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The Forefront of Space Science

Monitor of All-sky X-ray Image (MAXI) Starting to Watch the Dynamically Changing Universe
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An outline of the Monitor of All-sky X-ray Image (MAXI) and the significance of all-sky X-ray monitoring were reported in ISAS News of Aug. 2008 (Japanese only) under the title ďScience on the Kibo.ĀEThis article reports the latest status of MAXI after its activation.

Start of MAXI operation

At 9:41 on Aug. 3, 2009, Japan Standard Time (JST) (hereinafter, JST is used), power for experiments was input into MAXI, which had been installed on the Japanese Experiment Module (JEM) ďKiboĀEof the International Space Station (ISS) in the early morning on Jul. 24, 2009. When the telemetry data from MAXI started to display on the operation control deskís screen at the Tsukuba Space Center (TKSC), everyone there burst into applause to celebrate the birth of MAXI.

On Aug. 8, we began to apply high-voltage power on the detectors (a total of 12 proportional counters) of the X-ray Gas Slit Camera (GSC) onboard MAXI. Once the high voltage (1,650 volts) is applied, MAXI starts to detect X-ray photons. With MAXIís successful detection of the Crab Nebula, one of the bright X-ray sources, we confirmed that the detectors and data-processing system onboard MAXI were functioning properly. On Aug. 13, we completed the high-voltage application to all 12 detectors.

On Aug. 15, we initiated test operation of another observation instrument, an X-ray CCD camera called Solid-state Slit Camera (SSC), onboard MAXI. To detect X-ray photons, we need to cool a total of 32 CCD elements. On 18, we started to apply electric current to electronic refrigerators (Peltier elements) mounted on the rear of the CCD elements and, then, succeeded in cooling them to a target temperature of -60 deg. C.

All-sky X-ray images observed by MAXI

Fig. 1 (a) is the first light image by MAXI, which was released to the press on Aug. 18. This was produced from the X-ray photon data detected by GSC while the ISS orbited the earth once (about 90 min.). The horizontal axis in the center is the galactic plane (Milky-Way). Although exposure time and positional error had not yet been corrected , this image clearly shows about 30 major X-ray objects. The faintest one has an X-ray intensity of about 1/40 that of Crab Nebula. Thus, we verified that the system attained its target sensitivity in a single revolution around earth, as predicted by our computer simulation before its launch. The brightest object in the image is a binary star system consisting of a neutron star and a small ordinary star known as Scorpius X-1. The image also shows Cygnus X-1, which was first discovered as a possible black hole, and Crab Nebula, a supernova remnant discovered in 1054.

Fig. 1 (b) is an image produced from data collected over one day, Oct. 28. Since we gradually narrowed the solar-avoidance angle setting after the first-light retrieval, the width of the unobserved region decreased from 50 deg. to 10 deg., eventually accomplishing 96% of all-sky coverage ratio. The unobserved region moves on the celestial sphere. For the region not covered at the time of Fig. 1 (b) observation, MAXI came to see 10 days later.

Figure 1
Figure 1. All-sky images by X-ray Gas Slit Camera (GSC)
Since exposure time and background of images are not yet corrected, roughness of image such as stripes and uneven color is prominently seen. Corrected images of all the sky are now under preparation.

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