Astron. Astrophys. 356, L5-L8 (2000)

1. Introduction

The Galactic center (GC) is a strong source of diffuse X-ray emission in the 2-10 keV energy range and in lines from several ions (Kawai et al., 1988; Sunyaev et al., 1993; Koyama et al., 1996; Koyama et al., 1996; Sidoli and Mereghetti, 1999). Recently, the ASCA satellite mapped the X-ray emission from the GC (Koyama et al., 1989). One of the most interesting results is the spatial distribution and the intensity of the iron K lines. The Fe K lines of highly ionized ions (He-like at 6.70 keV and H-like at 6.97 keV) arise from hot gas with temperatures of 9 keV. This emission is concentrated towards Sgr A-West and symmetrically distributed along the galactic plane with a spatial distribution similar to that of the radio continuum emission and the molecular clouds (Maeda and Koyama, 1996). This is in sharp contrast with the K line from neutral or low ionized Fe atoms at 6.40 keV (hereafter Fe^o line) which shows emission only towards the Sgr A and Sgr B complexes (Koyama et al., 1996).

The Fe^o line emission is caused by fluorescence and appears when neutral cold molecular clouds are exposed to a strong source of hard X-rays. In X-ray irradiated molecular clouds like those in the GC, it is expected that the X-rays will influence the heating, the ionization and the chemistry of these clouds (see e.g. Hollenbach et al., 1997). It is well known that the physical conditions and the chemistry of the molecular clouds in the GC differ substantially from those in the galactic disk (see e.g. Morris & Serabyn, 1996). High gas kinetic temperature (Huettemeister et al., 1993), and large abundance of SiO are typical in the GC (Minh et al., 1992; Martin-Pintado et al., 1997; Huettemeister et al., 1998). The origin of these unusual characteristics is unclear, but it is believed to be due to strong shocks in the GC (Wilson et al., 1982; Martin-Pintado et al., 1997). In this letter we present a correlation between the SiO radio emission and the Fe^o line, suggesting that X-rays may play an important role in the heating and the chemistry of the GC molecular clouds.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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