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.
ABSTRACT
Blazars research is one of the hot topics of contemporary extragalactic astrophysics. That is because these sources are the most abundant type of extragalactic γ-ray sources and are ...suspected to play a central role in multimessenger astrophysics. We have used Swift$\_$xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count rates and flux estimations in several energy bands of over 31 000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multifrequency archival data to assemble the broad-band Spectral Energy Distributions (SEDs) and the long-term light curves of all sources in the sample. Our study shows that large X-ray luminosity variability on different time-scales is present in all objects. Spectral changes are also frequently observed with a ‘harder-when-brighter’ or ‘softer-when-brighter’ behaviour depending on the SED type of the blazars. The peak energy of the synchrotron component (νpeak) in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.
A soft X-ray excess above the 2–10 keV power-law extrapolation is generally observed in the X-ray spectra of active galactic nuclei. The origin of this excess is still not well understood. Presently ...there are two competitive models: blurred ionized reflection and warm Comptonization. In the case of warm Comptonization, observations suggest a corona temperature in the range 0.1–2 keV and a corona optical depth of about 10–20. Moreover, radiative constraints from spectral fits with Comptonization models suggest that most of the accretion power should be released in the warm corona and the disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. However, the true radiative properties of such a warm and optically thick plasma are not well known. For instance, the importance of the Comptonization process, the potential presence of strong absorption and/or emission features, and the spectral shape of the output spectrum have been studied only very recently. Here, we present simulations of warm and optically thick coronae using the
TITAN
radiative transfer code coupled with the
NOAR
Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by hard X-ray emission and from below by an optically thick accretion disk are taken into account, as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering a non-dissipative accretion disk almost entirely. These results confirm that warm Comptonization is a valuable model that can explain the origin of the soft X-ray excess.
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.
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.
Context. The primary X-ray emission in active galactic nuclei (AGNs) is widely believed to be due to Comptonisation of the thermal radiation from the accretion disc in a corona of hot electrons. The ...resulting spectra can, in first approximation, be modelled with a cut-off power law, the photon index and the high-energy roll-over encoding information on the physical properties of the X ray emitting region. The photon index and the high-energy curvature of AGNs (Γ, Ec) have been largely studied since the launch of X-ray satellites operating above 10 keV. However, high-precision measurements of these two observables have only been obtained in recent years thanks to the unprecedented sensitivity of NuSTAR up to 79 keV. Aims. We aim at deriving relations between Γ, Ec phenomenological parameters and the intrinsic properties of the X-ray-emitting region (the hot corona), namely the optical depth and temperature. Methods. We use MoCA (Monte Carlo code for Comptonisation in Astrophysics) to produce synthetic spectra for the case of an AGN with MBH = 1.5 × 108 M⊙ and ṁ = 0.1 and then compared them with the widely used power-law model with an exponential high-energy cutoff. Results. We provide phenomenological relations relating Γ and Ec with the opacity and temperature of the coronal electrons for the case of spherical and slab-like coronae. These relations give origin to a well defined parameter space which fully contains the observed values. Exploiting the increasing number of high-energy cut-offs quoted in the literature, we report on the comparison of physical quantities obtained using MoCA with those estimated using commonly adopted spectral Comptonisation models. Finally, we discuss the negligible impact of different black hole masses and accretion rates on the inferred relations.
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.
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.
It is widely believed that the primary X-ray emission of AGN is due to the Comptonization of optical-UV photons from a hot electron corona, while the origin of the ‘soft-excess’ is still uncertain ...and matter of debate. A second Comptonization component, called warm corona, was therefore proposed to account for the soft-excess, and found in agreement with the optical-UV to X-ray emission of a sample of Seyfert galaxies. In this context, we exploit the broad-band XMM–Newton and NuSTAR simultaneous observations of the Seyfert galaxy NGC 4593 to further test the so-called ‘two corona model’. The NGC 4593 spectra are well reproduced by the model, from the optical/UV to the hard X-rays. Moreover, the data reveal a significant correlation between the hot and the warm corona parameters during our monitoring campaign.
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