713 research outputs found
Chandra monitoring of UGC 4203: the structure of the X-ray absorber
We present a Chandra monitoring campaign of the highly variable Seyfert
galaxy UGC 4203 (the "Phoenix Galaxy") which revealed variations in the X-ray
absorbing column density on time scales of two weeks. This is the third, clear
case, after NGC 1365 and NGC 7582, of dramatic N_H variability on short time
scales observed in a "changing look" source, i.e. an AGN observed in the past
in both a reflection-dominated and a Compton-thin state. The inferred limits on
the distance of the X-ray absorber from the center suggest that the X-ray
"torus" could be one and the same with the broad emission line region. This
scenario, first proposed for an "ad-hoc" picture for NGC 1365, may be the
common structure of the circumnuclear medium in AGN.Comment: 5 Pages, 4 figures. Accepted for publication in MNRAS. Missing
references added and typos correcte
The XMM-Newton long look of NGC 1365: uncovering of the obscured X-ray source
We present an analysis of the extreme obscuration variability observed during an XMM–Newton 5-d continuous monitoring of the active galactic nuclei (AGN) in NGC 1365. The source was in a reflection-dominated state in the first ∼1.5 d, then a strong increase in the 7–10 keV emission was observed in ∼10 h, followed by a symmetric decrease. The spectral analysis of the different states clearly shows that this variation is due to an uncovering of the X-ray source. From this observation, we estimate a size of the X-ray source DS < 1013 cm, a distance of the obscuring clouds R∼ 1016 cm and a density n∼ 1011 cm−3. These values suggest that the X-ray absorption/reflection originates from the broad-line region clouds. This is also supported by the resolved width of the iron narrow Kα emission line, consistent with the width of the broad Hβ line
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&
Variable partial covering and a relativistic iron line in NGC 1365
We present a complete analysis of the hard X-ray (2-10 keV) properties of the
Seyfert galaxy NGC 1365, based on a 60 ks XMM-Newton observation performed in
January 2004. The two main results are: 1) We detect an obscuring cloud with
N_H~3.5x10^23 cm^(-2) crossing the line of sight in ~25 ks. This implies a
dimension of the X-ray source not larger than a few 10^13 cm and a distance of
the obscuring cloud of the order of 10^16 cm. Adopting the black hole mass
M(BH) estimated from the M(BH)-velocity dispersion relation, the source size is
D_S<20 R_G and the distance and density of the obscuring clouds are
R~3000-10000 R_G and n~10^(10) cm^(-3), i.e. typical values for broad line
region clouds. 2) An iron emission line with a relativistic profile is detected
with high statistical significance. A time integrated fit of the line+continuum
reflection components suggests a high iron abundance (~3 times solar) and an
origin of these components in the inner part (~10 R_G) of the accretion disk,
in agreement with the small source size inferred from the analysis of the
absorption variability.Comment: 31 pages, 8 figs. Accepted for publication in the Astrophysical
Journa
A strong excess in the 20-100 keV emission of NGC 1365
We present a new Suzaku observation of the obscured AGN in NGC 1365,
revealing an unexpected excess of X-rays above 20 keV of at least a factor ~2
with respect to the extrapolation of the best-fitting 3-10 keV model.
