Simultaneous optical and ROSAT X-ray observations of the classical T Tauri star BP Tauri

The classical T Tauri star BP Tauri has been simultaneously observed with UBVRI high-speed photometry at a time resolution of 2 sec and with the ROSAT PSPC detector during five nights. BP Tauri showed brightness variations on time scales ranging from nights to hours both in the optical and in the X-ray band, however, the night-to-night variations in the optical and X-ray spectral regions were not correlated. On one occasion, a short term optical event with an amplitude in U of similar to 0.05(m) and a time duration of 1.2 hours occurred, with no corresponding increase in the X-ray count rate during the decay of the event. In conclusion, the observations show that there are no detectable correlations between the optical and X-ray variability of BP Tau on time scales ranging from 1 hour to days. We discuss the possibility that the optical variability of BP Tau is related to accretion of circumstellar material onto the central star, while the X-ray emission presumably comes from magnetically active regions.