Abstract
The X-ray spectrum of the famous starburst galaxy M82 consists of three components: soft, medium, and hard (Tsuru et al. 1997, AAA 68.157.702). The spectrum of the hard component, which is spatially unresolved, is well represented by an absorbed thermal bremsstrahlung, or an absorbed power-law model. However, the origin of the hard component is unclear. Thus, we conducted a monitoring observation with ASCA in 1996. Although the X-ray flux of the soft and medium components remained constant, a significant time variability of the hard component was found between 3×1040 erg s−1 and 1†1041 erg s−1 at various time scales from 10 ks to one month. The temperature or photon index of the hard component also changed. We proved that the spatial position of the hard component is the center of M82. The spectrum of the variable source obtained by subtracting the spectrum of the lowest state from the highest state suggests strong absorption of NH ∼ 1022 cm2, which means a variable source is embedded in the center of M82. All of these observations suggest that a low-luminosity AGN exists in M82.
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