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

Abstract

Information about the X-ray luminosity of the supermassive black hole located at the Galactic center (GC), Sgr A*, and its temporal variations in the past is imprinted in the scattered emission observed today in the direction towards giant molecular clouds (GMCs) located in our Galaxy. Due to light travel time effects these clouds probe the activity of Sgr A* at different times in the past depending on their position relative to the GC and the observer. In this paper 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 in a given direction, with data from CO surveys of the same regions. These CO surveys map the position and mass of the molecular gas which the GMCs are made up 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 Sgr A* 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, which also scatters the radiation emitted by Sgr A*, allows us to extend the time coverage further into the past, back to about 110.000 years, albeit the limits are becoming less tight. We thereby can rule out a long term X-ray activity phase of Sgr A* at one per cent of its Eddington level ending less than about 80.000 years ago. The limits presented in this paper can be improved by observations of emission in the fluorescent iron K_alpha-line. We study the feasibility of these methods to investigate past nuclear activity in other spiral galaxies observed with the angular resolution of X-ray telescopes like Chandra and XMM-Newton.

20 pages, 21 figures, submitted to Astronomy and Astrophysics

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• Traces of past activity in the Galactic Centre
• Not that long time ago in the nearest galaxy: 3D slice of molecular gas revealed by a 110 years old flare of Sgr A*
• Highlights and Discoveries from the Chandra X-ray Observatory
• Sagittarius A* as an Origin of the Galactic PeV Cosmic Rays?
• Coronal radiation of a cusp of spun-up stars and the X-ray luminosity of Sgr A*
• Hadronic origin of multi-TeV gamma rays and neutrinos from low-luminosity active galactic nuclei: implications of past activities of the Galactic center
• Is SS 433 a misaligned ultraluminous X-ray source? Constraints from its reflected signal in the Galactic plane
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• Probing the clumping structure of Giant Molecular Clouds through the spectrum, polarisation and morphology of X-ray Reflection Nebulae
• On the Origin of the 6.4 keV Line in the Galactic Center Region
• Scattering in the vicinity of relativistic jets: a method for constraining jet parameters
• Observation of Very High Energy Gamma Rays from HESS J1804-216 with CANGAROO-III Telescopes
• Evidence for an interaction between the Galactic Center clouds M0.10-0.08 and M0.11-0.11
• Sgr A* as probe of the theory of supermassive compact objects without event horizon
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