Paris, 27 May 2010

The turbulent past of the Milky Way's black hole

The supermassive black hole at the center of our Galaxy went through turbulent times over the past centuries. This we know thanks to its surrounding molecular clouds, whose varying X-ray and gamma-ray luminosity reflects a major flare in the past. These findings, obtained by an international team of researchers led by French astrophysicists, are published in The Astrophysical Journal.

The black holes located in active galactic nuclei usually show intense activity.  But this is not true of our own Galaxy's black hole, Sgr A*, which shows very low activity. However, an international team of researchers, led by astrophysicists from the Astroparticle and Cosmology Laboratory in Paris (CNRS / Université Paris-Diderot / CEA / Observatoire de Paris), has succeeded in showing that the black hole was active in the very recent past, and that it is therefore not as unusual as it seemed.

XMM-Newton and INTEGRAL, two ESA satellites to which CNES, CEA and CNRS-INSU have made a major contribution, have enabled researchers to observe X-ray and gamma-ray emissions from molecular clouds in the region of the galactic center, near Sgr A*. The scientists were surprised to discover that the emissions showed spectacular variation, such as the progressive brightening of one of the clouds between 2004 and 2009, at an apparent speed three times greater than that of light (Fig. 1).

The phenomenon shows that the clouds reflect the high-energy radiation produced by the intense activity of the supermassive black hole. The light echo that reaches us is delayed in comparison with the direct light from the black hole. This delay depends on the position of the cloud and on the time the radiation takes to travel the distance from the black hole to the cloud. By observing the decay of the gamma-ray emission from another cloud (Fig. 2), the researchers estimated the duration and intensity of the flare, which began around four centuries ago and ended at the beginning of the twentieth century.  For three hundred years, our Galaxy's supermassive black hole was therefore a million times brighter than now.

XMM-Newton images of the emission of the neutral iron fluorescent line in the molecular clouds around Sgr A*

© Ponti G., et al., 2010, The Astrophysical Journal, V. 714, p. 732-747

Figure 1. XMM-Newton images of the emission of the neutral iron fluorescent line in the molecular clouds around Sgr A* (Bridge, MC1 et MC2) between 2004 and 2008. The progression of the emission in the sub-regions indicated by ellipses 1,2, 3, and 4 of The Bridge are clearly shown. The distance traveled by the emission is 15 light years in less than 5 years. The angular scale (2 arc-minutes = 15 light years at the distance of the galactic center) and the direction towards Sgr A* are also shown.

The region of the galactic center seen by INTEGRAL in low-energy gamma rays

© Terrier R., et al., 2010, The Astrophysical Journal, forthcoming

Figure 2. The region of the galactic center seen by INTEGRAL in low-energy gamma rays (20-60 keV) at different periods, each strip showing a different year from 2003 to 2009. The different sources are generally associated with variable X-ray binary systems. The green circle in the top image shows the position of the Sgr B2 molecular cloud. The associated gamma-ray source clearly shows decay over 7 years.

To find out more:


Discovery of a Superluminal Fe K Echo at the Galactic Centre: The Glorious Past of Sgr A* Preserved by Molecular Clouds. View web site
Ponti G. (1,2), Terrier R. (1), Goldwurm A. (1,3), Belanger G. (4) and Trap G. (1,3)
The Astrophysical Journal, 2010 May 1, Vol 714, p. 732.
(1) APC - Paris (F), (2) University of Southampton (UK), (3) SAp / IRFU - Saclay (F), (4) ESAC/ESA - Villanueva de la Canada (SP)

Fading Hard X-ray Emission from the Galactic Centre Molecular Cloud Sgr B2
Terrier R. (1), Ponti G. (1,2), Bélanger G. (3), Decourchelle A. (4,5), Tatischeff V. (6), Goldwurm A. (1,4), Trap G. (1,4), Morris M. R. (7) and Warwick R. (8)
The Astrophysical Journal, 2010
(1) APC - Paris (F), (2) University of Southampton (UK), (3) ESAC/ESA - Villanueva de la Canada (SP), (4) SAp/IRFU - Saclay (F), (5) AIM - Saclay (F), (6) CSNSM/CNRS - Orsay (F), (7) UCLA - Los Angeles (US), (8) University of Leicester (UK)


Regis Terrier l T 01 57 27 60 79l
Andrea Goldwurm l T 01 57 27 60 58 l
Gabriele Ponti l T +44 (0)23 8059 2089 l

CNRS press office:
Julien Guillaume l T 01 44 96 46 35 l

APC / INP2P3 Communications office:
Jean Luc Robert l T 01 57 27 61 53 l


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