210 research outputs found
The nature of X-ray spectral variability in MCG-6-30-15
The flux-flux plot (FFP) method can provide model-independent clues regarding
the X-ray variability of active galactic nuclei. To use it properly, the bin
size of the light curves should be as short as possible, provided the average
counts in the light curve bins are larger than . We apply the FFP
method to the 2013, simultaneous XMM-Newton and NuSTAR observations of the
Seyfert galaxy MCG6-30-15, in the 0.3-40 keV range. The FFPs above keV are well-described by a straight line. This result rules out spectral
slope variations and the hypothesis of absorption driven variability. Our
results are fully consistent with a power-law component varying in
normalization only, with a spectral slope of , plus a variable,
relativistic reflection arising from the inner accretion disc around a rotating
black hole. We also detect spectral components which remain constant over days (at least). At energies above keV, the stable component is
consistent with reflection from distant, neutral material. The constant
component at low energies is consistent with a blackbody spectrum of eV. The fluxes of these components are of the
average continuum flux (in the respective bands). They should always be
included in the models that are used to fit the spectrum of the source. The
FFPs below 1.6 keV are non-linear, which could be due to the variable warm
absorber in this source.Comment: 18 pages, 12 figures, 6 tables. Accepted for publication in MNRA
Testing the accuracy of reflection-based supermassive black hole spin measurements in AGN
X-ray reflection is a very powerful method to assess the spin of supermassive
black holes (SMBHs) in active galactic nuclei (AGN), yet this technique is not
universally accepted. Indeed, complex reprocessing (absorption, scattering) of
the intrinsic spectra along the line of sight can mimic the relativistic
effects on which the spin measure is based. In this work, we test the
reliability of SMBH spin measurements that can currently be achieved through
the simulations of high-quality XMM-Newton and NuSTAR spectra. Each member of
our group simulated ten spectra with multiple components that are typically
seen in AGN, such as warm and (partial-covering) neutral absorbers,
relativistic and distant reflection, and thermal emission. The resulting
spectra were blindly analysed by the other two members. Out of the 60 fits, 42
turn out to be physically accurate when compared to the input model. The SMBH
spin is retrieved with success in 31 cases, some of which (9) are even found
among formally inaccurate fits (although with looser constraints). We show
that, at the high signal-to-noise ratio assumed in our simulations, neither the
complexity of the multi-layer, partial-covering absorber nor the input value of
the spin are the major drivers of our results. The height of the X-ray source
(in a lamp-post geometry) instead plays a crucial role in recovering the spin.
In particular, a success rate of 16 out of 16 is found among the accurate fits
for a dimensionless spin parameter larger than 0.8 and a lamp-post height lower
than five gravitational radii.Comment: 20 pages, 9 figures, 4 tables. Accepted for publication in A&
Variability of the soft X-ray excess in IRAS 13224-3809
We study the soft excess variability of the narrow line Seyfert 1 galaxy IRAS
13224-3809. We considered all five archival XMM-Newton observations, and we
applied the 'flux-flux plot' (FFP) method. We found that the flux-flux plots
were highly affected by the choice of the light curves' time bin size, most
probably because of the fast and large amplitude variations, and the intrinsic
non-linear flux--flux relations in this source. Therefore, we recommend that
the smallest bin-size should be used in such cases. Hence, We constructed FFPs
in 11 energy bands below 1.7 keV, and we considered the 1.7-3 keV band, as
being representative of the primary emission. The FFPs are reasonably well
fitted by a 'power-law plus a constant' model. We detected significant positive
constants in three out of five observations. The best-fit slopes are flatter
than unity at energies below keV, where the soft excess is
strongest. This suggests the presence of intrinsic spectral variability. A
power-law-like primary component, which is variable in flux and spectral slope
(as ) and a soft-excess component, which varies
with the primary continuum (as ),
can broadly explain the FFPs. In fact, this can create positive `constants',
even when a stable spectral component does not exist. Nevertheless, the
possibility of a stable, soft--band constant component cannot be ruled out, but
its contribution to the observed 0.2-1 keV band flux should be less than %. The model constants in the FFPs were consistent with zero in one
observation, and negative at energies below 1 keV in another. It is hard to
explain these results in the context of any spectral variability scenario, but
they may signify the presence of a variable, warm absorber in the source.Comment: Accepted for publication in A&A, 10 pages, 7 figure
A physical model for the broadband energy spectrum of X-ray illuminated accretion discs: fitting the spectral energy distribution of NGC 5548
We develop a new physical model for the broadband spectral energy
distribution (SED) of X-ray illuminated accretion discs, that takes into
account the mutual interaction of the accretion disc and the X-ray corona,
including all relativistic effects. We assume a Keplerian, optically thick and
geometrically thin accretion disc and an X-ray source in the lamp-post geometry
that emits an isotropic power-law spectrum with a high-energy cut-off. We
assume that all the energy that would be released by thermal radiation in the
standard disc model in its innermost part, is transported to the corona,
effectively cooling the disc in this region. We include the disc heating due to
thermalisation of the absorbed part of the disc illumination by X-ray corona.
