Search for a Stochastic Gravitational-wave Background using a pair of Torsion-bar Antennas

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We have set a new upper limit on the stochastic gravitational wave background (SGWB) using two prototype Torsion-bar Antennas (TOBAs). TOBA is a low-frequency gravitational-wave detector with bar-shaped test masses rotated by the tidal force of gravitational waves. As a result of simultaneous 7-hour observations with TOBAs in Tokyo and Kyoto in Japan, our upper limit with a confidence level of 95% is Ω gwh₀² < 1.9 × 10¹⁷ at 0.035 - 0.830 Hz, where h₀ is the Hubble constant in units of 100 km/s/Mpc and Ω gw is the gravitational wave energy density per logarithmic frequency interval in units of the closure density. We successfully updated the upper limit and extended the explored frequency band.