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Information about the X-ray luminosity of the supermassive black hole locatedat the Galactic center (GC), Sgr A*, and its temporal variations in the past isimprinted in the scattered emission observed today in the direction towardsgiant molecular clouds (GMCs) located in our Galaxy. Due to light travel timeeffects these clouds probe the activity of Sgr A* at different times in thepast depending on their position relative to the GC and the observer. In thispaper we combine results of recent ASCA observations along the Galactic plane,providing upper limits for the scattered flux in the 4-10 keV range produced ina given direction, with data from CO surveys of the same regions. These COsurveys map the position and mass of the molecular gas which the GMCs are madeup of. Demanding the scattered flux to be not larger than the observed one,this data enables us to derive upper limits for the 4-10 keV luminosity of SgrA* at certain times during the last 40.000 years down to about 8*10^40 erg/s.At other times the limits are less tight, of the order of 10^41-10^42 erg/s.The more extended and continuous HI distribution in the Galactic disk, whichalso scatters the radiation emitted by Sgr A*, allows us to extend the timecoverage further into the past, back to about 110.000 years, albeit the limitsare becoming less tight. We thereby can rule out a long term X-ray activityphase of Sgr A* at one per cent of its Eddington level ending less than about80.000 years ago. The limits presented in this paper can be improved byobservations of emission in the fluorescent iron K_alpha-line. We study thefeasibility of these methods to investigate past nuclear activity in otherspiral galaxies observed with the angular resolution of X-ray telescopes likeChandra and XMM-Newton.Comment: 20 pages, 21 figures, submitted to Astronomy and Astrophysic

Interstellar gas in the Galaxy and the X-ray luminosity of Sgr A*in the recent past

Interstellar gas in the Galaxy and the X-ray luminosity of Sgr A^*in the recent past