  IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs)
discovered so far. We report on the aperiodic timing and color analysis of its
X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top
noise and two harmonically related quasi-periodic oscillations, all of them at
very low frequencies (0.01-0.1 Hz), were present in the power spectra during
most of the outburst as well as a very high fractional variability (~50%).
These properties are atypical not only for AMPs but also for neutron star
low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable
similarities with the variability observed in black hole systems, reinforcing
the connections between these two types of LMXB, as well as some interesting
differences. We note finally that the results of this paper are difficult to
reconcile with interpretations where any break frequency of power density
spectra scales inversely with the mass of the central object at an accuracy
sufficient to distinguish between the masses of neutron stars and black holes
in LMXBs.

Astrophysics