X-ray and Observational Astronomy

Massive outflows from Active Galactic Nuclei: a new paradigm

The ratio of X-ray counts observed from PG1211+143 compared to those that would be due to a simple power law spectrum. Absorption lines at 1.47 keV (Mg), 2.89 keV (S), 7.02 and 8.05 (Fe) can be seen, as well as an emission line from iron redshifted from 6.4 to 5.9 keV.	The decomposed spectrum of PG1211+143 showing the various components: green (bottom) emission line; red ionised absorbed power law, blue highly ionised absorbed power law; green total model spectrum.	The low energy X-ray end of the big blue bump in PG1211+143.The ratio of the data to the extrapolated power law is shown, the power law being fit above 1 keV.

While it has been known for many years that the central region (the nucleus) of some galaxies are unusually bright, and are hence called Active Galactic Nuclei (AGN), the origin of this extreme brightness has only become clear with observations from space. We now know that the brightness of the AGN is due to a massive black hole accreting gas at the centre of the galaxy, this accretion making the gas hot and causing it to shine brightly before falling into the black hole. The brightest AGN have central black holes a million to a billion times the mass of the sun. Probably all substantial galaxies have such central black holes (see the HST press release), but in many cases (including our own Galaxy and its near neighbour the Andromeda galaxy) the massive black hole is faint, possibly because of a lack of gas for it to accrete.

Active Galactic Nuclei are generally bright X-ray sources because of the very high energy acquired by the gas falling into the black hole. For this reason, X-ray observations can tell us much about the conditions close to the black hole, and have been important in building up the current understanding of these objects. The X-ray emission from AGN is typically dominated by a hard power law spectrum, thought to be due to low energy photons gaining energy in collisions with hot electrons (so-called inverse Compton scattering). A common picture adopted by astronomers is of an optically bright disk of gas spiralling into the black hole, with very hot thin gas above and below (possibly supported by magnetic fields). Low density hot gas cools very inefficiently, and so the electrons have the energy to convert optical photons to X-ray photons. Many AGN are very bright at optical to extreme ultra-violet wavelengths, and this 'Big Blue Bump' in the spectrum is supposed to be the light from the accretion disk.

Recent observations with the European Space Agency's XMM-Newton X-ray observatory by Professor Ken Pounds, Dr James Reeves, Professor Andrew King, Miss Kim Page, Dr Paul O'Brien and Dr Martin Turner have shown that this simple picture needs to be changed, at least for some of the brightest AGN. Using the high sensitivity and spectral resolution of the EPIC and RGS instruments on XMM, Prof Pounds has found narrow absorption lines superimposed on the underlying power law spectrum in the bright quasar PG1211+143. These lines are due to highly ionised iron, sulfur and other lighter elements, and indicate that there is a large amount of gas (equivalent to 5x10²³ H atoms.cm^-3) surrounding the primary X-ray source. This gas has not been detected before because it is almost transparent due to being so highly ionised. The absorption lines are seen to be blue-shifted with respect to their laboratory wavelengths, indicating that the the Doppler effect is important and that the gas is flowing away from the black hole with a speed around 24,000 km.s^-1.

The massive outflow implies that closer to the black hole the gas is likely to be opaque. This has important consequences, leading to an extremely a bright surface at the distance from the black hole where this gas become transparent. This bright surface (a 'photosphere' like the visible surface of the sun) will have a temperature of around 100,000 degrees and a radius of approximately 1 billion kilometers. The hundred thousand degree photosphere implied then provides a natural explanation for the Big Blue Bump which dominates the radiation from many AGN.

A massive outflow, similar to that seen in PG1211+143 has also now been seen in the quasar PG0844+349 and in 3 other objects. In PG0844+349 the 'black hole wind' is even faster, reaching 20% of the speed of light. These black hole winds carry a large amount of gas away from the black hole, about a solar mass per year. This is in fact exactly what would be expected for a black hole accreting as much as it can (at the Eddington limit), higher accretion rates resulting in a radiation pressure sufficient to halt and reverse the direction of the inflowing gas. Previously it was suspected that the black hole was able to swallow large amounts of gas without emitting the expected radiation (ie the radiation was advected into the hole), but the newly discovered massive winds suggest that instead the gas is actually not accreted in the first place.

The new understanding of the X-ray spectrum of high accretion rate AGN has one further impact. It had been thought that the iron X-ray emission lines seen in some AGN were broadened by relativistic effects due the strong gravity close to the black hole. This now seems less certain, firstly because the underlying continuum spectrum is more complex than was assumed, and also because the region close to the black hole where relativity is important is hidden by the black hole wind photosphere.

A report on earlier AGN research at Leicester can be found here. The papers detailing the results from PG1211+143 and PG0844+349 are available from the arXiv service. They will be published in the Monthly Notices of the Royal Astronomical Society (PG1211+143 accepted, PG0844+349 submitted).

Other news items are available here.

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Last updated: 2003 August 11 by Julian Osborne