Evidence for coherent emission with a 12 millisecond period during a burst from MXB 1728-34

Four bursts of MXB 1728--34 have been observed with the HEAO A-1 instrument using 5 ms timing resolution. The rapid initial rise has been resolved in all four bursts. Fourier analysis indicates the presence of a periodic component with a 12.2 ms period during one of the bursts. Epoch-folding analysis confirms this period and further indicates that the period decreases with time, with dP/dt = 10/sup -6/. The signal has a probability of 2.5 x 10/sup -9/ to occur at random, and an expectation of 3 x 10/sup -3/ to occur in the number of epoch folding performed.