Limits on I-Band Microvariability of the Galactic Bulge Mira Variables

We search for microvariability in a sample of 485 Mira variables with high-quality I-band light curves from the second-generation Optical Gravitational Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1 mag range lasting hours to days were discovered in Hipparcos data by de Laverny et al. Our search is primarily sensitive to events with timescales of ~1 day but retains a few percent efficiency (per object) for detecting unresolved microvariability events as short as 2 hr. We do not detect any candidate events. Assuming that the distribution of the event time profiles is identical to that from the Hipparcos light curves, we derive a 95% confidence level upper limit of 0.038 yr-1 star-1 for the rate of such events (1 per 26 yr per average object of the ensemble). The high event rates of the order of ~1 yr-1 star-1 implied by the Hipparcos study in the HP band are excluded with high confidence by the OGLE-II data in the I band. Our nondetection could still be explained by much redder spectral response of the I filter compared to the HP band or by population differences between the bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide the first limit on the rate of the postulated microvariability events in Mira stars and offer new quantitative constraints on their properties. Similar limits are obtained for other pulse shapes and a range of the assumed timescales and size-frequency distributions.