Discovery of Quasi‐periodic Oscillations in the AM Herculis Object BL Hydri

Abstract

We obtained high-speed optical photometry of the AM Her object BL Hyi at the Las Campanas Observatory and the Cerro Tololo Inter-American Observatory during 1989-1996. BL Hyi was in its faint-luminosity state in 1989; it subsequently brightened and was in its high-luminosity state for our 1994-1996 observations. We discovered broad, 0.2-0.8 Hz quasi-periodic oscillations (QPOs) and narrower QPOs superposed on the broad QPOs when BL Hyi was in its high-luminosity state. The broad QPOs had widths of Δf/fp ~ 0.5-1 and rms pulsed amplitudes of ~1%-4%, where fp is the frequency of the QPO peak. The narrow QPOs had widths of Δf/fp < 0.1 and rms pulsed amplitudes of less than 1%. BL Hyi showed stronger QPOs and was slightly brighter in 1994 than in 1995-1996. The amplitudes of the broad and narrow QPOs were modulated on the orbital period of the system; they were strongest during the bright orbital phase. BL Hyi is the fifth AM Her system to show the short-period QPOs discovered by Middleditch.