My Research

Abstract.

So far no diffuse emissions in dwarf spheroidal satellites of the Milky Way have ever been observed. Given that dwarf galaxies are predominantly composed of Dark Matter, the discovery of these signals could offer valuable insights into understanding the nature of Dark Matter. We present "diffSph", a Python tool which in its present version provides fast predictions of such diffuse signals in radio frequencies. It also features a very comprehensive module for the computation of "J" and "D" factors that are relevant for indirect Dark Matter detection using gamma rays. Routines are coupled to parton-shower algorithms and Dark Matter halo mass functions from state-of-the-art kinematic fits. This code is also useful for testing generic hypotheses (not necessarily associated with any Dark Matter candidate) about the cosmic-ray electron/positron sources in the dwarf galaxies. The diffSph tool has already been employed in searches for diffuse signals from dwarf spheroidal galaxies using the LOw Frequency ARray (LOFAR).

Abstract.

Context. Weakly interacting massive particles (WIMPs) can self-annihilate, thus providing us with a way to indirectly detect dark matter (DM). Dwarf spheroidal (dSph) galaxies are excellent places to search for annihilation signals because they are rich in DM and background emission is low. If Ο(0.1 – 10 μG) magnetic fields in dSph galaxies exist, the particles produced in DM annihilation emit synchrotron radiation in the radio band.

Aims. We used the non-detection of 150 MHz radio continuum emission from dSph galaxies with the LOw Frequency ARray (LOFAR) to derive constraints on the annihilation cross section of WIMPs in electron–positron pairs. Our main underlying assumption is that the transport of the cosmic rays can be described by the diffusion approximation, which necessitates the existence of magnetic fields.

Methods. We used observations of six dSph galaxies in the LOFAR Two-metre Sky Survey (LoTSS). The data were reimaged, and a radial profile was generated for each galaxy. We also used stacking to increase the sensitivity. In order to derive upper limits on the WIMP cross section, we injected fake Gaussian sources into the data, which were then detected with 2σ significance in the radial profile. These sources represent the lowest emission we would have been able to detect.

Results. We present limits from the observations of individual galaxies as well as from stacking. We explored the uncertainty due to the choice of diffusion and magnetic field parameters by constructing three different model scenarios: optimistic (OPT), intermediate (INT), and pessimistic (PES). Assuming monochromatic annihilation into electron–positron pairs, the limits from the INT scenario exclude thermal WIMPs (❬σv❭≈2.2 x 10^-26 cm³ s^-1)

below 20 GeV, and the limits from the OPT scenario even exclude thermal WIMPs below 70 GeV. The INT limits can compete with limits set by Fermi-LAT using γ-ray observations of multiple dwarf galaxies, and they are especially strong for low WIMP masses.

Abstract.

Constraining properties of Dark Matter particles, such as WIMPs, is one of the biggest tasks in physics during the last decades. There are many approaches for finding these limits, both of astrophysical and non-astrophysical nature. Astrophysical approaches are for example g-ray observations or more recently radio continuum observations. For this project, we use 144 MHz radio data from the LOFAR Two-metre Sky Survey (LoTSS) to analyze radio continuum emission from six dwarf spheroidal galaxies. More precisely, we use the non-detection of synchrotron emission that are related to the selfannihilation of WIMPs into primary cosmic-ray electrons and positrons to put upper limits on the self-annihilation cross-section. These cosmic rays interact with the magnetic fields in the Dark Matter halos of these galaxies. We assume that this process is driven only by diffusion and hence use typical values for the magnetic field strength (Ο(µG)) and the diffusion coefficient (Ο(10²⁷ cm²s^-1)). To constrain the self-annihilation cross-section of WIMPs, we insert a fake-source with a known flux density into the LOFAR (u,v)-data. Background emission from AGN and large-scale foreground emission from the Milky Way require us to pre-process the data before measuring radial intensity profiles of the dwarf galaxies. Fitting Gaussians to these profiles, that contain the inserted fake-source, together with the non-detection of WIMP related signals in the observational data enable us to put upper limits on the self-annihilation cross-section. We define three scenarios with different magnetic field strengths and diffusion coefficients to handle the uncertainties of both parameters. In the optimistic scenario, where WIMP signals are most visible, we use a magnetic field strength of 10 µG and a diffusion coefficient of 10²⁶ cm²s^-1. We find limits on the WIMP self-annihilation cross-section of ❬σv❭ ~ 10^-29 cm³s^-1 for particle masses of a few GeV. In the intermediate scenario, we use Milky Way-like values for both parameters with 1 µG and 10²⁷ cm²s^-1. We find limits of ❬σv❭ ~ 10^-27 cm³s^-1 for the same mass range. In the pessimistic scenario, we choose rather conservative values for the magnetic field strength and the diffusion coefficient with 0.1 µG and 10²⁹ cm²s^-1. We find limits on the self-annihilation cross-section of ❬σv❭ ~ 10^-23 cm³s^-1 for a particle mass of a few GeV. Compared to recent limits from Fermi-LAT, our limits in the intermediate scenario matches well for these particle masses. Our results from the optimistic scenario improves the limits from Fermi-LAT by a few order of magnitudes in the same mass range.

Abstract.

The most obvious properties of radiogalaxies are the bipolar jets made of relativistic plasma and their radiolobes that are made when the plasma hits the cluster environment. The synchrotron radiation that is emitted of the jets and lobes is linearly polarized which allows us to use the Faraday-effect to make conclusions about the magnetic fields along the line of sight. The aim of this thesis is to apply the Faraday-effect to the synchrotron radiation of a radiogalaxy to measure magnetic fields in the intergalactic medium and their influence on the plane of polarization. For that, LOFAR-data with a wavelength of 1.8 m to 2.5 m was used mainly. Furthermore, data of the VLA with a wavelength of 21 cm was used. To calculate the rotation measure (RM) of the northern and southern hotspot of the radiogalaxy, a Fourier-transformation of the linear polarization vector was made with the LOFAR-data. The difference in the RM of the hotspots is 1.9 rad m^-2. Subsequently, the contribution of the surrounding cluster environment to the RM difference was estimated to be smaller than 0.9 rad m^-2. Assuming a constant contribution to the RM from the Milky Way, we assign the remaining RM contribution to the intergalactic medium to be bigger than 1 rad m^-2. With this value, the field strength of the intergalactic magnetic field along the line of sight was calculated for two different situations. In the first situation, the whole intergalactic medium was filled with a voidlike structure with a electron density that is similar to the average electron density in the universe. The corresponding line-of-sight magnetic field is smaller than 1.2 µG. In the second situation, the reason for the difference is a magnetised filament of the cosmic web with a electron density that is a hundred times higher than the average electron density in the universe. For a total length of the filament of 1 Mpc, the corresponding magnetic field strength along the line of sight is smaller than 0.4 µG.