877 research outputs found
The iron K line complex in NGC1068: implications for X-ray reflection in the nucleus
We report a new analysis of ASCA data on the iron K line complex in NGC1068.
The line complex basically consists of three components, as previously
reported. A weak red wing of the 6.4 keV fluoresence iron K line is found. A
plausible explanation is Compton scattering in optically thick, cold matter
which can be identified with an obscuring torus or cold gas in the host galaxy.
We also show that this `Compton shoulder' should be observable with ASCA using
a simulated reflection spectrum. In order to explain the two higher energy
lines as well as the cold 6.4 keV line, we fit the ASCA data with a composite
model of cold and warm reflection. This shows that cold reflection dominates
the observed X-ray emission above 4 keV. The two higher energy lines have large
equivalent width with respect to the warm-scattered continuum, suggesting that
efficient resonant scattering operates. The line energies are systematically
lower than those expected from resonant lines for FeXXV and FeXXVI by 100 eV.
The redshifts may be due to either the ionized gas of the warm mirror receding
at a radial velocity of 4000-5000 km/s, or effects of Compton scattering in a
complicated geometry.Comment: 7 pages, 4 figures, to appear in MNRA
The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX
Results from BeppoSAX observations of Compton--thick Seyfert 2 galaxies are
summarized and reviewed, and their general properties derived and discussed. In
five out of the seven observed sources, the nucleus is directly visible at high
X-ray energies, where the photons penetrate absorbers with column densities in
the range 1.1-4.3x10^{24} cm-2 (in the other two sources, NGC 1068 and NGC
7674, the nucleus is instead totally obscured at all energies, implying even
larger column densities). In most sources there is unambiguous evidence of a
reflection component from optically thick, cold matter, while in two (or maybe
four) cases there is also evidence of reflection from ionized matter. For the
sources with a measured X-ray luminosity, a comparison with the infrared
luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the
IR emission appears to be dominated by starburst activity, in the other three
sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by
reprocessing of the UV and X-ray photons emitted by an AGN.Comment: Accepted for publication in MNRA
XMM-Newton study of the complex and variable spectrum of NGC 4051
We study the X-ray spectral variability of the Narrow Line Seyfert 1 galaxy
NGC 4051 as observed during two XMM-Newton observations. The data show evidence
for a neutral and constant reflection component and for constant emission from
photoionized gas, which are included in all spectral models. The nuclear
emission can be modelled both in terms of a ``standard model'' (pivoting power
law plus a black body component for the soft excess) and of a two--component
one (power law plus ionized reflection from the accretion disc). The standard
model results indicate that the soft excess does not follow the standard black
body law. Moreover, although the spectral slope is correlated with flux, which
is consistent with spectral pivoting, the hardest photon indexes are so flat as
to require rather unusual scenarios. These problems can be solved in terms of
the two-component model in which the soft excess is not thermal, but due to the
ionized reflection component. The variability of the reflection component from
the inner disc closely follows the predictions of the light bending model,
suggesting that most of the primary nuclear emission is produced in the very
innermost regions, only a few gravitational radii from the central black hole.
(abridged)Comment: accepted for publication in MNRA
A search for changing-look AGN in the Grossan catalog
We observed with XMM-Newton 4 objects selected from the Grossan catalog, with
the aim to search for new 'changing-look' AGN. The sample includes all the
sources which showed in subsequent observations a flux much lower than the one
measured with HEAO A-1: NGC 7674, NGC 4968, IRAS 13218+0552 and NGC 1667. None
of the sources was caught in a high flux state during the XMM-Newton
observations, whose analysis reveal they are all likely Compton-thick objects.
We suggest that, for all the sources, potential problems with the HEAO A-1
source identification and flux measurement prevent us from being certain that
the HEAO A-1 data represent a putative 'high' state for these objects.
