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Register a new userThe relation between optical and X-ray variability in Seyfert galaxies
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P. Uttley
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Studying simultaneous optical and X-ray light curves of radio-quiet AGN can help to probe the relationship between very different physical components - the cool, optically thick disk and hot, optically thin corona. Here, we review the relationship between optical and X-ray variability in Seyfert galaxies, which due to observing constraints was difficult to study for many years, but was given a huge boost with the launch of the RXTE satellite in 1995. We summarise the diverse results of several monitoring campaigns, which pose a challenge for standard theories relating optical and X-ray variability, with sources showing either correlated optical and X-ray flux variations, correlated optical flux and X-ray spectral variations, or no correlation at all. We discuss possible explanations for these results, some of which may be explained using a more standard AGN picture, while others may require additional components, such as the 2-phase accretion flows suggested to explain black hole X-ray binary behaviour.
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The optical classification of a Seyfert galaxy and whether it is considered X-ray absorbed are often used interchangeably. But there are many borderline cases and also numerous examples where the optical and X-ray classifications appear to be in conflict. In this article we re-visit the relation between optical obscuration and X-ray absorption in AGNs. We make use of our dust color method (Burtscher et al. 2015) to derive the optical obscuration A_V and consistently estimated X-ray absorbing columns using 0.3--150 keV spectral energy distributions. We also take into account the variable nature of the neutral gas column N_H and derive the Seyfert sub-classes of all our objects in a consistent way. We show in a sample of 25 local, hard-X-ray detected Seyfert galaxies (log L_X / (erg/s) ~ 41.5 - 43.5) that there can actually be a good agreement between optical and X-ray classification. If Seyfert types 1.8 and 1.9 are considered unobscured, the threshold between X-ray unabsorbed and absorbed should be chosen at a column N_H = 10^22.3 / cm^2 to be consistent with the optical classification. We find that N_H is related to A_V and that the N_H/A_V ratio is approximately Galactic or higher in all sources, as indicated previously. But in several objects we also see that deviations from the Galactic ratio are only due to a variable X-ray column, showing that (1) deviations from the Galactic N_H/A_V can simply be explained by dust-free neutral gas within the broad line region in some sources, that (2) the dust properties in AGNs can be similar to Galactic dust and that (3) the dust color method is a robust way to estimate the optical extinction towards the sublimation radius in all but the most obscured AGNs.
Seyfert 1.8/1.9 are sources showing weak broad H-alpha components in their optical spectra. We aim at testing whether Seyfert 1.8/1.9 have similar properties at UV and X-ray wavelengths to Seyfert 2. We use the 15 Seyfert 1.8/1.9 in the Veron Cetty and Veron catalogue with public data available from the Chandra and/or XMM-Newton archives at different dates, with timescales between observations ranging from days to years. Our results are homogeneously compared with a previous work using the same methodology applied to a sample of Seyfert 2 (Hernandez-Garcia et al. 2015). X-ray variability is found in all 15 nuclei over the aforementioned ranges of timescales. The main variability pattern is related to intrinsic changes in the sources, which are observed in ten nuclei. Changes in the column density are also frequent, as they are observed in six nuclei, and variations at soft energies, possibly related to scattered nuclear emission, are detected in six sources. X-ray intraday variations are detected in six out of the eight studied sources. Variations at UV frequencies are detected in seven out of nine sources. A comparison between the samples of Seyfert 1.8/1.9 and 2 shows that, even if the main variability pattern is due to intrinsic changes of the sources in the two families, these nuclei exhibit different variability properties in the UV and X-ray domains. In particular, variations in the broad X-ray band on short time-scales (days/weeks), and variations in the soft X-rays and UV on long time-scales (months/years) are detected in Seyfert 1.8/1.9 but not in Seyfert 2. Overall, we suggest that optically classified Seyfert 1.8/1.9 should be kept separated from Seyfert 2 galaxies in UV/X-ray studies of the obscured AGN population because their intrinsic properties might be different.
The rapid and seemingly random fluctuations in X-ray luminosity of Seyfert galaxies provided early support for the standard model in which Seyferts are powered by a supermassive black hole fed from an accretion disc. However, since EXOSAT there has been little opportunity to advance our understanding of the most rapid X-ray variability. Observations with XMM-Newton have changed this. We discuss some recent results obtained from XMM-Newton observations of Seyfert 1 galaxies. Particular attention will be given to the remarkable similarity found between the timing properties of Seyferts and black hole X-ray binaries, including the power spectrum and the cross spectrum (time delays and coherence), and their implications for the physical processes at work in Seyferts.
We have explored the relationship between the [O III] $lambda$5007 and the 2--10 keV luminosities for a sample of Broad- and Narrow-Line Seyfert 1 galaxies (BLSy1 and NLSy1, respectively). We find that both types of Seyferts span the same range in luminosity and possess similar [O III]/X-ray ratios. The NLSy1s are more luminous than BLSy1s, when normalized to their central black hole masses, which is attributed to higher mass accretion rates. However, we find no evidence for elevated [O III]/X-ray ratios in NLSy1s, which would have been expected if they had excess EUV continuum emission compared to BLSy1s. Also, other studies suggest that the gas in narrow-line regions (NLR) of NLSy1s and NLSy1s span a similar range in ionization, contrary to what is expected if those of the former are exposed to a stronger flux of EUV radiation. The simplest interpretation is that, like BLSy1s, a large EUV bump is not present in NLSy1s. However, we show that the [OIII]/X-ray ratio can be lowered as a result of absorption of the ionizing continuum by gas close to the central source, although there is no evidence that intrinsic line-of-sight absorption is more common among NLSy1s, as would be expected if there were a larger amount of circumnuclear gas. Other possible explanations include: 1) anisotropic emission of the ionizing radiation, 2) higher gas densities in the NLR of NLSy1s, resulting in lower average ionization, or 3) the presence of strong winds in the the nuclei of NLSy1s which may drive off much of the gas in the narrow-line region, resulting in lower cover fraction and weaker [O III] emission.
We discuss the origin of the optical variations in the Narrow line Seyfert 1 galaxy NGC 4051 and present the results of a cross-correlation study using X-ray and optical light curves spanning more than 12 years. The emission is highly variable in all wavebands, and the amplitude of the optical variations is found to be smaller than that of the X-rays, even after correcting for the contaminating host galaxy flux falling inside the photometric aperture. The optical power spectrum is best described by an unbroken power law model with slope $alpha=1.4^{+0.6}_{-0.2}$ and displays lower variability power than the 2-10 keV X-rays on all time-scales probed. We find the light curves to be significantly correlated at an optical delay of $1.2^{+1.0}_{-0.3}$ days behind the X-rays. This time-scale is consistent with the light travel time to the optical emitting region of the accretion disc, suggesting that the optical variations are driven by X-ray reprocessing. We show, however, that a model whereby the optical variations arise from reprocessing by a flat accretion disc cannot account for all the optical variability. There is also a second significant peak in the cross-correlation function, at an optical delay of $39^{+2.7}_{-8.4}$ days. The lag is consistent with the dust sublimation radius in this source, suggesting that there is a measurable amount of optical flux coming from the dust torus. We discuss the origin of the additional optical flux in terms of reprocessing of X-rays and reflection of optical light by the dust.
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