Einstein@Home All-sky “Bucket” Search for Continuous Gravitational Waves in LIGO O3a Public Data

Abstract We present results from the most sensitive all-sky search to date for continuous gravitational waves with frequencies 30.0 Hz ≤ f ≤ 250.0 Hz and frequency derivatives − 2.7 × 1 0 − 9 Hz s − 1 ≤ f ̇ ≤ 0.2 × 1 0 − 9 Hz s −1 . We deploy this search on the Einstein@Home volunteer-computing project and on three supercomputer clusters. At the end of a multistage approach there are four surviving candidates: three from “hardware injections,” i.e., signals “added” by moving the instruments’ mirrors, and one due to line disturbances in the data. The high sensitivity of our search enabled the first-ever detection of hardware injection 11. 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 × 10 −26 , at the 90% confidence level, which improve by about 70% with respect to the LIGO-Virgo-KAGRA Collaboration’s most stringent O3 upper limits, and might well be competitive with results from O4 searches.