  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 magnetic fields in dSph
galaxies exist, the particles produced in DM annihilation emit synchrotron
radiation. 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. We used observations of six dSph galaxies in the LOFAR Two-metre Sky
Survey (LoTSS). The data were reimaged and radial profiles were generated. 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 2$\sigma$ significance in the radial profile.
These sources represent the lowest emission we would have been able to detect.
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 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 $\gamma$-ray observations of multiple dwarf
galaxies, and they are especially strong for low WIMP masses.
