulate strongly relativistic blast waves, fireballs, and jets in 1D, 2D, and 3D.

Steady, Ideal Compressible Flows

Observations with the Fermi Large Area Telescope (LAT) indicate a possible small photon signal originating from the dwarf galaxy Reticulum II that exceeds the ex- pected background between 2GeV and 10GeV. We have investigated two specific scenarios for annihilating WIMP dark matter within the phenomenological Minimal Supersymmetric Standard Model (pMSSM) framework as a possible source for these photons. We find that the same parameter ranges in pMSSM as reported by an earlier paper to be consistent with the Galactic Center excess, are also consistent with the excess observed in Reticulum II, resulting in a J-factor of log10(J(�int = 0.5 ◦ )) ≃ (20.3 − 20.5) +0.2 −0.3 GeV 2 cm −5 . This J-factor is consistent with log 10 (J(�int = 0.5 ◦ )) = 19.6 +1.0 −0.7 GeV 2 cm −5 , which was derived using an optimized spherical Jeans analysis of kinematic data obtained from the Michigan/Magellan Fiber System (M2FS).