Upper limit on gravitational wave backgrounds at 0.2 Hz with a torsion-bar antenna.

We present the first upper limit on gravitational wave (GW) backgrounds at an unexplored frequency of 0.2 Hz using a torsion-bar antenna (TOBA). A TOBA was proposed to search for low-frequency GWs. We have developed a small-scaled TOBA and successfully found Ω(gw)(f)<4.3×10(17) at 0.2 Hz as demonstration of the TOBA's capabilities, where Ω(gw)(f) is the GW energy density per logarithmic frequency interval in units of the closure density. Our result is the first nonintegrated limit to bridge the gap between the LIGO band (around 100 Hz) and the Cassini band (10(-6)-10(-4) Hz).