Additional Swift-BAT and Integral-IBIS observations show that the 20-100 keV is
concentrated within ~1.5 arcmin from the center of the galaxy, and is not
significantly variable on time scales from days to years. A comparison of this
component with the 3-10 keV emission, which is characterized by a rapidly
variable absorption, suggests a complex structure of the circumnuclear medium,
consisting of at least two distinct components with rather different physical
properties, one of which covering >80% of the source with a column density
NH~3-4x10^24 cm^(-2). An alternative explanation is the presence of a double
active nucleus in the center of NGC 1365.Comment: 13 pages, 3 figures. Accepted for publication in ApJ Letter
A Chandra view of the clumpy reflector at the heart of the Circinus galaxy
We present a spectral and imaging analysis of the X-ray reflecting structure
at the heart of the Circinus galaxy, investigating the innermost regions
surrounding the central black hole. By studying an archival 200 ks Chandra
ACIS-S observation, we are able to image the extended clumpy structure
responsible for both cold reflection of the primary radiation and neutral iron
Ka line emission. We measure an excess of the equivalent width of the iron Ka
line which follows an axisymmetric geometry around the nucleus on a hundred pc
scale. Spectra extracted from different regions confirm a scenario in which the
dominant mechanism is the reflection of the nuclear radiation from
Compton-thick gas. Significant differences in the equivalent width of the iron
Ka emission line (up to a factor of 2) are found. It is argued that these
differences are due to different scattering angles with respect to the line of
sight rather than to different iron abundances.Comment: 6 pages, 4 figures, accepted for publication on MNRA
Hidden Broad Line Seyfert 2 Galaxies in the CfA and 12micron Samples
We report the results of a spectropolarimetric survey of the CfA and 12micron
samples of Seyfert 2 galaxies (S2s). Polarized (hidden) broad line regions
(HBLRs) are confirmed in a number of galaxies, and several new cases
(F02581-1136, MCG -3-58-7, NGC 5995, NGC 6552, NGC 7682) are reported. The
12micron S2 sample shows a significantly higher incidence of HBLR (50%) than
its CfA counterpart (30%), suggesting that the latter may be incomplete in
hidden AGNs. Compared to the non-HBLR S2s, the HBLR S2s display distinctly
higher radio power relative to their far-infrared output and hotter dust
temperature as indicated by the f25/f60 color. However, the level of
obscuration is indistinguishable between the two types of S2. These results
strongly support the existence of two intrinsically different populations of
S2: one harboring an energetic, hidden S1 nucleus with BLR, and the other, a
``pure S2'', with weak or absent S1 nucleus and a strong, perhaps dominating
starburst component. Thus, the simple purely orientation-based unification
model is not applicable to all Seyfert galaxies.Comment: 5 pages with embedded figs, ApJ Letters, in pres
Obscuring clouds playing hide-and-seek in the Active Nucleus H0557-385
This paper reports on two XMM-Newton observations of the Seyfert 1 Galaxy
H0557-385 obtained in 2006, which show the source at an historical low flux
state, more than a factor of 10 lower than a previous XMM-Newton look in 2002.
The low flux spectrum presents a strong Fe Kalpha line associated to a Compton
reflection continuum. An additional spectral line around 6.6 keV is required to
fit Kalpha emission from Fe XXV. The spectral curvature below 6 keV implies
obscuration by neutral gas with a column density of 8*10^{23}cm^{-2} partially
covering the primary emission, which still contributes for a few percent of the
soft X-ray emission. Absorption by ionised material on the line of sight is
required to fit the deep trough below 1 keV. The comparison of the two spectral
states shows that the flux transition is to be ascribed entirely to intervening
line-of-sight clouds with high column density.Comment: 5 pages, accepted for publication on MNRAS Letter
A Multi-Wavelength Study of the Nature of Type 1.8/1.9 Seyfert Galaxies
We focus on determining the underlying physical cause of a Seyfert galaxy's
appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical
Seyfert 1 nuclei with reddened broad-line regions? Or are they objects with
intrinsically weak continua and broad emission lines? We compare measurements
of the optical reddening of the narrow and broad-line regions with each other
and with the X-ray column derived from XMM-Newton 0.5-10 keV spectra to
determine the presence and location of dust in the line of sight. We also
searched the literature to see if the objects showed evidence for broad-line
variability, and determined if the changes were consistent with a change in
reddening or a change in the intrinsic ionizing continuum flux. We find that 10
of 19 objects previously classified as Seyfert 1.8/1.9s received this
designation due to their low continuum flux. In four objects the classification
was due to BLR reddening, either by the torus or dust structures in the
vicinity of the NLR; in the remaining five objects there is not sufficient
evidence to favor one scenario over the other. These findings imply that, in
general, samples of 1.8/1.9s are not suitable for use in studies of the gas and
dust in the central torus.Comment: 85 pages, accepted by Ap
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