Not that long time ago in the nearest galaxy: 3D slice of molecular gas revealed by a 110 yr old flare of Sgr A*

Abstract

A powerful outburst of X-ray radiation from the supermassive black hole Sgr A* at the centre of the Milky Way is believed to be responsible for the illumination of molecular clouds in the central ∼100 pc of the Galaxy (Sunyaev, Markevitch & Pavlinsky; Koyama et al.). The reflected/reprocessed radiation comes to us with a delay corresponding to the light propagation time that depends on the 3D position of molecular clouds with respect to Sgr A*. We suggest a novel way of determining the age of the outburst and positions of the clouds by studying characteristic imprints left by the outburst in the spatial and time variations of the reflected emission. We estimated the age of the outburst that illuminates the Sgr A molecular complex to be ∼110 yr. This estimate implies that we see the gas located ∼10 pc further away from us than Sgr A*. If the Sgr B2 complex is also illuminated by the same outburst, then it is located ∼130 pc closer than our Galactic Center. The outburst was short (less than a few years) and the total amount of emitted energy in X-rays is ∼10^{48}ρ _3^{-1} erg, where ρ<SUB>3</SUB> is the mean hydrogen density of the cloud complex in units of 10<SUP>3</SUP> cm<SUP>-3</SUP>. Energetically, such fluence can be provided by a partial tidal disruption event or even by a capture of a planet. Further progress in more accurate positioning and timing of the outburst should be possible with future X-ray polarimetric observations and long-term systematic observations with Chandra and XMM-Newton. A few hundred years long X-ray observations would provide a detailed 3D map of the gas density distribution in the central ∼100 pc region.