Context. The well established negative correlation between the αOX spectral slope and the optical/ultraviolet (UV) luminosity, a by-product of the relation between X-rays and optical/UV luminosity, ...is affected by relatively large dispersion. The main contributors to this dispersion can be variability in the X-ray/UV ratio and/or changes in fundamental physical parameters. Aims. We want to quantify the contribution from variability within single sources (intra-source dispersion) and that from variations of other quantities different from source to source (inter-source dispersion). Methods. We use archival data from the XMM-Newton Serendipitous Source Catalog (XMMSSC) and from the XMM-OM Serendipitous Ultraviolet Source Survey (XMMOM-SUSS3). We select a sub-sample in order to decrease the dispersion of the relation due to the presence of radio-loud and broad absorption line objects, and that due to absorptions in both X-ray and optical/UV bands. We use the structure function (SF) to estimate the contribution from variability to the dispersion. We analyse the dependence of the residuals of the relation on various physical parameters in order to characterise the inter-source dispersion. Results. We find a total dispersion of σ ∼ 0.12 and find that intrinsic variability contributes 56% of the variance of the αOX − LUV relation. If we select only sources with a larger number of observational epochs (≥3) the dispersion of the relation decreases by approximately 15%. We find weak but significant dependencies of the residuals of the relation on black-hole mass and on Eddington ratio, which are also confirmed by a multivariate regression analysis of αOX as a function of UV luminosity and black-hole mass and/or Eddington ratio. We find a weak positive correlation of both the αOX index and the residuals of the αOX − LUV relation with inclination indicators, such as the full width at half maximum (Hβ) and the equivalent width (EW)OIII, suggesting a weak increase of X-ray/UV ratio with the viewing angle. This suggests the development of new viewing angle indicators possibly applicable at higher redshifts. Moreover, our results suggest the possibility of selecting a sample of objects, based on their viewing angle and/or black-hole mass and Eddington ratio, for which the αOX − LUV relation is as tight as possible, in light of the use of the optical/UV – X-ray luminosity relation to build a distance modulus (DM)-z plane and estimate cosmological parameters.
Context. 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. Aims. 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 super(42) erg/s and ~10 super(47) erg/s. Methods. 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, taking account of the light curve duration in the rest frame on the basis of the knowledge of the variability behaviour gained by SF studies. Results. We find an ensemble increase of the X-ray variability with the rest-frame time lag tau, given by SF is proportional to tau super(0.12). We confirm an inverse dependence on the X-ray luminosity, approximately as SF is proportional to L sub(X) super(-0.19). We analyse the SF in different X-ray bands, finding a dependence of the variability on the frequency as SF is proportional to nu super(-0.15), corresponding to a so-called 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 (< or =15%) shift upwards (V-correction). Conclusions. 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.
Improving our understanding of the nuclear properties of high-Eddington-ratio (
λ
Edd
) active galactic nuclei (AGN) is necessary since at this regime the radiation pressure is expected to affect the ...structure and efficiency of the accretion disc-corona system. This may cause departures from the typical nuclear properties of low-
λ
Edd
AGN, which have been largely studied so far. We present here the X-ray spectral analysis of 14 radio-quiet,
λ
Edd
≳ 1 AGN at 0.4 ≤
z
≤ 0.75, observed with
XMM-Newton
. Optical/UV data from simultaneous Optical Monitor observations have also been considered. These quasars were selected to have relatively high values of black hole mass (
M
BH
∼ 10
8 − 8.5
M
⊙
) and bolometric luminosity (
L
bol
∼ 10
46
erg s
−1
) in order to complement previous studies of high-
λ
Edd
AGN at lower
M
BH
and
L
bol
. We studied the relation between
λ
Edd
and other key X-ray spectral parameters, such as the photon index (Γ) of the power-law continuum, the X-ray bolometric correction (
k
bol, X
), and the optical/UV-to-X-ray spectral index (
α
ox
). Our analysis reveals that, despite the homogeneous optical and supermassive black hole accretion properties, the X-ray properties of these high-
λ
Edd
AGN are quite heterogeneous. We indeed measured values of Γ between 1.3 and 2.5, at odds with the expectations based on previously reported Γ −
λ
Edd
relations, for which Γ ≥ 2 would be a ubiquitous hallmark of AGN with
λ
Edd
∼ 1. Interestingly, we found that ∼30% of the sources are X-ray weak, with an X-ray emission about a factor of ∼10 − 80 fainter than that of typical AGN at similar UV luminosities. The X-ray weakness seems to be intrinsic and not due to the presence of absorption along the line of sight to the nucleus. This result may indicate that high-
λ
Edd
AGN commonly undergo periods of intrinsic X-ray weakness. Furthermore, results from follow-up monitoring with
Swift
of one of these X-ray weak sources suggest that these periods can last for several years.
