99 research outputs found
Wide-Band X-Ray Spectra and Images of the Starburst Galaxy M82
The ASCA results of the starburst galaxy M82 are presented. The X-rays in the
0.5--10 keV band exhibit a thin thermal spectrum with emission lines from
highly ionized magnesium, silicon, and sulfur, as well as a hard tail extending
to higher than 10keV energy. The soft X-rays are spatially extended, while the
hard X-rays show an unresolved point-like structure with possible a long-term
flux variability. The flux ratio of the emission lines and the spatially
extended structure in the low-energy band indicate that at least
two-temperature thin thermal plasmas are present. The abundances of the oxygen,
neon, magnesium, silicon, sulfur, and iron in the thin thermal plasmas are
found to be significantly lower than the cosmic value. Neither type-Ia nor
type-II supernova explosions can reproduce the observed abundance ratio. The
origin of the unresolved hard X-rays is uncertain, but is probably an obscured
low-luminosity AGN.Comment: 25 pages, 8 figure
Luminosity-dependent unification of Active Galactic Nuclei and the X-ray Baldwin effect
The existence of an anti-correlation between the equivalent width (EW) of the
narrow core of the iron Kalpha line and the luminosity of the continuum (i.e.
the X-ray Baldwin effect) in type-I active galactic nuclei has been confirmed
over the last years by several studies carried out with XMM-Newton, Chandra and
Suzaku. However, so far no general consensus on the origin of this trend has
been reached. Several works have proposed the decrease of the covering factor
of the molecular torus with the luminosity (in the framework of the
luminosity-dependent unification models) as a possible explanation for the
X-ray Baldwin effect. Using the fraction of obscured sources measured by recent
X-ray and IR surveys as a proxy of the half-opening angle of the torus, and the
recent Monte-Carlo simulations of the X-ray radiation reprocessed by a
structure with a spherical-toroidal geometry by Ikeda et al. (2009) and
Brightman & Nandra (2011), we test the hypothesis that the X-ray Baldwin effect
is related to the decrease of the half-opening angle of the torus with the
luminosity. Simulating the spectra of an unabsorbed population with a
luminosity-dependent covering factor of the torus as predicted by recent X-ray
surveys, we find that this mechanism is able to explain the observed X-ray
Baldwin effect. Fitting the simulated data with a log-linear L_{2-10keV}-EW
relation, we found that in the Seyfert regime (L_{2-10keV}< 10^44.2 erg s^-1)
luminosity-dependent unification produces a slope consistent with the
observations for average values of the equatorial column densities of the torus
of log N_H^T > 23.1. In the quasar regime (L_{2-10 keV}> 10^44.2 erg s^-1) a
decrease of the covering factor of the torus with the luminosity slower than
that observed in the Seyfert regime (as found by recent hard X-ray surveys) is
able to reproduce the observations for 23.2 < log N_H^T < 24.2.Comment: 9 pages, 9 figures, 1 table. Accepted for pubblication in A&
X-Ray Spectral Variability in NGC 7469
We present analyses of two Ginga observations and two observations from the ROSAT database of NGC 7469, focusing on the spectral variability observed on timescales of days and longer. During the 1988 Ginga observation, the hardness ratio (8-21 keV/3.4-5.7 keV) increased significantly as the total flux decreased by 30%. As the spectrum is well fit by the reflection model and since the spectra variability dominates the higher energy band, this could be explained by either a variation in the power law index or in the effective covering fraction of the reflecting material. This ambiguity is inherent in reflection modeling of Ginga spectra from moderate flux Seyfert 1 galaxies. Assuming that the power law index did not change, we find that the reflected flux is consistent with being constant, suggesting that much of the reflecting material may be located more than 3 light-days from the continuum source with the molecular torus being a plausible site. This scenario is also supported by the report of a narrow rather than broad iron K-alpha line in the ASCA data by Guainazzi et al. NGC 7469 was faint during the 1989 Ginga observation, but variability was observed with doubling timescale of 5 hr, and the spectrum was harder. A reflection component could not be constrained, and the change