INTERMITTENT MILLISECOND X-RAY PULSATIONS FROM THE NEUTRON STAR X-RAY TRANSIENT SAX J1748.9-2021 IN THE GLOBULAR CLUSTER NGC 6440

摘要

We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron star X-ray binary SAX J1748.9-2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type I X-ray bursts and the appearance of the pulsations, but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1-2455. The reason for the intermittence of the pulsations remains unclear. However, it is now evident that a strict division between pulsating and nonpulsating neutron star systems does not exist. By studying the Doppler shift of the pulsation frequency we determine an orbit with a period of 8.7 hr and a projected semimajor axis of 0.39 lt-s. The companion star might be a main-sequence or a slightly evolved star with a mass of ～1 M_☉. Therefore, SAX J1748.9—2021 has a longer period and may have a more massive companion star than all the other accreting millisecond X-ray pulsars except for Aql X-1.