Multi-messenger astronomy with continuous gravitational waves

Multi-messenger astronomy is an umbrella term for using different forms of energy at different frequencies – light (radio, UV, optical, etc.) and gravitational waves) – to observe the same source. This field became massively intereting after GW170817, the first inspiral of two neutron stars ever detected. The signal lasted for ~100 seconds in-band, and resulted in an array of constraints on fundamental physics, as well as provided explanations for the production of heavy elements and some information about what happens after the merger of two neutron stars.

However, there was not much time to warn astronomers that this merger was imminent, since LIGO and Virgo are only begin being sensitive to these systems when they are sufficiently close to merger (revolving around each other at around 10 Hz). In the future, however, new detectors, such as Einstein Telescope, will come online, and be sensitive to oribital frequencies of around 1 Hz, which means that signals will stay in-band for much longer. Such long signals will then allow us to both localize the source, since we can use the relative motion of the earth w.r.t. the source, and warn astronomers well in advance of when, and possibly where, the system will merge. To that end, we have shown that astronmers could be warned hours in advance, depending on the distance from the source, with decent sky localizations. However, there is much work to be done to deal with realistic problems that we will face in the future: e.g. (1) the presence of many overlapping signals, (2) the estimation of the noise background, (3) the degree to which we can localize, and how much time we require for this, (4) determine spins and tidal deformability will greatly affect the phase evolution at low frequencies, and (5) whether we can achieve the same sensitivity of our method as matched filtering through the clever use of lower thresholds on our detection statistics.