in the spectrum could be explained by an increase in neutral absorption. The brighter of two ROSAT spectra was significantly softer, and in both spectra there was evidence of spectral complexity, as has been previously reported by Turner, George, & Mushotzky and Brandt et al. The spectrum could be fit by a variety of two-component models, including a warm absorber model, an ionized disk model, and a thermal model with single-component blackbody spectrum, but joint fitting of the 1988 average Ginga spectrum and the nonsimultaneous ROSAT spectra favored thermal models, and other models required an anomalously high reflection ratio. This model is supported by the observation of a soft excess component and the lack of ionized absorption edges in the ASCA spectrum by Guainazzi et al. The long-term spectral variability could be explained by relative variability between the power-law and soft excess component normalizations, perhaps implying that hard X-ray reprocessing in thermal material does not dominate on long timescales
A Candidate Active Galactic Nucleus with a Pure Soft Thermal X-ray Spectrum
We report the discovery of a candidate active galactic nucleus (AGN), 2XMM
J123103.2+110648 at z = 0.13, with an X-ray spectrum represented purely by soft
thermal emission reminiscent of Galactic black hole (BH) binaries in the
disk-dominated state. This object was found in the second XMM serendipitous
source catalogue as a highly variable X-ray source. In three separate
observations, its X-ray spectrum can be represented either by a multicolor disk
blackbody model with an inner temperature of kT_in~0.16-0.21 keV or a Wien
spectrum Comptonized by an optically thick plasma with kT~0.14-0.18 keV. The
soft X-ray luminosity in the 0.5--2 keV band is estimated to be (1.6-3.8)x10^42
erg/s. Hard emission above ~2 keV is not detected. The ratio of the soft to
hard emission is the strongest among AGNs observed thus far. Spectra selected
in high/low flux time intervals are examined in order to study spectral
variability. In the second observation with the highest signal-to-noise ratio,
the low energy (below 0.7 keV) spectral regime flattens when the flux is high,
while the shape of the high energy part (1-1.7 keV) remains unchanged. This
behavior is qualitatively consistent with being caused by strong
Comptonization. Both the strong soft excess and spectral change consistent with
Comptonization in the X-ray spectrum imply that the Eddington ratio is large,
which requires a small BH mass (smaller than ~10^5M_solar.Comment: To Appear in ApJ, 8 pages, 7 figure
The narrow Fe K line and the molecular torus in active galactic nuclei - an IR/X-ray view
The narrow component of the iron K is an almost ubiquitous feature in
the X-ray spectra of active galactic nuclei (AGN) and is believed to originate
in neutral material, possibly located in the molecular torus. This would imply
a tight connection between the Fe K equivalent width (EW) and the
physical properties of the torus. In a recent work we have shown that the
decrease of the covering factor of the torus with the luminosity, as expected
by luminosity-dependent unification models, would be able to explain the
decrease of Fe K EW with the luminosity (i.e., the X-ray Baldwin
effect). Recent developments in the study of the mid-IR (MIR) spectrum of AGN
allow important parameters of the torus to be deduced, such as its covering
factor () and equatorial column density (),
by applying clumpy torus models. Using XMM-Newton/EPIC observations of a sample
of 24 type-I AGN, we investigate the relation between the physical parameters
of the torus obtained by recent MIR works and the properties of the Fe
K line. We correct the values of the Fe K EW by taking the
inclination angle, the photon index, the equatorial column density, and
half-opening angle of the torus into account using a physical torus model of
X-ray reprocessed radiation. We find that the relation between Fe K EW
and shows a slope that is consistent with the expected value,
albeit with a low statistical significance. A trend that is consistent with the
theoretical prediction is also found when comparing the Fe K EW to
. Our work seems to confirm that the bulk of the narrow Fe
K line is produced by the same material responsible for the MIR
emission.Comment: A&A in press, 15 pages, 5 Figures, 3 tables - Few references update
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