The first Japanese X-ray astronomy satellite, launched in 1979 February, known before launch as Corsa-B (Corsa-A was a launch failure). A spinning satellite, it carried proportional counters which viewed along the spin axis and studied the luminosities of X-ray bursts. The mission ended in 1985 April.

http://heasarc.gsfc.nasa.gov/docs/hakucho/hakucho.html Information page at Goddard Space Flight Center.