1,269 research outputs found
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
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
Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365
We present multiple Chandra and XMM-Newton observations of the type 1.8
Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changes
observed so far in an AGN: the source switched from reflection dominated to
transmission dominated and back in just 6 weeks. During this time the soft
thermal component, arising from a ~1 kpc region around the center, remained
constant. The reflection component is constant at all timescales, and its high
flux relative to the primary component implies the presence of thick gas
covering a large fraction of the solid angle. The presence of this gas, and the
fast variability time scale, suggest that the Compton-thick to Compton thin
change is due to variation in the line-of-sight absorber, rather than to
extreme intrinsic emission variability. We discuss a structure of the
circumnuclear absorber/reflector which can explain the observed X-ray spectral
and temporal properties.Comment: 12 pages, 2 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
NGC454: unveiling a new "changing look" AGN
We present a detailed analysis of the X-ray spectrum of the Seyfert 2 galaxy
NGC454E, belonging to the interacting system NGC454. Observations performed
with Suzaku, XMM-Newton and Swift allowed us to detect a dramatic change in the
curvature of the 2-10 keV spectrum, revealing a significant variation of the
absorbing column density along the line of sight (from ~ 1 x10^{24}cm^{-2} to ~
1x10^{23}cm^{-2}). Consequently, we propose this source as a new member of the
class of "changing look" AGN, i.e. AGN that have been observed both in
Compton-thin (NH =10^{23 cm^{-2}) and reflection dominated states
(Compton-thick, NH >10^{24} cm^{-2}). Due to the quite long time lag (6 months)
between the Suzaku and XMM-Newton observations we cannot infer the possible
location of the obscuring material causing the observed variability. In the 6-7
keV range the XMM-Newton observation also shows a clear signature of the
presence of an ionized absorber. Since this feature is not detected during the
Suzaku observation (despite its detectability), the simplest interpretation is
that the ionized absorber is also variable; its location is estimated to be
within ~10^{-3} pc from the central black hole, probably much closer in than
the rather neutral absorber.Comment: 10 page
PCA of PCA: Principal Component Analysis of Partial Covering Absorption in NGC 1365
We analyse 400 ks of XMM-Newton data on the active galactic nucleus NGC 1365
using principal component analysis (PCA) to identify model independent spectral
components. We find two significant components and demonstrate that they are
qualitatively different from those found in MCG?6-30-15 using the same method.
As the variability in NGC 1365 is known to be due to changes in the parameters
of a partial covering neutral absorber, this shows that the same mechanism
cannot be the driver of variability in MCG-6-30-15. By examining intervals
where the spectrum shows relatively low absorption we separate the effects of
intrinsic source variability, including signatures of relativistic reflection,
from variations in the intervening absorption. We simulate the principal
components produced by different physical variations, and show that PCA
provides a clear distinction between absorption and reflection as the drivers
of variability in AGN spectra. The simulations are shown to reproduce the PCA
spectra of both NGC 1365 and MCG-6-30-15, and further demonstrate that the
dominant cause of spectral variability in these two sources requires a
qualitatively different mechanism.Comment: 8 pages, 10 figures. Accepted for publication in MNRA
Analysis of Spitzer-IRS spectra of hyperluminous infrared galaxies
Hyperluminous infrared galaxies (HLIRG) are the most luminous persistent
objects in the Universe. They exhibit extremely high star formation rates, and
most of them seem to harbour an AGN. They are unique laboratories to
investigate the most extreme star formation, and its connection to
super-massive black hole growth. The AGN and SB relative contributions to the
total output in these objects is still debated. Our aim is to disentangle the
AGN and SB emission of a sample of thirteen HLIRG. We have studied the MIR low
resolution spectra of a sample of thirteen HLIRG obtained with the IRS on board
Spitzer. The 5-8 {\mu}m range is an optimal window to detect AGN activity even
in a heavily obscured environment. We performed a SB/AGN decomposition of the
continuum using templates, successfully applied for ULIRG in previous works.
The MIR spectra of all sources is largely dominated by AGN emission. Converting
the 6 {\mu}m luminosity into IR luminosity, we found that ~80% of the sample
shows an IR output dominated by the AGN emission. However, the SB activity is
significant in all sources (mean SB contribution ~30%), showing star formation
rates ~300-3000 solar masses per year. Using X-ray and MIR data we estimated
the dust covering factor (CF) of these HLIRG, finding that a significant
fraction presents a CF consistent with unity. Along with the high X-ray
absorption shown by these sources, this suggests that large amounts of dust and
gas enshroud the nucleus of these HLIRG, as also observed in ULIRG. Our results
are in agreement with previous studies of the IR SED of HLIRG using radiative
transfer models, and we find strong evidence that all HLIRG harbour an AGN.
This work provides further support to the idea that AGN and SB are both crucial
to understand the properties of HLIRG. Our study of the CF supports the
hypothesis that HLIRG can be divided in two different populations.Comment: 17 pages, 9 figures, 4 tables. Accepted for publication in A&
Spectral decomposition of starbursts and AGNs in 5-8 micron Spitzer IRS spectra of local ULIRGs
We present an analysis of the 5-8 micron Spitzer-IRS spectra of a sample of
68 local Ultraluminous Infrared Galaxies (ULIRGs). Our diagnostic technique
allows a clear separation of the active galactic nucleus (AGN) and starburst
(SB) components in the observed mid-IR emission, and a simple analytic model
provides a quantitative estimate of the AGN/starburst contribution to the
bolometric luminosity. We show that AGNs are ~30 times brighter at 6 micron
than starbursts with the same bolometric luminosity, so that even faint AGNs
can be detected. Star formation events are confirmed as the dominant power
source for extreme infrared activity, since ~85% of ULIRG luminosity arises
from the SB component. Nonetheless an AGN is present in the majority (46/68) of
our sources.Comment: 5 Pages, 3 figures. MNRAS Letters, Accepte
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
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