Abstract
We report the spectral analysis of a long XMM-Newton observation of the well-studied, moderate luminosity Broad Line Seyfert 1 galaxy NGC 5548. The source was at an historically average brightness and we find the hard (3-10 keV) spectrum can be well fitted by a power law of photon index gamma ~ 1.75, together with reflection. The only feature in the hard X-ray spectrum is a narrow emission line near 6.4 keV, with an equivalent width of ~ 60 eV. The energy and strength of this line is consistent with fluorescence from `neutral' iron distant from the central continuum source. We find no evidence for a broad Fe K line, with an upper limit well below previous reports, suggesting the inner accretion disc is now absent or highly ionised. The addition of simultaneous BeppoSAX data allows the analysis to be extended to 200 keV, yielding important constraints on the total reflection. Extrapolation of the hard X-ray power law down to 0.3 keV shows a clear `soft excess' below ~ 0.7 keV. After due allowance for the effects of a complex warm absorber, measured with the XMM-Newton RGS, we find the soft excess is better described as a smooth upward curvature in the continuum flux below ~ 2 keV. The soft excess can be modelled either by Comptonised thermal emission or by enhanced reflection from the surface of a highly ionised disc.
Highlights
It is widely believed that the primary luminosity of an active galactic nucleus (AGN) originates in an accretion disc around a supermassive black hole
In the X-ray band, a hard power-law component generally dominates above ∼2 keV in the well studied broad line Seyfert 1 galaxies and is considered to arise in a hot corona above the surface of the accretion disc, where optical/ultraviolet (UV) photons from the disc are Comptonized to X-ray energies
The key importance of measuring continuum reflection is underlined in our study of NGC 5548 where it has a significant effect on the deduced hard X-ray power-law slope, which in turn is critical in quantifying any soft excess component
Summary
It is widely believed that the primary luminosity of an active galactic nucleus (AGN) originates in an accretion disc around a supermassive black hole. In the X-ray band, a hard power-law component generally dominates above ∼2 keV in the well studied broad line Seyfert 1 galaxies and is considered to arise in a hot corona above the surface of the accretion disc, where optical/ultraviolet (UV) photons from the disc are Comptonized to X-ray energies These X-rays in turn illuminate the disc, being either ‘reflected’ towards the observer or thermalized back into optical/UV emission (Mushotzky, Done & Pounds 1993). In a long BeppoSAX observation in 1997, Nicastro et al (2000) found the spectral variability in NGC 5548 was primarily due to a change in the power-law slope, with no requirement for a soft excess Other results of this BeppoSAX study were: a high-energy cut-off in the range 90–165 keV; low-energy absorption well-modelled by O VII and O VIII edges; an unresolved Fe K emission line and a continuum ‘hump’ around 20–30 keV consistent with reflection R ∼ 0.5 from ‘cold’ matter
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