Context.
Ultra-fast outflows (UFOs) are the most powerful disk-driven winds in active galactic nuclei (AGNs). Theoretical and observational evidence shows that UFOs play a key role in the AGN ...feedback mechanism. The mechanical power of the strongest UFOs may be enough to propagate the feedback to the host galaxies and ultimately shape the AGN-galaxy coevolution. It is therefore of paramount importance to fully characterize UFOs, their location, and energetics.
Aims.
We study two
XMM-Newton
archival observations of the narrow-line Seyfert 1 galaxy PG 1448+273. We concentrate on the latest observation, whose spectrum is characterized by a strong absorption feature in the Fe K band. This feature represents the spectral imprint of a UFO, as confirmed by other independent analyses. We study this feature in detail with a novel modeling tool.
Methods.
In order to constrain the physical properties of the UFO, we implemented the novel model called wind in the ionized nuclear environment (WINE) to fit the photoionized emission and absorption lines from a disk wind in X-ray spectra. WINE is a photoionization model that allows us to self-consistently calculate absorption and emission profiles. It also takes special relativistic effects into account.
Results.
Our detection of the UFO in PG 1448+273 is very robust. The outflowing material is highly ionized, log
ξ
= 5.53
−0.05
+0.04
erg s
−1
cm, has a high column density,
N
H
= 4.5
−1.1
+0.8
× 10
23
cm
−2
, is ejected with a maximum velocity
v
0
= 0.24
−0.06
+0.08
c
(90% confidence level errors), and attains an average velocity
v
avg
= 0.152
c
. WINE succeeds remarkably well to constrain a launching radius of
r
0
= 77
−19
+31
r
S
from the black hole. We also derive a lower limit on both the opening angle of the wind (
θ
> 72°) and the covering factor (
C
f
> 0.69). We find a mass outflow rate
Ṁ
out
= 0.65
−0.33
+0.44
M
⊙
yr
−1
= 2.0
−1.0
+1.3
Ṁ
acc
and a high instantaneous outflow kinetic power
Ė
out
= 4.4
−3.6
+4.4
× 10
44
erg s
−1
= 24%
L
bol
= 18%
L
Edd
(1
σ
errors). We find that a major error contribution on the energetics is due to
r
0
, stressing the importance of an accurate determination through proper spectral modeling, as done with WINE. Finally, using 20
Swift
(UVOT and XRT) observations together with the simultaneous Optical Monitor data from
XMM-Newton
, we also find that
α
ox
varied strongly, with a maximum excursion of Δ
α
ox
= −0.7, after the UFO was detected, leading to a remarkable X-ray weakness. This may indicate a starving of the inner accretion disk due to the removal of matter through the wind, and it may have repercussions for the larger population of observed X-ray weak quasars.
Context. Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band, flux variations occur on short timescales (hours) as well ...as on larger timescales. While short timescale variability is often investigated in single source studies, only a few works are able to explore flux variation on very long timescales. Aims. This work aims to provide a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to approximately 20 yr rest-frame for a sample of 220 sources. Moreover, we study variability for 2700 quasars up to approximatley eight years rest-frame in the same (soft) band. Methods. We built our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. To ensure that we selected AGN only, we used the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigated variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations. Results. Our work shows an increase of the structure function up to 20 yr. We find no evidence of a plateau in the structure function on these long timescales. Conclusions. The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long timescales.
