Ray Luminous Molecular Clouds 1941 XRay Reflection Nebulae

With the imaging capability in the wide energy band up to ten kiloelectronvolt, ASCA found 6.4 keV line emission in the Galactic Center region [8]. This emission is largely coincident with giant molecular clouds, e.g., Sgr B2, the first cloud from which the 6.4keV line was detected (Fig. 19.7). Because the 6.4keV line is a characteristic radiation from neutral iron, it is natural to believe that X-rays are emitted by these molecular clouds. However, clouds are cold and cannot emit X-rays by themselves. Therefore, it has been proposed that these clouds are irradiated by an external X-ray source, and emit fluorescent X-rays. To explain the strength of the 6.4 keV line from the Sgr B2 molecular cloud, a strong X-ray source with a luminosity of «1039ergs_1 is required. Since there is not such a bright source in the vicinity, it has been argued that the nucleus of our Galaxy was much brighter than today some hundred years ago [24]. This corresponds to a light travel time between Sgr A* and Sgr B2 which is located about 100 pc away from the Galactic Center but probably at an only slightly shorter distance from us (Fig. 19.8).

Using archival data of ASCA, BeppoSAX, Chandra Observatory, and XMM-Newton, no significant variability of the line flux during the period 1993-2001 is found. This excludes alternative explanations with a transient source inside of Sgr B2. Recently, a hard X-ray source associated with Sgr B2 has been found with the IBIS instrument onboard of INTEGRAL (IGR J17475-2822). The broad band (3-200 keV) spectrum of the source constructed from data of different observatories strongly supports the idea that the X-ray emission of Sgr B2 is Compton scattered and reprocessed radiation emitted in the past by Sgr A* [21]. Radiation 6.4 keV was also found from other molecular clouds, e.g., Sgr C and clouds within the radio arc region. From their brightnesses and their respective distances from Sgr A*, its putative luminosity in the past could be estimated. It turns out from these studies that the luminosity of Sgr A* decreases continously over the last several hundreds of years [14].

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Sgr B2 region

Units: x10 x counts/sec/0.106min: 10'

Sgr B2 region

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Fig. 19.7 The distribution of the 6.4-keV Line intensity in the GC region observed with ASCA [14] 102 10

400 relative R.A.

Fig. 19.7 The distribution of the 6.4-keV Line intensity in the GC region observed with ASCA [14] 102 10

100 10 Time (year)

Fig. 19.8 The past luminosity of Sgr A* estimated from the luminosity of 6.4 keV line of clouds at different distances from Sgr A* (Sgr B2, Sgr C, M0.11-0.08, from left to right; the right-most data point is the current luminosity of Sgr A*) [14]

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