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HETE J1900.1--2455 is a peculiar accreting millisecond X-ray pulsar (AMXP)because it has shown intermittent pulsations after 22 days from the beginningof its outburst. The origin of intermittent pulses in accreting systems remainsto be understood. To better investigate the phenomenon of intermittentpulsations here we present an analysis of 7 years of X-ray data collected withthe Rossi X-Ray Timing Explorer and focus on the aperiodic variability. We showthat the power spectral components follow the same frequency correlations asthe non-pulsating atoll sources. We also study the known kHz QPO and we showthat it reaches a frequency of up to approximately 900 Hz, which is the highestfrequency observed for any kHz QPO in an AMXP. We also report the discovery ofa new kHz QPO at ~500 Hz. Finally, we discuss in further detail the known pulsephase drift observed in this source, which so far has no explanation. Weinterpret the behavior of the aperiodic variability, the high frequency of the900 kHz QPO and the presence of the pulse drift as three independent pieces ofevidence for a very weak neutron star magnetosphere in HETE J1900.1--2455.

Further Evidence for a Weak Neutron Star Magnetosphere in the Accreting Millisecond X-Ray Pulsar HETE J1900.1-2455