Astro-C, renamed Ginga (Japanese for 'galaxy') when it reached orbit, was launched from the Kagoshima Space Center on 5 February 1987. It was put into an orbit with an orbital period of ~96 minutes. Ginga was the third Japanese X-ray astronomy mission, following Hakucho and Tenma. It had two main science objectives:
- Study the time variability on timescales of milliseconds to years for all types of X-ray sources.
- Study the spectra of sources.
After a highly successful mission, the Ginga satellite reentered the Earth's atmosphere on 1 November 1991.
Ginga was approximately 1000 x 1000 x 1550 mm in size. It weighed about 420 kg. There were 3 science instruments on board: a large area proportional counter (LAC), an all-sky monitor (ASM), and a gamma-ray burst detector (GBD).
The LAC was the main scientific instrument aboard Ginga. It was designed and built under a Japan-United Kingdom collaboration (ISAS, U. Tokyo, Nagoya U., U. Leicester, Rutherford Appleton Lab). The LAC consisted of 8 identical proportional counters, with a total effective area of ~ 4000 sq-cm. The instrument was sensitive to ~1-37 keV X-ray photons.
The ASM consisted of 2 identical gas proportional counters, and was sensitive to 1-20 keV X-rays. The aim of the ASM was to create an all-sky survey every 1-2 days to look for transient events (for alarm to the LAC) and to collect a long-term record for X-ray sources.
The purpose of the GBD was to detect gamma-ray bursts in the energy range 1 - 400 keV with a high energy resolution. The GBD could also operate as a radiation belt monitor for high particle backgrounds which could harm the other 2 experiments.
The Science Results
Ginga performed approximately 1000 observations of about 350 different objects. The observing program included all known classes of X-ray sources and produced many significant results. Some highlights are:
- The ASM discovered 2 very bright transients GS2023+338 and GS2000+25.
- The LAC found cyclotron features in several X-ray pulsars, such as 4U1538-522, V0332+53, and Cep X-4.
- A large number of pulsar transients in the galactic ridge were observed.
Published: February 1998
Text Reviewed: September 2018