Intermittent Pulsations in an Accretion-powered Millisecond Pulsar

We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455, made with the Rossi X-Ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of ≈10 days. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, nonpulsing low-mass X-ray binary (LMXB); we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars.