Neutral iron Kalpha diagnostic -- X-ray archaeology

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When neutral matter containing an admixture of iron is illuminated in the laboratory by hard X-ray radiation a characteristic spectrum is formed that contains a bright fluorescent line at 6.4 keV (known in X-ray spectroscopy as neutral iron Kalpha-line). Whenever this line is observed from an astronomical object we know that cold gas shares neighboring regions of space with extremely hot matter emitting X-rays. The Japanese satellite ASCA recently found that the central region of our Galaxy is very bright in this line. The presence of the line was in fact expected, since this region is known to host a huge mass of molecular gas along with many X-ray sources.

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Fig. 1: Molecular clouds in the center of the galaxy. The intensity of the 6.4 keV iron-line is shown colour-coded in this picture.


One of the potentially interesting possibilities is the "X-ray archaeology" based on observations of Kalpha-photons generated by a flaring compact X-ray source surrounded by neutral matter. If the light crossing time of the region occupied by neutral matter is long enough, Kalpha-photons generated by the primary X-ray radiation may be observable long after the fading of the compact source. This is very similar to the usual echo, but in the X-ray band. The Kalpha from the central region of our own Galaxy may be due to this kind of X-ray echo, produced by an outburst of X-ray radiation of the supermassive black hole (known as Sgr A* object) lurking in the gas and dust at the dynamic center of the Galaxy.

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Fig. 2: Points of equal light-travel times between source and observer are located on a parabola. Information about a source being active for only a short period of time is contained within the area between the two parabolas.

In the case of a short flare of the Sgr A* emission (e.g. few hundred years ago) one can expect that molecular clouds, which are exposed to primary radiation at a given moment of time, are located along the surface of the parabola with the focus at Sgr A*. The geometry of the parabola reflects the time delay associated with the propagation of primary hard X-ray photons from Sgr A* to the neutral matter and further propagation of the Kalpha- photons to the observer. Clouds located not at the surface of this parabola will not produce Kalpha-emission at the given moment of time. The whole picture will not be static: the geometry of the parabola will change in time and some molecular clouds may fade, while others will become brighter in the Kalpha-line. It is therefore an exciting application for imaging X-ray telescopes to detect changes of the Kalpha- morphology in the central region of our Galaxy on time scales of years or tens of years.


E. Churazov, M. Gilfanov, R. Sunyaev







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