X-ray and Observational Astronomy

News October 2002: Extreme X-ray variability in quasar PDS 456 suggests cascading magnetic reconnection near spinning black hole

X-ray light curves of the bright quasar PDS 456 from the low and medium energy telescopes on the BeppoSAX satellite (0.3-2 & 1-10 keV). The very rapid changes in intensity imply a small emission region.	An XMM-Newton EPIC MOS X-ray image of the quasar, 6 arcminutes across. The X-rays come from a point-like source, the rays extending from the centre are due to the X-ray telescope support structure.	An image taken in red light of the quasar. PDS 456 is the bright object in the centre of this 1 arcminute DSS2 image. The galaxy which hosts this bright nucleus is too faint to be visible here.

Quasars are 'quasi-stellar radio sources' (or QSOs). They were discovered in 1963 as point-like emitters of radio energy, and soon realised to be extraordinarily luminous at all wavelengths. It is this massive luminosity coming from a small region which makes quasars so interesting. Many thousands of quasars are now known, some have strong radio emission and long radio jets, but most do not.

A quasar is an extreme form of an Active Galactic Nucleus. Our own Galaxy, and our nearby neighbour, the Andromeda galaxy have a smooth distribution of light coming from their inner regions, whereas galaxies containing an AGN have very bright emission from the very centre of the galaxy. In quasars this contrast is so extreme that it is only recently, using the Hubble Space Telescope, that it has been possible to see the host galaxy at all.

We now suspect that all substantial galaxies contain a massive black hole at their centre. For some reason it is very faint in our Galaxy and in Andromeda, but in the AGN and particularly in quasars this black hole is radiating vast amounts of energy, presumably by accreting gas from the inner regions of the host galaxy (as in this diagram). Just how this emission occurs is unclear, but recent X-ray observations may provide a clue.

James Reeves, Graham Wynn, Paul O'Brien and Ken Pounds at the University of Leicester have used the BeppoSAX and XMM-Newton X-ray observatories to extend their earlier work showing that PDS 456 is the most luminous 'local' quasar accreting at an unusually high rate (ie a 10^9 solar mass black hole accreting at the Eddington limit, producing 10^47 erg/sec). The BeppoSAX X-ray light curve (shown top left) exhibits extreme variability, with the low energy flux increasing by 4 times in just 11 hours. This rise time defines a maximum size of the emission region (which must be smaller than the time it takes the light to cross it). This size is the size of the solar system (ie 10^15 cm). The rate of change of luminosity is actually greater than the theoretically allowed maximum for a stationary black hole, and so requires that the black hole be rapidly spinning. Because PDS 456 does not have substantial radio emission, it does not have a jet, and so relativistic boosting of the luminosity cannot be responsible for the rapid and massive luminosity changes.

The very rapid variability is faster than orbital period of matter close to the black hole, leading the authors to suggest that the flares are magnetic reconnection events due to buoyant magnetic flux tubes such as give rise to flares on the Sun. Magnetic flux would be amplified by the differential rotation of the accretion disk, tubes of flux rising into the disk corona when the field strength exceeded ~5000 Gauss. These will rapidly accelerate and heat the plasma contained within them when oppositely directed field lines come into contact high above the accretion disk. Although the disk corona can contain a very large magnetic energy reservoir, individual reconnection events are likely to be much less energetic than the massive flares seen in the X-ray light curve, due the limited space close to the black hole. Thus a triggered cascade of magnetic reconnection events is required to account for the large coherent flares seen, such a chain reaction can occur due to the high field filling factor and because the changes in field geometry due to the first reconnection cause a rearrangement of the flux tubes so allowing further reconnection events to occur.

Magnetic reconnection cascades, and hence large individual flares in the X-ray light curve, are favoured when the disk corona is rapidly filled with magnetic flux tubes. This occurs when the accretion rate is high, and so this model predicts that the highest accretion rate AGN (ie those close to the Eddington limit ) will show the greatest X-ray variability. The large flares seen in the lower luminosity (and lower black hole mass) narrow line Seyfert 1 galaxies appears to be consistent with this picture.

This work was recently accepted for publication in Monthly Notices of the Royal Astronomical Society.

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Last updated: 2002 October 09 by Julian Osborne