Extreme X-ray variability in the luminous quasar PDS 456

We present evidence from BeppoSAX and XMM–Newton of extreme X-ray variability in the high-luminosity radio-quiet quasar PDS 456, the most luminous known active galactic nucleus (AGN) at z < 0.3. Repeated X-ray flaring is found in PDS 456, over the duration of the 340-ks long BeppoSAX observation. The X-ray flux doubles in just 30 ks, whilst the total energy output of the flaring events is as high as 1051 erg. Under the assumption of isotropic emission at the Eddington limit, this implies that the size of the X-ray emitting region in PDS 456 is less than 3 Schwarzschild radii, for a 109-M⊙ black hole. From the rates of change of luminosity observed during the X-ray flares, we calculate lower limits for the radiative efficiency limit between 0.06 and 0.41, implying that accretion on to a Kerr black hole is likely in PDS 456. We suggest that the rapid variability is from X-ray flares produced through magnetic reconnection above the disc and calculate that the energetics and time-scale of the flares are plausible if the quasar is accreting near to the maximum Eddington rate. A similar mechanism may account for the extreme, rapid X-ray variability observed in many narrow line Seyfert 1s. In the case of PDS 456, we show that the X-ray flaring could be reproduced through a self-induced cascade of ∼1000 individual flares over a time-scale of the order 1 d.