Some topics on the x-ray pulsars and the x-ray bursts observed by the satellite Hakucho

Several subjects are selected from observational results on X-ray pulsars and X-ray burst sources obtained by the Japanese X-ray astronomy satellite ``Hakucho.'' Results on Vela X-1 and A0535+262 which exhibited transient or time vaiable pulsar behavior are discussed. We are lead to speculate that the category of Class-II X-ray sources, represented by X-ray sources in globular clusters, very bright sources within 30° of the galactic center, GCX sources and soft transient sources, may be characterized by its potentiality for bursting whenever certain, yet unknown, conditions are fulfilled. The results on the X-ray burst may provide keys to evolve the thermonuclear flash model from the simplified original He-flash version to explain much involved dynamic processes in the neutron star atomsphere. Necessity of reservoirs at the neutron star to store matter as the nuclear fuel or the source of gravitational energy appears to be evident in order to explain observational facts of the bursts. Some of the difficulties associated with the model are growing to be definitive even to the extent that the thermonuclear flash model of the Type I burst may have to be rereviewed.