127 research outputs found
Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state
We present an analysis of the 2013 Suzaku campaign on the nearby luminous
quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455
ks. During these observations, the X-ray flux was suppressed by a factor of >10
in the soft X-ray band when compared to other epochs. We investigated the
broadband continuum by constructing a spectral energy distribution, making use
of the optical/UV photometry and hard X-ray spectra from the later
XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state
cannot be accounted for by self-consistent accretion disc and corona models
without attenuation by absorbing gas, which partially covers a substantial
fraction of the line of sight towards the X-ray source. Two absorption layers
are required, of column density and ,
with average covering factors of ~80% (with typical 5% variations) and 60%
(10-15%), respectively. In these observations PDS 456 displays significant
short term X-ray spectral variability, on timescales of ~100 ks, which can be
accounted for by variable covering of the absorbing gas. The partial covering
absorber prefers an outflow velocity of at
the >99.9% confidence level over the case where . This is
consistent with the velocity of the highly ionised outflow responsible for the
blueshifted iron K absorption profile. We therefore suggest that the partial
covering clouds could be the denser, or clumpy part of an inhomogeneous
accretion disc wind. Finally we estimate the size-scale of the X-ray source
from its variability. The radial extent of the X-ray emitter is found to be of
the order ~15-20 , although the hard X-ray (>2 keV) emission may
originate from a more compact or patchy corona of hot electrons, which is ~6-8
in size.Comment: 38 pages, 13 figures, accepted for publication in MNRA
Interpreting the long-term variability of the changing-look AGN Mrk 1018
We present a thorough study of the Changing-Look Active Galactic Nucleus
(CL-AGN) Mrk 1018, utilizing an extensive dataset spanning optical, UV, and
X-ray spectro-photometric data from 2005 to 2019. We analysed X-ray spectra and
broad-band photometry, and performed optical-to-X-ray spectral energy
distribution (SED) fitting to comprehend the observed changing-look behaviour.
We found that over the 14 years in analysis, significant changes in X-ray
spectra occurred, as the hardness ratio increases by a factor of ~2. We
validated also the broad-band dimming, with optical, UV, and X-ray luminosities
decreasing by factors of >7, >24 and ~9, respectively. These dims are
attributed to the declining UV emission. We described the X-ray spectra with a
two-Comptonization model, revealing a consistent hot comptonizing medium but a
cooling warm component. This cooling, linked to the weakening of the magnetic
fields in the accretion disk, explains the UV dimming. We propose that the
weakening is caused by the formation of a jet, in turn originated from the
change of state of the inner accretion flow. Our optical-to-X-ray SED fitting
supports this conclusion, as the normalised accretion rate is super-critical
(0.06>0.02) in the bright state and sub-critical (0.01<0.02) in the
faint state. Instabilities arising at the interface of the state-transition are
able to reduce the viscous timescale to the observed ~10 years of Mrk 1018
variability. We explored a possible triggering mechanism for this state
transition, involving gaseous clouds pushed onto the AGN sub-pc regions by a
recent merging event or by cold chaotic accretion. This scenario, if validated
by future simulations, could enhance our understanding of CL-AGN and raises
questions about an accretion rate of ~0.02, coupled with minor disturbances in
the accretion disk, being the primary factor in the changing-look phenomenon.Comment: 18 pages, 8 figure
Evidence for a clumpy disc-wind in the star forming Seyfert\,2 galaxy MCG--03--58--007
We report the results of a detailed analysis of a deep simultaneous \textit{XMM-Newton & NuSTAR} observation of the nearby () and
bright () starburst-AGN
Seyfert\,2 system: MCG--03--58--007. From the broadband fitting we show that
most of the obscuration needs to be modeled with a toroidal type reprocessor
such as \texttt{MYTorus} \citep{MurphyYaqoob09}. Nonetheless the signature of a
powerful disc-wind is still apparent at higher energies and the observed rapid
short-term X-ray spectral variability is more likely caused by a variable zone
of highly ionized fast wind rather than by a neutral clumpy medium. We also
detect X-ray emission from larger scale gas as seen from the presence of
several soft narrow emission lines in the RGS, originating from a contribution
of a weak star forming activity together with a dominant photoionized component
from the AGN.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Multiplicity of Positive Solutions for an Obstacle Problem in R
In this paper we establish the existence of two positive solutions for the
obstacle problem \displaystyle \int_{\Re}\left[u'(v-u)'+(1+\lambda
V(x))u(v-u)\right] \geq \displaystyle \int_{\Re} f(u)(v-u), \forall v\in \Ka
where is a continuous function verifying some technical conditions and
\Ka is the convex set given by \Ka =\left\{v\in H^{1}(\Re); v \geq \varphi
\right\}, with having nontrivial positive part with
compact support in .
