Journal Article

A strong magnetic field around the supermassive black hole at the centre of the Galaxy

Eatough, R. P., Desvignes, G., Falcke, H., Freire, P. C. C., Lee, K. J., Schnitzeler, D. H. F. M., Kraus, A., Keane, E. F., Bassa, C., Spitler, L. G., Noutsos, A., Brunthaler, A., Cognard, I., Stappers, B., Lyne, A. G., Bower, G. C., Karuppusamy, R., Champion, D. J., Klein, B., Demorest, P. B., Wex, N., Deller, A. T., Kramer, M.

2013

Overview

The unusually large Faraday rotation of a newly discovered pulsar indicates that there is a dynamically important magnetic field near the supermassive black hole believed to lie at the centre of the Milky Way. Strong magnetic field at Galactic Centre Sagittarius A * , at the centre of the Milky Way, harbours a strong candidate for a supermassive black hole. The source is thought to be powered by radiatively inefficient accretion of gas, with magnetic fields possibly helping the flow to occur. In most galaxies the accretion flow cannot be resolved directly and accretion models lack information about the initial conditions of the flow. Ralph Eatough et al . have determined magnetic field strength indirectly by measuring the rotation of radio emission from a new pulsar that erupted as an X-ray source near Sag A * . The results point to the presence of dynamically important magnetic field near the black hole. Earth’s nearest candidate supermassive black hole lies at the centre of the Milky Way 1 . Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment 2 , which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed 3 . The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas 4 , expel matter through relativistic jets 5 and lead to synchrotron emission such as that previously observed 6 , 7 , 8 . Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre 9 , 10 , 11 , 12 and show that the pulsar’s unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission—from radio to X-ray wavelengths—from the black hole.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/864 / Accretion / Astronomical research / Astrophysics / Black holes / Black holes (Astronomy) / Discovery and exploration / Emissions / Galactic Astrophysics / Humanities and Social Sciences / letter / Magnetic fields / Magnetic fields (Cosmic physics) / Milky Way / multidisciplinary / Observations / Radiation / Science / Sciences of the Universe / Wavelengths