Torsion-Bar Antenna for Low-Frequency Gravitational-Wave Observations

We propose a novel type of gravitational-wave antenna, formed by two bar-shaped test masses and laser-interferometric sensors to monitor their differential angular fluctuations. This antenna has a fundamental sensitivity to low-frequency signals below 1 Hz, even with a ground-based configuration. In addition, it is possible to expand the observation band to a lower limit determined by the observation time, by using modulation and up-conversion of gravitational-wave signals by rotation of the antenna. The potential sensitivity of this antenna is superior to those of current detectors in a 1 mHz–10 Hz frequency band and is sufficient for observations of gravitational waves radiated from in-spiral and merger events of intermediate-mass black holes.

Figure 1

Principle of a torsion-bar antenna. Two orthogonal bars feel differential torques by incident gravitational waves.

Figure 2

Estimated sensitivity of a torsion-bar antenna (thick black curve). Limitations by fundamental noises are shown together.

Figure 3

Comparison of sensitivities in $h_c$, the characteristic amplitude of GWs defined as an amplitude with a bandwidth equal to the observation frequency [1]. The sensitivity of the TOBA is represented by a bold curve. Thick gray curves correspond to the current detectors [3, 4, 5, 21], and the dotted curves correspond to designs of future projects [6, 7, 8, 9, 10, 11, 12].

Figure 4

Estimated SNR for a merger of equal-mass black holes with equal and parallel spin parameters of 0.5 [19], shown as a function of the total mass and the luminosity distance.