Detectability of continuous gravitational waves from planetary-mass companions orbiting compact stars

TL;DR

This study assesses the potential for future gravitational wave detectors to observe signals from planetary-mass companions orbiting compact stars, identifying specific sources with promising detectability prospects.

Contribution

It provides the first detailed analysis of GW detectability from planetary-mass companions orbiting pulsars and white dwarfs, highlighting sources detectable by future observatories.

Findings

Fourteen sources achieve S/N rom 5 within four years.

Three sources have pulsar primaries; eleven have white dwarf primaries.

Detection is unlikely with near-term missions but possible with future detectors.

Abstract

Binary systems with ultrashort-period planetary-mass companions are expected to radiate continuous gravitational waves (GWs). However, earlier studies found that the detectability of such systems by the Laser Interferometer Space Antenna (LISA) is unlikely. In this study, we investigate the detectability of GWs from planetary-mass companions orbiting pulsars (PSRs) or white dwarfs (WDs) whose fundamental parameters, essential for calculating GW properties, have been measured. We compare the GW signals from our sample with the sensitivity curves of space-based GW detectors. We find that fourteen sources achieve a signal-to-noise ratio (\(\text{S/N}\)) of \(\gtrsim 5\) within four years of observations. Among these, three sources have PSR primaries (2S 0918-549 b, 4U 0513-40 b, and 4U 1543-62), and eleven systems possess WD primaries (BW Scl b, CP Eri b, CR Boo b, EF Eri b, GP Com b, GW Lib b, SDSS J0926+3624 b, SDSS J1507+5230 b, SMSS J1606-1000 b, SRGeJ0453 b, and WZ Sge b). We note that their detectability is less probable with near-term missions such as LISA, TianQin, and Taiji. Nevertheless, they could be detected by more advanced, future-generation observatories, such as the Deci-hertz Interferometer Gravitational wave Observatory (DECIGO) and the Big Bang Observer (BBO). This offers the potential to investigate the formation and evolution of ultrashort-period planetary-mass companions around compact stars through joint GW and electromagnetic surveys.