Nonetheless, based on the high flux state and Compton-thin spectrum of its
GINGA observation, NGC 7674 represents probably the sixth known case of a
'changing-look' Seyfert 2 galaxy. From the X-ray variability pattern, we can
estimate a likely lower limit of a few parsec to the distance of the inner
walls of the torus in this object. Remarkably, IRAS 13218+0552 was not detected
by XMM-Newton, despite being currently classified as a Seyfert 1 with a large
[OIII] flux. However, the original classification was likely to be affected by
an extreme velocity outflow component in the emission lines. The object likely
harbors an highly obscured AGN and should be re-classified as a Type 2 source.Comment: 11 pages, 12 figures, accepted for publication in A&
Probing variability patterns of the Fe K line complex in bright nearby AGNs
The unprecedented sensitivity of current X-ray telescopes allows for the
first time to address the issue of the Fe K line complex variability patterns
in bright, nearby AGNs. We examine XMM-Newton observations of the brightest
sources of the FERO sample of radio-quiet type 1 AGNs with the aim of
characterizing the temporal behaviour of Fe K complex features. A systematic
mapping of residual flux above and below the continuum in the 4-9 keV range is
performed in the time vs energy domain, with the purpose of identifying
interesting spectral features in the three energy bands: 5.4-6.1 keV, 6.1-6.8
keV and 6.8-7.2 keV, corresponding respectively to the redshifted, rest frame
and blueshifted or highly ionized Fe Kalpha line bands. The variability
significance is assessed by extracting light curves and comparing them with
MonteCarlo simulations. The time-averaged profile of the Fe K complex revealed
spectral complexity in several observations. Red- and blue-shifted components
(either in emission or absorption) were observed in 30 out of 72 observations,
with an average ~90 eV for emission and ~ -30 eV for absorption
features. We detected significant line variability (with confidence levels
ranging between 90% and 99.7%) within at least one of the above energy bands in
26 out of 72 observations on time scales of ~6-30 ks. Reliability of these
features has been carefully calculated using this sample and has been assessed
at ~3sigma confidence level. This work increases the currently scanty number of
detections of variable, energy shifted, Fe lines and confirms the reliability
of the claimed detections. We found that the distribution of detected features
is peaked at high variability significances in the red- and blue-shifted energy
bands, suggesting an origin in a relativistically modified accretion flow.Comment: Accepted for publication in Astronomy & Astrophysic
Accretion Disk Illumination in Schwarzschild and Kerr Geometries: Fitting Formulae
We describe the methodology and compute the illumination of geometrically
thin accretion disks around black holes of arbitrary spin parameter exposed
to the radiation of a point-like, isotropic source at arbitrary height above
the disk on its symmetry axis. We then provide analytic fitting formulae for
the illumination as a function of the source height and the black hole
angular momentum . We find that for a source on the disk symmetry axis and
, the main effect of the parameter is allowing the disk to extend
to smaller radii (approaching as ) and thus allow the
illumination of regions of much higher rotational velocity and redshift. We
also compute the illumination profiles for anisotropic emission associated with
the motion of the source relative to the accretion disk and present the
fractions of photons absorbed by the black hole, intercepted by the disk or
escaping to infinity for both isotropic and anisotropic emission for
and . As the anisotropy (of a source approaching the disk) increases
the illumination profile reduces (approximately) to a single power-law, whose
index, , because of absorption of the beamed photons by the black hole,
saturates to a value no higher than . Finally, we compute the
fluorescence Fe line profiles associated with the specific illumination and
compare them among various cases.Comment: 26 pages, 21 b/w figures, accepted for publication in the
Astrophysical Journal as of 4/16/200
The ASCA spectrum of the z=4.72 blazar, GB 1428+4217
The X-ray luminous quasar GB 1428+4217 at redshift 4.72 has been observed
with ASCA. The observed 0.5-10 keV flux is 3.2E-12 erg/s/cm2. We report here on
the intrinsic 4-57 keV X-ray spectrum, which is very flat (photon index of
1.29). We find no evidence for flux variability within the ASCA dataset or
between it and ROSAT data. We show that the overall spectral energy
distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars
and present models which fit the available data. The Doppler beaming factor is
likely to be at least 8. We speculate on the number density of such high
redshift blazars, which must contain rapidly-formed massive black holes.Comment: 5 pages, 3 Postscript figures, to appear in MNRA
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