The X-ray Background - a 40-years mystery

The X-ray background, strong radiation observed from every direction in the entire sky, was discovered by Dr. Riccardo Giacconi and his collaborators at the birth of X-ray astronomy. The cosmological importance of the radiation was clearly recognized at that time and revealing its origin has been a primary theme of X-ray astronomy for over 40 years. The energy spectrum of the X-ray background has a peak around 30 keV, and the most of its energy is emitted at photon energies above 2 keV (the hard X-ray background). Pre-ASCA surveys in soft X-ray band detected mainly unabsorbed AGNs (Type 1 AGNs). Their spectrum is, however, far softer than that of the X-ray background. Thus, it proved impossible to explain the origin of the X-ray background by a superposition of emission from the same type of objects. This contradiction is known as the "spectral paradox", and has been one of the biggest mystery in the pursuit of the origin of the X-ray background.

Using ASCA at its then-highest sensitivity in the hard X-ray band, we conducted a survey observation of a continuous region covering seven square degrees in the direction of the Coma Berenices for several periods from 1993 to 1995: the ASCA Large Sky Survey (ALSS). Consequently, we resolved about 30% of the X-ray background in the 2-10 keV band into discrete sources and discovered a population of X-ray sources with hard spectra, which are considered the main contributors of the X-ray background. We find evidence that they are heavily absorbed and, as a result, the average spectrum of faint X-ray sources is significantly harder than that of Type 1 AGN. Further, we succeeded in optically identifying all the 33 X-ray sources detected in the energy band above 2 keV. Interestingly, we found that most of the absorbed X-ray sources exist in the relatively nearby universe at redshifts less than 0.5.

At around the beginning of the ALSS project, we started close, strong joint research with optical astronomers represented by Masayuki Akiyama (the National Astronomical Observatory of Japan; NAOJ), Koji Ohta (the Faculty of Science, Kyoto University), and Toru Yamada (NAOJ). The collaboration continues to this day. The central galaxy in Fig. 1 is the optical image of an AGN discovered in the ALSS project, taken with the SUBARU telescope. The error circle of the X-ray source is shown in the small figure upper left, taken with the University of Hawaii 88-inch telescope. As you can imagine, optical identification is not a smooth process. It is a struggle against the position accuracy of ASCA sources, which has been improved to 20 arcsec by the final calibration. The identification process is something like work by skilled craftsmen. We had to make real-time judgments and seek the most efficient way to identifying as many sources as possible within the limited time of the telescope. Like a band of guerrillas, we visited observatories around the world.

Looking for more quantitative evaluation, our research moved forward to the much larger area survey, the ASCA Medium Sensitivity Survey (AMSS). Using all the archive data of ASCA, we systematically search serendipitous sources that came by chance into the GIS detector's field of view. Naturally, this required the analysis of a huge amount of data. Finally we were able to publish the results as an X-ray catalog, although, for a certain period of time, our almost total occupation of the X-ray group's computer caused trouble to others. We then defined a flux-limited, hard-band selected sample from this catalog and initiated a systematic optical identification program. This project took several years. Recently we have succeeded in optically identifying about 100 X-ray sources almost completely. The area covered by the AMSS is still the largest in the world, providing a unique X-ray sample compared with other surveys.

Fig. 1 Example of the optical image of an AGN discovered by the ASCA hard X-ray survey
(the galaxy in the center is the object in question).
Taken with the Subaru telescope's Supreme-Cam. Image circled is a finding chart taken with the University of Hawaii 88-inch telescope.