Publications (4)1.01 Total impact
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Article: AGN in the XMM fields of Mrk 205 and QSO 0130–403
Astronomische Nachrichten 02/2003; 324(1‐2):167 - 167. · 1.01 Impact Factor -
Article: Extreme X-ray variability in the luminous quasar PDS 456
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ABSTRACT: We present evidence from Beppo-SAX and XMM-Newton of extreme X-ray variability in the high luminosity radio-quiet quasar PDS 456, the most luminous known AGN at z<0.3. Repeated X-ray flaring is found in PDS 456, over the duration of the 340 ksec long Beppo-SAX observation. The X-ray flux doubles in just 30 ksec, whilst the total energy output of the flaring events is as high as 10^51 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 10^9 solar mass black hole. From the rates of change of luminosity observed during the X-ray flares, we calculate lower limits for the radiative efficiency between 0.06 and 0.41, implying that accretion onto 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 timescale 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 about 1000 individual flares over a timescale of the order 1 day. Comment: 5 pages. Accepted for publication in MNRAS Letters09/2002; -
Article: Studies of the high luminosity quasar, PDS 456
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ABSTRACT: X-ray and multi-wavelength observations of the most luminous known local (z<0.3) AGN, the recently discovered radio-quiet quasar PDS 456, are presented. The spectral energy distribution shows that PDS 456 has a bolometric luminosity of 1e47 erg/s, peaking in the UV. The X-ray spectrum obtained by ASCA and RXTE shows considerable complexity. The most striking feature observed is a deep, highly-ionised, iron K edge (8.7 keV, rest-frame), originating via reprocessing from highly ionised material, possibly the inner accretion disk. PDS 456 was found to be remarkably variable for its luminosity; in one flare the X-ray flux doubled in just about 15 ksec. If confirmed this would be an unprecedented event in a high-luminosity source, with a light-crossing time corresponding to about 2RS. The implications are that either flaring occurs within the very central regions, or else that PDS 456 is a super-Eddington or relativistically beamed system.06/2000; -
Article: High velocity X-ray outflows in Active Galactic Nuclei
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ABSTRACT: Evidence for high velocity, massive outflows in AGN and quasars will be presented. The increased sensitivity of XMM-Newton has allowed us to detect the highest known ionisation components to the AGN warm absorber, in the form of deep iron K-shell absorption lines and edges. In the Seyfert 1 galaxy NGC 3783, we detect high ionisation absorption from He-like iron, which appears to vary on short timescales of <1 day, constraining the location of the high ionisation absorber to <0.1 pc from the nucleus. The warm absorber also adds sufficient opacity to the X-ray spectrum above 2 keV, reducing the requirement for the relativistic iron line from the accretion disc. In some higher luminosity AGN, such as in the quasars PG 1211+143 and PDS 456, we find extreme X-ray absorbers, with large column densities (1024 cm-2) and high ionisation parameters (log ξ˜3), whilst the outflow velocities can reach 0.1-0.15c. For these AGN, a large fraction of the total energy budget arises from the bulk kinetic outflow, driven from the accretion disc, with mass outflow rates near Eddington. We will also discuss the impact of these outflows on the local AGN environment.35:3660.
