Euclid

Abstract:

The absolute luminosity of the Fe Kα emission line from matter illuminated by X-rays in astrophysical sources is non-trivial to calculate except when the line-emitting medium is optically thin to absorption and scattering. We characterize the Fe Kα line flux using a dimensionless efficiency, defined as the fraction of continuum photons above the Fe K shell absorption edge threshold energy that appear in the line. The optically thin approximation begins to break down even for column densities as small as 2 × 1022 cm-2. We show how to obtain reliable estimates of the Fe Kα line efficiency in the case of cold, neutral matter, even for the Compton-thick regime. We find that, regardless of geometry and covering factor, the largest Fe Kα line efficiency is attained well before the medium becomes Compton-thick. For cosmic elemental abundances it is difficult to achieve an efficiency higher than a few per cent under the most favourable conditions and lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe Kα line efficiencies that are orders of magnitude less than the maximum possible for that geometry. Configurations that allow unobscured views of a Compton-thick reflecting surface are capable of yielding the highest efficiencies. Our results can be used to estimate the predicted flux of the narrow Fe Kα line at ∼6.4 keV from absorption models in active galactic nucleus (AGN). In particular we show that contrary to a recent claim in the literature, absorption-dominated models for the relativistic Fe Kα emission line in MCG-6-30-15 do not overpredict the narrow Fe Kα line for any column density or covering factor. © 2009 RAS.

Abstract:

A Suzaku X-ray observation of NGC 4051 taken during 2005 November reveals line emission at 5.44keV in the rest frame of the galaxy which does not have an obvious origin in known rest-frame atomic transitions. The improvement to the fit statistic when this line is accounted for establishes its reality at >99.9% confidence: we have also verified that the line is detected in the three X-ray Imaging Spectrometer units independently. Comparison between the data and Monte Carlo simulations shows that the probability of the line being a statistical fluctuation is p < 3.3 × 10-4. Consideration of three independent line detections in Suzaku data taken at different epochs yields a probability p < 3 × 10-11 and thus conclusively demonstrates that it cannot be a statistical fluctuation in the data. The new line and a strong component of Fe Kα emission from neutral material are prominent when the source flux is low, during 2005. Spectra from 2008 show evidence for a line consistent with having the same flux and energy as that observed during 2005, but inconsistent with having a constant equivalent width against the observed continuum. The stability of the line flux and energy suggests that it may not arise in transient hotspots, as has been suggested for similar lines in other sources, but could arise from a special location in the reprocessor, such as the inner edge of the accretion disk. Alternatively, the line energy may be explained by spallation of Fe into Cr, as discussed in a companion paper. © 2010. The American Astronomical Society.

Abstract:

Long-exposure Suzaku X-ray observations of the nearby active galaxy NGC 4051 from 2005 and 2008 are analysed, in an attempt to reach a self-consistent understanding of both the spectral variability on long time-scales and the broad-band variability at high time resolution. The techniques of principal components analysis and a maximum likelihood method of power spectrum analysis are used. In common with other type I active galactic nuclei (AGN), the spectral variability is dominated by a varying-normalization power-law component together with a quasi-steady, hard-spectrum offset component that contains Fe K atomic features. NGC 4051 displays a strong excess over a power law at energies of above 20 keV, some fraction of which also appears to vary with the power-law continuum. The fast time-scale power spectrum has a shape generally consistent with previous determinations, with the previously known dependence on broad-band photon energy, but in the new data significant differences are found between the low and high flux states of the source, demonstrating that the power spectrum is non-stationary. Frequency-dependent time lags between the hard and soft bands of up to 970± 225 s are measured. The existence of the observed time lags excludes the possibility that the hard spectral component originates as reflection from the inner accretion disc. We instead show that the time lags and their frequency and energy dependence may be explained simply by the effects of reverberation in the hard band, caused by reflection from a thick shell of material with maximum lags of about 10000 s. If the reflecting material surrounds the AGN, it extends to a distance of about 1.5× 1014cm, 600 gravitational radii, from the illuminating source and the global covering factor is Cg≳0.44, confirming previous suggestions that type I AGN have high covering factors of absorbing and reflecting material. Given the spectral and timing similarities with other type I AGN, we infer that this source structure is common in the type I population. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

The narrow-line Seyfert 1 galaxy 1H 0707-495 has previously been identified as showing time lags between flux variations in the soft-energy (0.3-1 keV) and medium-energy (1-4 keV) X-ray bands that oscillate between positive and negative values as a function of the frequency of the mode of variation. Here we measure and analyse the lags also between a harder X-ray band (4-7.5 keV) and the soft and medium bands, using existing XMM-Newton data, and demonstrate that the entire spectrum of lags, considering both the full energy range, 0.3-7.5 keV, and the full frequency range, 10-5≲ν≲ 10-2 Hz, are inconsistent with previous claims of arising as reverberation associated with the inner accretion disc. Instead we demonstrate that a simple reverberation model, in which scattering or reflection is present in all X-ray bands, explains the full set of lags without requiring any ad hoc explanation for the time lag sign changes. The range of time delays required to explain the observed lags extends up to about 1800 s in the hard band. The results are consistent with reverberation caused by scattering of X-rays passing through an absorbing medium whose opacity decreases with increasing energy and that partially covers the source. A high covering factor of absorbing and scattering circumnuclear material is inferred. © 2010 The Authors. Journal compilation © 2010 RAS.