Present and Past Activity of the Galactic Center Super-massive Black Hole and its Impact on the Galaxy Central Molecular Zone

Overview

The centre of our galaxy hosts the nearest super-massive black hole (SMBH) to the solar system, identified with the compact radio source Sagittarius A* (Sgr A*). With an estimated mass of few million solar masses this SMBH is extremely weak at nearly all wavelengths so that its present bolometric luminosity is less than 10-8 times its estimated Eddington luminosity. This makes the Milky Way SMBH a particularly weak specimen, much fainter than the Low Luminosity Active Galactic Nuclei (LLAGN) observed in external galaxies. One specific question that we would like to tackle with the proposed project is whether the present quiet state is a normal condition for Sgr A* or if this source undergoes periods of significantly higher activity.

Much efforts has been dedicated by observers to understand its present behaviour, namely to understand the flaring activity that it displays in X-rays, in the infrared and at sub-mm/radio wavelengths. These flares are very interesting because they mark the radiation mechanisms and the behaviour of matter at only few gravitational radii from the BH horizon. However they increase the object luminosity only by, at most, a factor of about 100 for few hours a day.

A number of recent observational and theoretical results show instead that Sgr A* could have been active in the past at a level comparable to a typical LLAGN or even more. The main indication of such activity is the presence of variable X-ray and hard X-ray emission from the molecular clouds of the Central Molecular Zone of the Galaxy. Such emission and in particular the 6.4 keV line of neutral iron from some of the most dense molecular clouds of the region is indeed interpreted as due to reflection of X-rays generated by a powerful source, probably Sgr A*, in the recent past, some 100-300 years back.

Fluorescence lines from neutral metals and hard X-ray continuum can however be produced by accelerated particles interacting with molecular material and in order to correctly evaluate the past X-ray activity of the SMBH it is necessary to evaluate which part of the emission is rather due to localized particle acceleration. Moreover particle injection into the interstellar medium by the SMBH itself has been proposed as a mechanism to explain a number of high energy phenomena in the region, but a proper comparison of these theories with available data has not been done yet.

The project we present here is aimed at gathering an international team of specialists in the field to investigate the behaviour of the galactic center SMBH and study the possibility that Sgr A* has been a source, in the past, of both intense X/gamma-ray radiation and accelerated particles that interact in the surrounding interstellar medium and whose effects we presently observe in the Galactic center region.

Scientific items we intend to explore

The flaring behaviour of Sgr A*.

The characteristics of the large X-ray outburst at the origin of the radiation reflected by the molecular clouds of the region.

The effects of accelerated particles in local area of the Galactic center and injection of accelerated particles in the region by Sgr A*.