ABSTRACT
At present, most of the variability studies of active galactic nuclei (AGNs) are based on ensemble analyses. Nevertheless, it is interesting to provide estimates of the individual ...variability properties of each AGN, in order to relate them with intrinsic physical quantities. A useful data set is provided by the Catalina Surveys Data Release 2 (CSDR2), which encompasses almost a decade of photometric measurements of ∼500 million objects repeatedly observed hundreds of times. We aim to investigate the individual optical variability properties of 795 AGNs originally included in the Multi-Epoch XMM Serendipitous AGN Sample 2 (MEXSAS2). Our goals consist of (i) searching for correlations between variability and AGN physical quantities and (ii) extending our knowledge of the variability features of MEXSAS2 from the X-ray to the optical. We use the structure function (SF) to analyse AGN flux variations. We model the SF as a power law, $\text{SF}(\tau)=A\, (\tau /\tau _0)^\gamma$, and we compute its variability parameters. We introduce the V-correction as a simple tool to correctly quantify the amount of variability in the rest frame of each source. We find a significant decrease of variability amplitude with increasing bolometric, optical and X-ray luminosity. We obtain the indication of an intrinsically weak positive correlation between variability amplitude and redshift, z. Variability amplitude also appears to be positively correlated with αox. The slope of the power-law SF, γ, is weakly correlated with the bolometric luminosity Lbol and/or with the black hole mass MBH. When comparing optical to X-ray variability properties, we find that X-ray variability amplitude is approximately the same for those AGNs with larger or smaller variability amplitude in the optical. On the contrary, AGNs with steeper SF in the optical do present steeper SF in the X-ray, and vice versa.
ABSTRACT
A thorough study of radio emission in active galactic nuclei (AGNs) is of fundamental importance to understand the physical mechanisms responsible for the emission and the interplay between ...accretion and ejection processes. High-frequency radio observations can target the nuclear contribution of smaller emitting regions and are less affected by absorption. We present JVLA 22 and 45 GHz observations of 16 nearby (0.003 ≤ z ≤ 0.3) hard-X-ray-selected AGNs at the (sub)-kpc scale with tens μJy beam−1 sensitivity. We detected 15/16 sources, with flux densities ranging from hundreds μJy to tens Jy (specific luminosities from ∼1020 to ${\sim}10^{25}\, \mathrm{ W}\, \mathrm{ Hz}^{-1}$ at 22 GHz). All detected sources host a compact core, with eight being core-dominated at either frequencies, the others exhibiting also extended structures. Spectral indices range from steep to flat/inverted. We interpret this evidence as either due to a core+jet system (6/15), a core accompanied by surrounding star formation (1/15), to a jet oriented close to the line of sight (3/15), to emission from a corona or the base of a jet (1/15), although there might be degeneracies between different processes. Four sources require more data to shed light on their nature. We conclude that, at these frequencies, extended, optically thin components are present together with the flat-spectrum core. The LR/LX ∼ 10−5 relation is roughly followed, indicating a possible contribution to radio emission from a hot corona. A weakly significant correlation between radio core (22 and 45 GHz) and X-ray luminosities is discussed in the light of an accretion–ejection framework.
Context. X-ray spectral variability analyses of active galactic nuclei (AGN) with moderate luminosities and redshifts typically show a “softer when brighter” behaviour. Such a trend has rarely been ...investigated for high-luminosity AGNs (Lbol ≳ 1044 erg/s), nor for a wider redshift range (e.g. 0 ≲ z ≲ 5). Aims. We present an analysis of spectral variability based on a large sample of 2700 quasars, measured at several different epochs, extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue. Methods. We quantified the spectral variability through the parameter β defined as the ratio between the change in the photon index Γ and the corresponding logarithmic flux variation, β = −ΔΓ/Δlog FX. Results. Our analysis confirms a softer when brighter behaviour for our sample, extending the previously found general trend to high luminosity and redshift. We estimate an ensemble value of the spectral variability parameter β = −0.69 ± 0.03. We do not find dependence of β on redshift, X-ray luminosity, black hole mass or Eddington ratio. A subsample of radio-loud sources shows a smaller spectral variability parameter. There is also some change with the X-ray flux, with smaller β (in absolute value) for brighter sources. We also find significant correlations for a small number of individual sources, indicating more negative values for some sources.
