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New Findings in the 20th Year — Discovery of X-Ray Transients from the Ginga Archives
Today, we know that there are many X-ray objects in the universe, some of which vary their luminosities significantly. In fact, there are sources which are bright and observable only during short periods of time. Archival data records of such X-ray transients, when and where the X-ray sources appeared at which brightness, are extremely important. Also, when new sources are discovered, we can go back to the old data to investigate for their past activities, as long as the data are archived.
The Ginga satellite, launched in February 1987, observed many parts of the sky and discovered dozens of new X-ray sources, while all the data have not been fully analyzed yet. When the Ginga satellite was operational in late 1980's, it was virtually the only satellite in the world dedicated for X-ray astronomical observations. Hence the Ginga archival data, even though 20 year old, are still precious. Below we introduce recent results made from the Ginga archives.
Figure 1 shows distribution of the X-ray intensity along the Galactic plane obtained by scanning the Norma constellation region in April 1988 (scan angle=0 corresponds to the Galactic longitude of 335 degree). In the scan observation, the instrument is slowly rotated around the satellite spin axis. In this method, the detector counting rates get high and low when the X-ray source is in and out of the detector field of view, respectively. Thus, we can determine the location of the X-ray source from the scan profile. In Figure 1, bright peaks indicate the positions of known X-ray sources. Note the peak marked with dotted line and arrow. The dotted line gives the position of the well-known X-ray source X1608-522. In high energy X-rays (the third figure), the peak agrees with the dotted line well, while the peak shifts toward left, as the energy goes down (the second figure). In the lowest energy band (the top panel), we can clearly see the peak is off-set to the left. This indicates that there is such a new, unknown source in the "left" side of X1608-522, which emits preferentially low energy X-rays compared to X1608-522 . After careful data analysis, the location of the new source was determined as indicated by arrow. This region of the sky has been observed repeatedly since then, but the source has never been detected again. Studying the energy spectrum carefully, this new source is suggested to be a black hole candidate. Presumably, this is a black hole binary only having been shinning around April 1988. Who knows?
Figure 2 indicates the scan profile of the Galactic center region from 1987 to 1989 . In 1987, we see a bright peak (as bright as half the flux of Crab nebula) toward the Galactic center (the first diagram, indicated by arrow). However, in 1988 and 1989 (the second and third diagrams), the peak seen in 1987 has disappeared, and the X-ray fluxes are almost the same. The position of the transient source was determined at slightly north of the Galactic center, from several scan observations made in Spring 1987. Until when this new source was shining in X-rays? The archival data in August 1987 indicate the presence of the source. Also, in October 1987, the Galactic center region observation was carried out making use of the moon occultation technique, and this transient source was still detected  (see Note). Consequently, the source must have been bright from when the Ginga satellite started observation in Spring 1987 to, at least, September 1987, after which the source became dimmer by March 1988. This is also confirmed by observation by the All Sky Monitor onboard Ginga . Detailed X-ray spectral analysis of the source suggests that this is a binary composed of neutron star and low mass main-sequence star.
Some X-ray sources tend to brighten every tens of years. The Ginga transients may be bright again in future, when true nature of these sources may be revealed with much more sophisticated instruments.
Then, future scientists may look at the Ginga archival data again for reference.
Note: Moon occultation analysis gave two candidate source positions, while the present re-analysis of the scan data determined the source position uniquely between the two.
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Shigeo Yamauchi (Iwate University)