SIMBAD references

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.