Einstein@Home all-sky "bucket" search for continuous gravitational waves in LIGO O3 public data

We conduct an all-sky search for continuous gravitational waves using LIGO O3 public data from the Hanford and Livingston detectors. We search for nearly-monochromatic signals with frequencies $30\, \text{Hz} \leq f \leq 250\, \text{Hz}$ and spin-down $-2.7 \times 10^{-9}\, \text{Hz/s} \leq \dot{f} \leq 0.2 \times 10^{-9}\, \text{Hz/s}$. We deploy this search on the Einstein@Home volunteer-computing project and on three super computer clusters; the Atlas supercomputer at the Max Planck Institute for Gravitational Physics, and the two high performance computing systems Raven and Viper at the Max Planck Computing and Data Facility. Our results are consistent with a non-detection. We set upper limits on the gravitational wave amplitude $h_{0}$, and translate these to upper limits on the neutron star ellipticity and on the r-mode amplitude. The most stringent upper limits are at 173 Hz with $h_{0} = 6.5\times 10^{-26}$, at the 90% confidence level.