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 −1 . At other times the limits are less tight, of the order of 10 41 −10 42 erg s −1 . For two periods of time of about 2000 and 4000 years duration 8000 and 14 000 years ago the currently available CO data is insensitive to any enhanced activity of the GC. Flares lasting longer than 3000 years fill these time gaps and therefore can be excluded to have occurred during the last 40 000 years with a luminosity larger than a few 10 42 erg s −1 . 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 Kα-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.

Abstract. Only a small fraction of galactic nuclei in the nearby universe are luminous; most of them are currently dim. We investigate the feasibility of constraining the X-ray luminosity in the recent past (up to ∼10 6 years ago) of the nucleus of a cluster dominant galaxy by measuring the contribution of scattered radiation from the central source to the surface brightness of the intracluster gas dominated by thermal emission. We show that resonance X-ray lines present an advantage over the adjacent continuum, because the relative contribution of the scattered component is typically larger in the line case by a significant factor of 3-10. As an example, we estimate the level of constraint that could be derived from future fine spectroscopic observations on the past X-ray luminosity of the nearby M 87 and Cygnus A active galaxies. For instance, scattered line radiation should be detectable from the Virgo cluster if the X-ray luminosity of M 87 was a few times 10 44 erg s −1 , or 10 −3 of its Eddington luminosity, until a few times 10 5 years ago. For comparison, upper limits inferred from the available XMM-Newton and Chandra X-ray continuum data are typically 1-2 orders of magnitude weaker. The same method can be applied to distant powerful quasars (at redshifts z > ∼ 1) if they have cluster-like gaseous coronae, as suggested by Rosat and Chandra observations of active galaxies at z < ∼ 1. Their surface brightness profiles in the X-ray continuum above ∼10 keV kT/(1 + z) (where T is the gas temperature) should be dominated by redshifted scattered radiation from the quasar. Therefore, measurements with forthcoming mirror-optics hard X-ray telescopes could give information on the lifetime of quasars and parameters of the hot gas around them.

Abstract. Relativistic jets of radio loud active galactic nuclei (AGN) produce highly directed, intense beams of radiation. A fraction of this beamed radiation scatters on the thermal plasma generally surrounding an AGN. The morphology of the scattered emission can thus provide constraints on the physical properties of the jet. We present a model to study the feasibility of constraining the parameters of a jet, especially its inclination angle and bulk Lorentz factor in this way. We apply our model to the well studied jet of M 87 and the surrounding diffuse gas and find that the observational limits of the surface brightness measured in the region of the putative counterjet provide the tightest constraints on the jet parameters consistent with constraints derived by other methods. We briefly discuss the applicability of our model to other sources exhibiting relativistic motions.