Search for continuous gravitational waves from unknown neutron stars in binary systems with long orbital periods in O3 data

Gravitational waves emitted by asymmetric rotating neutron stars are the primary targets of continuous gravitational-wave searches. Neutron stars in binary systems are particularly interesting due to the potential for non-axisymmetric deformations induced by a companion star. However, all-sky searches for unknown neutron stars in binary systems are very computationally expensive and this limits their sensitivity and/or breadth. In this paper we present results of a search for signals with gravitational-wave frequencies between $50$ and $150$~Hz, from systems with orbital periods between $100$ and $1000$ days and projected semi-major axes between $40$ and $200$~light-seconds. This parameter-space region has never been directly searched before. We do not detect any signal, and our results exclude gravitational-wave amplitudes above $1.25\times {10}^{-25}$ at $144.32$~Hz with $95\mathrm{\%}$ confidence. Our improved search pipeline is more sensitive than any previous all-sky binary search by about $75\mathrm{\%}$.