X-RAY DETECTOR

The objective of the flight was to observe and document X-ray intensity variations from Cygnus X-1, the brightest celestial object in the X-ray region above 20 keV, specifically focusing on detecting and characterizing any sudden changes or flare events in its hard X-ray emission. The detector was developed through a collaboration between the Department of Physics at Osaka City University and the Cosmic Ray Laboratory at the University of Tokyo.

In the image at left we can see an scheme of the detectors. It was composed by a 3-inch NaI(Tl) crystal scintillator with a thickness of 3 mm as the primary detector, coupled with a plastic scintillator of the same area but 30 mm thickness and employed a cellular collimator that provided a 4° × 20° FWHM field of view to focus observations on the target region. The entire detector assembly was protected by a guarded shield made of lead and tin, along with magnetic shielding, except for the entrance window of the detector. To minimize background interference from charged particles, the system incorporated anti-coincidence shielding using the scintillator's signal combined with pulse rise-time discrimination methods.

Two identical detector systems were mounted in the gondola, with each detector axis inclined at 4° from the zenith. The detectors were designed to operate in two alternating modes controlled by command signals from Earth: ON-mode, where the detector axis pointed south and Cygnus X-1 lay within the field of view, and OFF-mode, where the axis pointed north and the source could not be observed. This switching arrangement allowed for background subtraction and verification of source detection. In-flight calibration was performed using radioactive americium-241 isotope, triggered by Earth commands whenever the detector switched between ON and OFF modes.

Balloon launched on: 10/7/1972 at 5:25 utc
Launch site: Sanriku Balloon Center, Iwate, Japan  
Balloon launched by: Institute of Space and Astronautical Science (ISAS)
Balloon manufacturer/size/composition: Zero Pressure Balloon model B15 15.000 m3
Flight identification number: B15-21
End of flight (L for landing time, W for last contact, otherwise termination time): 10/7/1972 at 11:31 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 6 h 6 m

The balloon was launched on October 7, 1972, at 5:25 UTC from the Sanriku Balloon Center in Japan. The gondola reached its ceiling altitude at 7:50 UTC and maintained a floating altitude that varied between 34.88 km and 35.38 km throughout the observation period. The observational coverage showed that Cygnus X-1 appeared at the edge of the collimator's field of view at 7:40 UTC, traversed through the field, crossed the meridian at 9:17 UTC, and disappeared from view at 10:55 UTC.

During the observation period, the researchers detected a dramatic sudden increase in X-ray intensity from Cygnus X-1 at approximately 10:10 UTC, which they termed a "flare-up." This flare event showed the intensity rising by a factor of 4 above the baseline level. The flare began near 9:50 UTC, reached its maximum in less than 15 minutes, and then decreased over the following 10 minutes, with the entire event lasting less than 25 minutes.

• An observation of X-ray Variation in Cygnus X-1 13th International Conference on Cosmic Rays, Vol.1, p.73
• Ten Years of Scientific Ballooning Activities in Japan (1966-1975) Tokyo University. ISAS: Bulletin, 12(2B):517-555, 1976
• Time variation of hard X-ray from CYG X-1 15th International Conference on Cosmic Rays, Vol.1, p.161