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The Forefront of Space Science

Black Hole Binaries Observed by MAXI
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Monitor of All-sky X-ray Image (MAXI) is an all-sky X-ray monitor onboard the International Space Station (ISS). Unlike Suzaku, an X-ray astronomical satellite conducting precise long-exposure observations of narrow fields in the sky, MAXI sweeps like radar almost the entire sky every 92 minutes, the period for the ISS to circle the earth, to watch for various activities of X-ray sources.

MAXI was delivered to the ISS by the Space Shuttle Endeavour in July 2009 and started full operation from August. The 2009 August and November editions of ISAS News outlined MAXI's observation instruments, transportation and installation on the ISS. This article focuses on black hole binaries to highlight the scientific results for the first 18 months of MAXI's mission.

Sky watched in X-ray

The sky observed in X-ray is very different from that observed in visible light. In the night sky seen in visible light, numerous stars are distributed almost evenly Eexcept for the Milky Way Eand always shine in the same way. In contrast, strong cosmic X-ray sources are few in number and most of them vary violently. Fig. 1 shows the X-ray all-sky image obtained by MAXI's first 10-month observation. The celestial sphere is represented on a plane just like a world map. The Galactic Plane (Milky Way) corresponds to the equator on a world map with the center of the figure to the direction of the Galactic Center.

Figure 1
Figure 1. X-ray all-sky image obtained by MAXI's first 10-month observation
Bright X-ray sources (mainly binaries comprising neutron stars and black holes) exist in large numbers around the Galactic Center (in the direction of Sagittarius) and along the Galactic Plane (Milky Way) and change from day to day. Colors indicate the "hardness" of X-ray spectrum. More than 200 X-ray sources including weak ones have been identified.

Why do visible stars and X-ray sources appear so different? Most visible stars shine with energies generated by nuclear fusion in their cores. In these stars, if the energy generated in their core increases more than usual, the whole object expands and eventually lowers the core temperature. In this way, negative feedback is activated to stabilize the nuclear reaction. For this reason, these stars shine very stably for most of their lifetime. On the other hand, the energy source of most intense X-ray sources is gravitational energy released when the gas surrounding extremely compact bodies like black holes and neutron stars is accreted onto them. The normal stars' stabilizing mechanism does not work in this process, and accordingly, X-ray intensity fluctuates in response to changes in the supply of gas from the surrounding area.

Furthermore, even when the matter supply from the surroundings is constant, great fluctuations in the accretion of gas onto black holes or neutron stars (i.e., accretion rate) and X-ray radiation can occur in the accretion disks, which are formed by gas whirling down onto compact bodies such as a black hole. Conversely, this means that research on X-ray fluctuation and its spectra will provide us with answers to the questions: a) What phenomena occur in the area near black holes or neutron stars; b) Is the object located at the center really a black hole; and c) If so, what is its nature?

However, for this research we need to catch the moment when interesting activities occur in X-ray sources. This is MAXI's most important mission. When a well-known X-ray source such as Cygnus X-1 begins to exhibit an unusual behavior, not to mention emergence of a new previously unknown X-ray source, we issue a notification to the world and request precise observation by scientific satellites (e.g., Suzaku) and ground telescopes.

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