Accretion-powered Millisecond Pulsars -- Anomalous Pulse Phase Variations and Energy Dependent Phase Lag----中国科学院紫金山天文台

　　Low Mass X-ray Binaries (LMXBs) are considered to be the progenitor of the millisecond pulsars detected in the radio band. However,　 the attempts to measure coherent millisecond pulsation from an LMXB, which provides a direct evolutionary link between radio millisecond pulsar and LMXB, were unsuccessful until the discovery of the first accretion-powered millisecond pulsar SAX 1808.4-3658 in 1998. To date, coherent millisecond pulsations have been detected in ten LMXBs with spin periods ranging from 1.67 ms to 5.5 ms. Although the material accreting onto the neutron star through the magnetic field lines is from the inner part of the disk, unlike the regular accreting pulsar (hundreds of millisecond to several second), their fast rotation speeds and the low magnetic fields (10 8 -10 9 G) imply that their radii of the inner disk edges are only several factors larger than the neutron star. This different type of accretion can further improve our knowledge for X-ray binaries, as well as the nature of the neutron star. In this talk, I will introduce interesting discoveries from our studies for several of these accretion-powered millisecond pulsars, including the anomalous pulse phase variations and the implication from the abnormal energy dependent phase lag.