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Register a new userEnsemble X-ray variability of Active Galactic Nuclei from serendipitous source catalogues
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The X-ray variability of the Active Galactic Nuclei (AGN) has been most often investigated with studies of individual, nearby, sources, and only a few ensemble analyses have been applied to large samples in wide ranges of luminosity and redshift. We want to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogues of XMM-Newton and Swift, with redshift between ~0.2 and ~4.5, and X-ray luminosities, in the 0.5-4.5 keV band, between ~10^43 erg/s and ~10^46 erg/s. We use the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. SF is also more appropriate than fractional variability and excess variance, because such parameters are biased by the duration of the monitoring time interval in the rest-frame, and thus by cosmological time dilation. We find statistically consistent results for the two samples, with the SF described by a power law of the time lag, approximately as SF propto tau^0.1. We do not find evidence of the break in the SF, at variance with the case of lower luminosity AGNs. We confirm a strong anti-correlation of the variability with X-ray luminosity, accompanied by a change of the slope of the SF. We find evidence in support of a weak, intrinsic, average increase of X-ray variability with redshift. The change of amplitude and slope of the SF with X-ray luminosity provides new constraints on both single oscillator models and multiple subunits models of variability.
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The variability of the X-ray spectra of active galactic nuclei (AGN) usually includes a change of the spectral slope. This has been investigated for a small sample of local AGNs by Sobolewska and Papadakis, who found that slope variations are well correlated with flux variations, and that spectra are typically steeper in the bright phase (softer when brighter behaviour). Not much information is available for the spectral variability of high-luminosity AGNs and quasars. In order to investigate this phenomenon, we use data from the XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray observations for a large number of active galactic nuclei in a wide luminosity and redshift range, for several different epochs. This allows to perform an ensemble analysis of the spectral variability for a large sample of quasars. We quantify the spectral variability through the spectral variability parameter $beta$, defined as the ratio between the change in spectral slope and the corresponding logarithmic flux variation. We find that the spectral variability of quasars has a softer when brighter behaviour, similarly to local AGNs.
The observed relation between the X-ray radiation from AGNs, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope alpha_ox and the UV luminosity. Many factors can affect this relation, including: enhanced X-ray emission associated with the jets of radio-loud AGNs; X-ray absorption associated with the UV Broad Absorption Line (BAL) outflows; other X-ray absorption not associated with BALs; intrinsic X-ray weakness; UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic alpha_ox-L_UV relation and its dispersion, constraining models of disk-corona coupling. We extract simultaneous data from the second XMM-Newton serendipitous source catalogue and the XMM-Newton Optical Monitor Serendipitous UV Source Survey Catalog, and derive the single-epoch alpha_ox indices. We use ensemble structure functions to analyse multi-epoch data. We confirm the anticorrelation of alpha_ox with L_UV, and do not find any evidence of a dependence of alpha_ox on z. The dispersion in our simultaneous data (0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that artificial alpha_ox variability introduced by non-simultaneity is not the main cause of dispersion. Intrinsic alpha_ox variability, i.e., the true variability of the X-ray to optical ratio, is instead important, and accounts for ~30% of the total variance, or more. Inter-source dispersion, due to intrinsic differences in the average alpha_ox values from source to source, is also important. The dispersion introduced by variability is mostly caused by the long timescale variations, which are expected to be driven by the optical variations.
Most investigations of the X-ray variability of active galactic nuclei (AGN) have been concentrated on the detailed analyses of individual, nearby sources. A relatively small number of studies have treated the ensemble behaviour of the more general AGN population in wider regions of the luminosity-redshift plane. We want to determine the ensemble variability properties of a rich AGN sample, called Multi-Epoch XMM Serendipitous AGN Sample (MEXSAS), extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue (XMMSSC-DR5), with redshift between 0.1 and 5, and X-ray luminosities in the 0.5-4.5 keV band between 10^42 and 10^47 erg/s. We urge caution on the use of the normalised excess variance (NXS), noting that it may lead to underestimate variability if used improperly. We use the structure function (SF), updating our previous analysis for a smaller sample. We propose a correction to the NXS variability estimator, accounting for the light curve duration in the rest frame on the basis of the knowledge of the variability behaviour gained by SF studies. We find an ensemble increase of the X-ray variability with the rest-frame time lag tau, given by tau^0.12. We confirm an inverse dependence on the X-ray luminosity, approximately as L_X^-0.19. We analyse the SF in different X-ray bands, finding a dependence of the variability on the frequency as nu^-0.15, corresponding to a softer when brighter trend. In turn, this dependence allows us to parametrically correct the variability estimated in observer-frame bands to that in the rest frame, resulting in a moderate shift upwards (V-correction). Ensemble X-ray variability of AGNs is best described by the structure function. An improper use of the normalised excess variance may lead to an underestimate of the intrinsic variability, so that appropriate corrections to the data or the models must be applied to prevent these effects.
We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The high-quality, echellette Magellan spectra reveal broad H$alpha$ emission in all the sources, allowing us to estimate robust viral BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median $M_{rm BH} = 1.2times 10^6M_odot$ and median $L_{rm bol}/L_{rm Edd}=0.44$. The sample follows the $M_{rm BH}-sigma_*$ relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O II] $lambda 3727$, [O III] $lambda 5007$, and X-rays.
We present results on a systematic study of flux variability on hourly time-scales in a large sample of active galactic nuclei (AGN) in the 3-79 keV band using data from Nuclear Spectroscopic Telescope Array. Our sample consists of 4 BL Lac objects (BL Lacs), 3 flat spectrum radio quasars (FSRQs) 24 Seyfert 1, 42 Seyfert 2 and 8 narrow line Seyfert 1 (NLSy1) galaxies. We find that in the 3-79 keV band, about 65% of the sources in our sample show significant variations on hourly time scales. Using Mann-Whitney U-test and Kolmogorov-Smirnov test, we find no difference in the variability behaviour between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our sample, are more variable than Seyfert galaxies that include Seyfert 1 and Seyfert 2 in the soft (3-10 keV), hard (10-79 keV) and total (3-79 keV) bands. NLSy1 galaxies show the highest duty cycle of variability (87%), followed by BL Lacs (82%), Seyfert galaxies (56%) and FSRQs (23%). We obtained flux doubling/halving time in the hard X-ray band less than 10 min in 11 sources. The flux variations between the hard and soft bands in all the sources in our sample are consistent with zero lag.
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