Tenma [Astro B]

Photo of Tenma

* Mission Overview

Tenma is the eighth Japanese scientific satellite from the Institute of Space and Astronautical Science (ISAS) and the second X-ray astronomy satellite after Hakucho. It was placed into a near circular orbit with a apogee of 501 km, a perigee of 497 km and an inclination angle of 31.5 degrees. Tenma was a spin-stabilized satellite and the FOV of the scientific payload was mostly aligned with the spin-axis (Z-axis). The main objective of Tenma was to perform spectral and temporal observations of galactic and extra-galactic sources with particular emphasis on energy spectra thanks to the improved energy resolution of the on-board instrumentation. The observing efficency was reduced greatly (only daytime operations) after the battery failed in July 1984. Observations continued intermittently until 11 November 1985, when operations were stopped because they were too inefficient. It re-entered on January 19 1989.

* Instrumentation

Tenma carried four high-energy experiments. They were:
  • Gas Scintillation Proportional Counter. This was the primary instrument on the satellite. Its main objective was the spectral and temporal study of cosmic X-ray sources in the range 2-60 keV. It consisted of 10 gas scintillation proportional counters (GSPC), with an effective area of 80 sq-cm each. The ten counters were divided into three groups SPC-A, B and C. The A and B consisted of 4 counters with honeycomb collimators with a FOV of 3.1 and 2.5° (FWHM), the C instead consisted in 2 counters with a 3.8° FOV (FWHM). The energy resolution was about 9.5% at 6 keV, about half of the conventional proportional counters used so far.
  • X-ray Focusing Collector. This instrument's main objective was the study of soft X-ray sources in the range 0.1-2 keV. The system consisted of 2 identical co-aligned subsystems, each with a position sensitive proportional counter. The peak effective area was ~ 7 sq-cm for each subsystem at 0.7 keV. The field of view was 5 degrees x 0.2 degrees (FWHM), divided into 7 resolution elements.
  • Transient Source Monitor. This instrument carried out continuous monitoring of a wide field of the sky (45 degree radius). It consisted of 2 detector groups: (1) a Hadamard X-ray telescope and (2) a scanning counter. The experiment was sensitive to the energy range 2-10 keV.
  • Radiation Belt Monitor/Gamma-Ray Burst Detector. This experiment consisted of 2 sets of scintillation counters with an effective area of 7 sq-cm each. Their primary purpose was to monitor the background variations. They were also used as alarms for satellite entry into the radiation belts. The system could also record gamma-ray bursts with 1/8 s resolution, with a detection limit of ~10-5 erg/sq-cm for the burst size.


The main science result is the discovery (and/or study) of the iron line region in several classes of X-ray sources. In particular, it discovered such emission from low-mass X -ray binaries (LMXB) and from the galactic ridge. The spectral resolution allowed a distinction to be made from "cold" iron line emission at 6.4 keV believed to arise from X-rays impinging on cold matter and "hot" 6.6-6.7 keV iron emission believed to arise from a hot plasma. The "cold" line was found in pulsars and AGN; the "hot" line was found in LMXB and the galactic ridge.

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