A deep search for radio pulsations from the 1.3 M$_{\odot}$ compact-object binary companion of young pulsar PSR J1906+0746

Abstract: Double pulsar systems offer unrivaled advantages for the study of both astrophysics and fundamental physics. But only one has been visible: PSR J0737$-$3039; and its component pulsar B has now rotated out of sight due to the general-relativistic effect of geodetic precession. We know, though, that these precession cycles can also pivot pulsars into sight, and that this precession occurs at similar strength in PSR J1906+0746. That source is a young, unrecycled radio pulsar, orbiting a compact object with mass $\sim $1.32 M$_{\odot}$. This work presents a renewed campaign to detect radio pulsations from this companion, two decades after the previous search. Two key reasons driving this reattempt are the possibility that the companion radio beam has since precessed into our line of sight, and the improved sensitivity now offered by the FAST radio telescope. In 28 deep observations, we did not detect a credible companion pulsar signal. After comparing the possible scenarios, we conclude the companion is still most likely a pulsar that is not pointing at us. We next present estimates for the sky covered by such systems throughout their precession cycle. We find that for most system geometries, the all-time beaming fraction is unity, i.e., observers in any direction can see the system at some point. We conclude it is still likely that PSR J1906+0746 will be visible as a double pulsar in the future.