Context. Outflows from active galactic nuclei (AGN) are often invoked to explain the co-evolution of AGN and their host galaxies, and the scaling relations between the central black hole mass and the ...bulge velocity dispersion. Nuclear winds are often seen in the X-ray spectra through Fe K shell transitions and some of them are called ultra fast outflows (UFOs) due to their high velocities, up to some fractions of the speed of light. If they were able to transfer some percentage of the AGN luminosity to the host galaxy, this might be enough to trigger an efficient feedback mechanism. Aims. We aim to establish new constraints on the covering fraction and on the kinematic properties of the UFO in the powerful (Lbol ∼ 1047 erg s−1) quasar PDS 456, an established Rosetta stone for studying AGN feedback from disk winds. This will allow us to estimate the mass outflow rate and the energy transfer rate of the wind, which are key quantities to understand the potential impact on the host galaxy. Methods. We analyze two sets of simultaneous XMM-Newton and NuSTAR observations taken in September 2013 and reported in Nardini et al. (2015, Science, 347, 860) as having similar broadband spectral properties. We fit the Fe K features with a P-Cygni profile between 5 and 14 keV, using a novel Monte Carlo model for the WINd Emission (WINE). Results. We find an outflow velocity ranging from 0.17 to 0.28 c, with a mean value of 0.23 c. We obtain an opening angle of the wind of 71−8+13 71 − 8 + 13 $ 71^{+13}_{-8} $ deg and a covering fraction of 0.7−0.3+0.2 0 . 7 − 0.3 + 0.2 $ 0.7^{+0.2}_{-0.3} $ , suggesting a wide-angle outflow. We check the reliability of the WINE model by performing extensive simulations of joint XMM-Newton and NuSTAR observations. Furthermore, we test the accuracy of the WINE model in recovering the geometrical properties of UFOs by simulating observations with the forthcoming Advanced Telescope for High-Energy Astrophysics (ATHENA) in the X-ray band.
Context. Variability, both in X-ray and optical/UV, affects the well-known anti-correlation between the alpha sub(ox) spectral index and the UV luminosity of active galactic nuclei, contributing part ...of the dispersion around the average correlation (intra-source dispersion) in addition to the differences among the time-average alpha sub(ox) values from source to source (inter-source dispersion). Aims. We aim to evaluate the intrinsic alpha sub(ox) variations in individual objects and their effect on the dispersion of the alpha sub(ox) - L sub(UV) anti-correlation. Methods. We used simultaneous UV/X-ray data from Swift observations of a low-redshift sample to derive the epoch-dependent alpha sub(ox)(t) indices. We corrected for the host galaxy contribution by a spectral fit of the optical/UV data. We computed ensemble structure functions to analyse the variability of multi-epoch data. Results. We find a strong intrinsic alpha sub(ox) variability, which significantly contributes (~40% of the total variance) to the dispersion of the alpha sub(ox) - L sub(UV) anti-correlation (intra-source dispersion). The strong X-ray variability and weaker UV variability of this sample are comparable to other samples of low-z active galactic nuclei, and are neither caused by the high fraction of strongly variable narrow line Seyfert 1 galaxies, nor by dilution of the optical variability by the host galaxies. Dilution instead affects the slope of the anticorrelation, which steepens, once corrected, and becomes similar to higher luminosity sources. The structure function of alpha sub(ox) increases with the time lag up to about one month. This indicates the important contribution of the intermediate-to-long timescale variations, which are possibly generated in the outer parts of the accretion disk.
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