  Dying pulse trains (DPTs) pulses of radiation with decreasing intensity and
decreasing intervals between them are predicted by General Relativity to occur
from material spiraling into an event horizon after detaching from the last
stable orbit in an accretion disk around a black hole. Two events resembling
DPTs were detected in 3 hours observation of Cyg X-1 in the far UV using the
High Speed Photometer on the Hubble Space Telescope (Dolan 2001). We observed
Cyg X-1, a leading candidate for a black hole, with the proportional counter
array on RXTE to seek such events in the low-energy X-ray region. No dying
pulse trains with a characteristic timescale between pulses of 1 - 40 ms were
detected in 10 hours of observation during Cyg X-1s high luminosity state, low
luminosity state, and transitions between states, although individual pulses
are clearly detectable in data with 1 ms temporal resolution. The 2 sigma upper
limit on the rate of DPTs in the X-ray region is less than half the rate
reported by Dolan (2001) in the UV. These negative results are consistent with
Cyg X-1 being an extreme Kerr black hole with a characteristic timescale
between DPT pulses less than 1 ms.
