Somewhere in the depths of the Milky Way, radio pulses flare up from time to time — rare, steady, like the ticks of a distant metronome. These long-period radio transients were discovered only recently, and they immediately puzzled astronomers. One hypothesis is that we are looking at a binary system where a compact object (say, a white dwarf or a neutron star) orbits a normal star and, once per orbital period, sends a narrow beam of radio emission our way.
If that is the case, the period of the radio bursts exactly matches the orbital period. Consequently, the system should emit gravitational waves, ripples in spacetime, at millihertz frequencies. Such ripples could be detected by the future space observatory LISA (a laser interferometer with arms 2.5 million kilometers long), scheduled for launch in the next decade.
The authors built a plausible population of such sources, based on known radio data and on models of similar systems — cataclysmic variables. They then estimated how many of these LISA could 'hear' over four years of operation. The numbers came out as follows: between 0.05 and 6 percent, with the spread depending on how optimistically one views the properties of the transients themselves.
For those systems that fall within the detector's reach, LISA could measure the gravitational signal's frequency to one part in a hundred thousand, the amplitude to within a factor of two, and the position on the sky to within 30 square degrees. That is roughly the area covered by an outstretched hand with fingers spread.
In this way, gravitational-wave astronomy becomes a tool for navigating the radio sky. Knowing the precise period and position from LISA, radio astronomers can point their antennas at the right time and the right spot. The metronome ceases to be mysterious: it becomes predictable.
The gravitational-wave frequency from such binary systems is thousands of times lower than the lowest sound the human ear can detect.