Observation by Suzaku and its findings
Launched on July 10, 2005, Suzaku is Japan’s fifth X-ray astronomy satellite. It carries two types of detectors: X-ray CCD cameras called X-ray Imaging Spectrometer (XIS) and Hard X-ray Detector (HXD), and performs high-sensitivity observations covering a wide energy band from soft to hard X-rays. Suzaku XIS is superior in spectral response, in particular for soft X-rays, compared to the X-ray CCD cameras of the Chandra and XMM-Newton satellites. Coupled with its X-ray telescope with a large effective area, Suzaku contributes greatly to research on soft X-ray background radiation.
From September 2 to 4, 2005, we, the Suzaku team, pointed the satellite to the northern ecliptic polar region to observe X-ray background radiation. The upper diagram of Fig. 2 shows the light-curve of X-rays during the observation period. From the diagram, we can see that X-ray intensity increased for about 10 hours in the early observation period. There were no bright X-ray objects in the region observed by Suzaku, nor were there X-ray objects that flared in the observation period. The enhancement occurred in the XIS’s entire field of view. The lower diagram of Fig. 2 shows the temporal variation profile in flux of solar wind (proton) measured by ACE satellite. This reveals that the proton’s flux increased during the period when the X-ray enhancement was observed. The mysterious X-ray enhancement happened to occur during the observation of the northern ecliptic pole region by Suzaku.
Suzaku’s superior feature is its good spectral response. We examined closely how the X-ray spectrum changed according to the X-ray enhancement. Fig. 3 shows the X-ray spectrum when there was no X-ray enhancement. The horizontal axis is energy (wavelength) while the vertical axis is X-ray’s intensity. The cross marks show the observed spectral data. The dotted lines show the contribution of active galactic nuclei (AGN) and/or galaxies and the thermal emission component (corresponding to about 2 million Kelvin) coming from hot gases inside and outside our Galaxy. It is clear that the spectrum from Suzaku is well expressed with these two components (green line in Fig. 3).
In Fig. 4, we expressed spectral data at X-ray enhancement using two models: the spectral model when no enhancement occurred and emission lines generated during the enhancement. Note that, even if monochromatic X-rays enters, its spectral line inevitably has a width that corresponds to XIS’s energy resolution and, accordingly, the model function of each emission line has a shape of the same width. Our analysis indicated that the X-ray spectrum at the enhancement could be well explained by adding nine emission lines (eight of the nine emission lines are shown in blue lines in Fig. 4) to the X-ray spectrum when no enhancement occurred. This means that at least nine emission lines emerged according to the X-ray enhancement.