X-Ray Evidence of an AGN in M82

An X-ray spectrum of the famous starburst galaxy M82 consists of threecomponents: soft, medium, and hard components (Tsuru et al. 1997). The spectrumof the hard component, which is spatially unresolved, is well represented by anabsorbed thermal bremsstrahlung, or an absorbed power-law model. However theorigin of the hard component was unclear. Thus, we made a monitoringobservation with ASCA in 1996. Although the X-ray flux of the soft and mediumcomponents remained constant, a significant time variability of the hardcomponent was found between $3\times10^{40}$ erg/s and $1\times10^{41}$ erg/sat various time scales from 10 ks to a month. The temperature or photon indexof the hard component also changed. We proved that the spatial position of thehard component is the center of M82. The spectrum of the variable sourceobtained by subtracting the spectrum of the lowest state from the highest statesuggests the strong absorption of $N_{\rm H} \sim 10^{22}$ cm$^2$, which meansthe variable source is embedded in the center of M82. All these suggest that alow-luminosity AGN exists in M82.Comment: 15pages for text and tables. 13pages for figure