Hierarchical follow-up of subthreshold candidates of an all-sky Einstein@Home search for continuous gravitational waves on LIGO sixth science run data

We report results of an all-sky search for periodic gravitational waves with frequency between 50 and 510 Hz from isolated compact objects, e.g., neutron stars. A new hierarchical multistage approach is taken, supported by the computing power of the Einstein@Home project, allowing us to probe more deeply than ever before. 16 million subthreshold candidates from the initial search [LIGO Scientific and Virgo Collaborations, Phys. Rev. D 94, 102002 (2016)] are followed up in four stages. None of those candidates is consistent with an isolated gravitational wave emitter, and 90% confidence level upper limits are placed on the amplitudes of continuous waves from the target population. Between 170.5 and 171 Hz, we set the most constraining 90% confidence upper limit on the strain amplitude ${h}_{0}$ at $4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$, while at the high end of our frequency range, we achieve an upper limit of $7.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$. These are the most constraining all-sky upper limits to date and constrain the ellipticity of rotating compact objects emitting at 300 Hz at a distance $D$ to less than $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ $[\frac{D}{100\text{ }\text{ }\mathrm{pc}}]$.