The X-ray reflection from the disc is also included. We compute the X-ray
luminosity and the low-energy X-ray cut-off through an iterative process,
taking full account of the interplay between the X-ray illumination of the disc
and the resulting accretion disc spectrum which enters the corona so that the
energy balance is preserved. The corona radius is also computed from the
conservation of the photon's number during Comptonization.
We discuss the model SEDs and their dependence on system parameters. The
disc-corona interaction has profound effects - it constrains the X-ray
luminosity and changes the shape and normalisation of the UV/optical blue bump.
We use the new code to fit the broad-band SED of a typical Seyfert 1 galaxy,
NGC 5548. We infer a high black-hole spin, an intermediate system inclination,
and an accretion rate below 10% of Eddington. The X-ray luminosity in this
source could be supported by 45-70% of the accretion energy dissipated in the
disc. The new model, named KYNSED, is publicly available to be used for fitting
AGN SEDs inside the XSPEC spectral analysis tool.Comment: 20 pages, 14 figures, accepted by Astronomy & Astrophysic
Revisiting UV/optical continuum time lags in AGN
In this paper, we present an updated version of our model (KYNXiltr) which
considers thermal reverberation of a standard Novikov-Thorne accretion disc
illuminated by an X-ray point-like source. Previously, the model considered
only two cases of black hole spins, and assumed a colour correction factor
. Now, we extend the model to any spin value and colour
correction. In addition, we consider two scenarios of powering the X-ray
corona, either via accretion, or external to the accretion disc. We use
KYNXiltr to fit the observed time lags obtained from intense monitoring of four
local Seyfert galaxies (NGC 5548, NGC 4395, Mrk 817, and Fairall 9). We
consider various combinations of black hole spin, colour correction, corona
height, and fraction of accretion power transferred to the corona. The model
fits well the overall time-lags spectrum in these sources (for a large
parameter space). For NGC 4593 only, we detect a significant excess of delays
in the U-band. The contribution of the diffuse BLR emission in the time-lags
spectrum of this source is significant. It is possible to reduce the large
best-fitting parameter space by combining the results with additional
information, such as the observed Eddington ratio and average X-ray luminosity.
We also provide an update to the analytic expression provided by Kammoun et
al., for an X-ray source that is not powered by the accretion process, which
can be used for any value of colour correction, and for two values of the black
hole spin (0 and 0.998).Comment: Accepted for publication in MNRA
Testing the accuracy of reflection-based supermassive black hole spin measurements in AGN
X-ray reflection is a very powerful method to assess the spin of supermassive
black holes (SMBHs) in active galactic nuclei (AGN), yet this technique is not
universally accepted. Indeed, complex reprocessing (absorption, scattering) of
the intrinsic spectra along the line of sight can mimic the relativistic
effects on which the spin measure is based. In this work, we test the
reliability of SMBH spin measurements that can currently be achieved through
the simulations of high-quality XMM-Newton and NuSTAR spectra. Each member of
our group simulated ten spectra with multiple components that are typically
seen in AGN, such as warm and (partial-covering) neutral absorbers,
relativistic and distant reflection, and thermal emission. The resulting
spectra were blindly analysed by the other two members. Out of the 60 fits, 42
turn out to be physically accurate when compared to the input model. The SMBH
spin is retrieved with success in 31 cases, some of which (9) are even found
among formally inaccurate fits (although with looser constraints). We show
that, at the high signal-to-noise ratio assumed in our simulations, neither the
complexity of the multi-layer, partial-covering absorber nor the input value of
the spin are the major drivers of our results. The height of the X-ray source
(in a lamp-post geometry) instead plays a crucial role in recovering the spin.
In particular, a success rate of 16 out of 16 is found among the accurate fits
for a dimensionless spin parameter larger than 0.8 and a lamp-post height lower
than five gravitational radii
A Hard Look at NGC 5347: Revealing a Nearby Compton-thick AGN
Current measurements show that the observed fraction of Compton-thick (CT) active galactic nuclei (AGN) is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 (z = 0.00792, D = 35.5 Mpc ) have alternately suggested a CT and Compton-thin source. Combining archival data from Suzaku, Chandra, andâmost importantlyânew data from NuSTAR, ... See full text for complete abstrac
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