Transient Sources in the Galactic Plane : First result of the ASCA Galactic Plane Survey Project

The only systematic attempts to search for galactic transient sources have been made with non-imaging instruments. This limited the survey sensitivity; typically sources with luminosity less than 10^{-11} erg/sec/cm^2 are below the detection limit. Hard X-ray imaging instruments, like ASCA, can greatly improve the sensitivity such that below 10^{-13} erg/sec/cm^2. Since the luminosity in quiescent state of neutron star binaries are generally found to be 10^{32-33} erg/s (10^{-12-13} erg/s/cm^2 at 3 kpc), this improved sensitivity would be potentially important to pin-point the on-set of the activity. This paper report the first result of the transient search as one of the subjects of the ASCA plane survey project, demonstrate the capability and discuss the nature of newly discovered transients as well as possible inferences given from the first results.