\vspace{0.2cm} \noindent \emph{2000 Mathematics Subject Classification} :
34B18, 35A15, 46E39.
\noindent \emph{Key words}: Obstacle problem, Variational methods, Positive
solutions.Comment: To appear in Progress in Nonlinear Differential Equations and their
Application
AGN X-ray spectroscopy with neural networks
We explore the possibility of using machine learning to estimate physical parameters directly from active galactic nucleus (AGN) X-ray spectra without needing computationally expensive spectral fitting. Specifically, we consider survey quality data, rather than long pointed observations, to ensure that this approach works in the regime where it is most likely to be applied. We simulate Athena Wide Field Imager spectra of AGN with warm absorbers, and train simple neural networks to estimate the ionization and column density of the absorbers. We find that this approach can give comparable accuracy to spectral fitting, without the risk of outliers caused by the fit sticking in a false minimum, and with an improvement of around three orders of magnitude in speed. We also demonstrate that using principal component analysis to reduce the dimensionality of the data prior to inputting it into the neural net can significantly increase the accuracy of the parameter estimation for negligible computational cost, while also allowing a simpler network architecture to be used
The properties of the X-ray corona in the distant (z = 3.91) quasar APM 08279+5255
We present new joint XMM-Newton and NuSTAR observations of APM08279+5255, a gravitationally-lensed, broad-absorption line quasar (z = 3:91). After showing a fairly stable flux ( f210 ' 45:5 1013 erg s1) from 2000 to 2008, APM08279+5255 was found in a fainter state in the latest X-ray exposures ( f210 ' 2:7 1013 erg s1), which can likely be ascribed to a lower X-ray activity. Moreover, the 2019 data present a prominent FeK emission line and do not show any significant absorption line. This fainter state, coupled to the first hard X-ray sampling of APM08279+5255, allowed us to measure X-ray reflection and the high-energy cuto in this source for the first time. From the analysis of previous XMM-Newton and Chandra observations, X-ray reflection is demonstrated to be a long-lasting feature of this source, but less prominent prior to 2008, possibly due to a stronger primary emission. The estimated high-energy cuto (Ecut = 99+91 35 keV) sets a new redshift record for the farthest ever measured and places APM08279+5255 in the allowed region of the compactness-temperature diagram of X-ray coronae, in agreement with previous results on high-z quasars
Detection of a possible multiphase ultra-fast outflow in IRAS 13349+2438 with NuSTAR and XMM-Newton
We present joint NuSTAR and XMM-Newton observations of the bright, variable
quasar IRAS 13349+2438. This combined dataset shows two clear iron absorption
lines at 8 and 9 keV, which are most likely associated with two layers of
mildly relativistic blueshifted absorption, with velocities of 0.14c and 0.27c.
We also find strong evidence for a series of Ly absorption lines at
intermediate energies in a stacked XMM-Newton EPIC-pn spectrum, at the same
blueshift as the lower velocity iron feature. This is consistent with a
scenario where an outflowing wind is radially stratified, so faster, higher
ionization material is observed closer to the black hole, and cooler, slower
material is seen from streamlines at larger radii.Comment: 5 pages, 2 figures, accepted for publication in MNRAS letter
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