670 research outputs found
X-ray Evidence for Multiple Absorbing Structures in Seyfert Galaxies
We have used X-ray spectra to measure attenuating columns in a large sample
of Seyfert galaxies. Over 30 of these sources have resolved radio jets,
allowing the relative orientation of the nucleus and host galaxy to be
constrained. We have discovered that the distribution of absorbing columns is
strongly correlated with the relative orientation of the Seyfert structures.
This result is inconsistent with unification models including only a torus and
is instead most readily explained if a second absorber is included: in addition
to a Compton-thick, parsec-scale torus there would also be a larger-scale
absorber with N_H < 10^{23} cm^{-2}. The second absorber is aligned with the
host galactic plane while the torus is arbitrarily misaligned.Comment: 2 pages, 1 figure, to appear in "Multiwavelength AGN Surveys"
(Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujica, Singapore:
World Scientific, 2004. Additional material may be found at
http://space.mit.edu/home/jonathan/research.htm
The XMM-Newton Iron Line Profile of NGC 3783
We report on observations of the iron K line in the nearby Seyfert 1 galaxy,
NGC 3783, obtained in a long, 2 orbit (240 ks) XMM-Newton observation. The line
profile obtained exhibits two strong narrow peaks at 6.4 keV and at 7.0 keV,
with measured line equivalent widths of 120 and 35 eV respectively. The 6.4 keV
emission is the K-alpha line from near neutral Fe, whilst the 7.0 keV feature
probably originates from a blend of the neutral Fe K-beta line and the H-like
line of Fe at 6.97 keV. The relatively narrow velocity width of the K-alpha
line (<5000 km/s), its lack of response to the continuum emission on short
timescales and the detection of a neutral Compton reflection component are all
consistent with a distant origin in Compton-thick matter such as the putative
molecular torus. A strong absorption line from highly ionized iron (at 6.67
keV) is detected in the time-averaged iron line profile, whilst the depth of
the feature appears to vary with time, being strongest when the continuum flux
is higher. The iron absorption line probably arises from the highest ionization
component of the known warm absorber in NGC 3783, with an ionization of logxi=3
and column density of 5x10^{22}cm{-2} and may originate from within 0.1pc of
the nucleus. A weak red-wing to the iron K line profile is also detected below
6.4 keV. However when the effect of the highly ionized warm absorber on the
underlying continuum is taken into account, the requirement for a relativistic
iron line component from the inner disk is reduced.Comment: 34 pages, including 11 figures. Accepted for publication in Ap
The X-Ray Spectral Variability of Mrk 766
Analysis results from ASCA and ROSAT observations of the narrow-line Seyfert
1 galaxy Mrk 766 are reported. In the ASCA observation we observed rapid
variability with a doubling time scale of 1000 seconds. A spectral variability
event was observed in which the spectrum softened and hardened above and below
~1 keV, respectively, as the flux increased. The spectra could be modeled with
5 components: a power law, warm absorber, iron K(alpha) line and soft excess
component flux. The spectral variability resulted from a highly significant
change in the intrinsic photon law index from Gamma ~1.6 to ~2.0, an increase
in the warm absorber ionization, and a marginally significant decrease in the
soft component normalization. A ~100 eV equivalent width narrow iron K(alpha)
line was detected in the high state spectrum. Spectral hardening during flux
increases was observed in three ROSAT observations.
The change in intrinsic photon index and disappearance of the soft excess
component in the ASCA spectra can be explained as a transition from a first
order pair reprocessed spectrum to a pair cascade brought about by a sudden
increase in the injected electron Lorentz factor. The change in the ionization
of the warm absorber, though model dependent, could correspond to the increase
in flux at the oxygen edges resulting from the spectral index change. The ROSAT
spectral variability can be interpreted by variable intensity hard power law
and a relatively nonvarying soft component, possibly primary disk emission.
These results are compared with those reported from other narrow-line Seyfert 1
galaxies.Comment: 29 pages using (AASTeX) aaspp4.sty and 18 Postscript figures. To
appear in the September 1, 1996, issue of The Astrophysical Journa
NGC 2992 in an X-ray high state observed by XMM: Response of the Relativistic Fe K Line to the Continuum
We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992.
The source was found in its highest level of X-ray activity yet detected, a
factor higher in 2--10 keV flux than the historical minimum. NGC
2992 is known to exhibit X-ray flaring activity on timescales of days to weeks,
and the XMM data provide at least factor of better spectral resolution
in the Fe K band than any previously measured flaring X-ray state. We find that
there is a broad feature in the \sim 5-7 keV band which could be interpreted as
a relativistic Fe K emission line. Its flux appears to have increased
in tandem with the 2--10 keV continuum when compared to a previous Suzaku
observation when the continuum was a factor of lower than that during
the XMM observation. The XMM data are consistent with the general picture that
increased X-ray activity and corresponding changes in the Fe K line
emission occur in the innermost regions of the putative accretion disk. This
behavior contrasts with the behavior of other AGN in which the Fe K
line does not respond to variability in the X-ray.Comment: 30 pages, 6 figures, Accepted to Ap
A Highly Doppler Blueshifted Fe-K Emission Line in the High-Redshift QSO PKS 2149-306
We report the results from an \asca observation of the QSO PKS 2149-306
(z=2.345). We detect an emission line centered at keV in the quasar
frame. Line emission at this energy has not been observed in any other active
galaxy or quasar to date. We present evidence rejecting the possibility that
this line is the result of instrumental artifacts, or a serendipitous source.
The most likely explanation is blueshifted Fe-K emission (the EW is 300+/-200
eV, QSO frame). Bulk velocities of the order of 0.75c are implied by the data.
We show that Fe-K line photons originating in an accretion disk and
Compton-scattering off a leptonic can account for the emission line. Curiously,
if the emission-line feature recently discovered in another quasar PKS
0637752, , is blueshifted Ovii, the Doppler factor is the same
(~2.7) for both.Comment: 15 pages plus 3 figures. Latex with separate .ps files (Accepted by
Astrophysical Journal Letters
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.Comment: 26 pages, 30 figures, accepted for publication in the Astrophysical
Journa
On the efficiency of production of the Fe Kalpha emission line in neutral matter
The absolute luminosity of the Fe Kalpha emission line from matter
illuminated by X-rays in astrophysical sources is nontrivial to calculate
except when the line-emitting medium is optically-thin to absorption and
scattering. We characterize the Fe Kalpha line flux using a dimensionless
efficiency, defined as the fraction of continuum photons above the Fe K shell
absorption edge threshold energy that appear in the line. The optically-thin
approximation begins to break down even for column densities as small as 2 x
10^22 cm^-2. We show how to obtain reliable estimates of the Fe Kalpha line
efficiency in the case of cold, neutral matter, even for the Compton-thick
regime. We find that, regardless of geometry and covering factor, the largest
Fe Kalpha line efficiency is attained well before the medium becomes
Compton-thick. For cosmic elemental abundances it is difficult to achieve an
efficiency higher than a few percent under the most favorable conditions and
lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe
Kalpha line efficiencies that are orders of magnitude less than the maximum
possible for that geometry. Configurations that allow unobscured views of a
Compton-thick reflecting surface are capable of yielding the highest
efficiencies. Our results can be used to estimate the predicted flux of the
narrow Fe Kalpha line at ~6.4 keV from absorption models in AGN. In particular
we show that contrary to a recent claim in the literature, absorption dominated
models for the relativistic Fe Kalpha emission line in MCG -6-30-15 do not
over-predict the narrow Fe Kalpha line for any column density or covering
factor.Comment: Accepted for publication in MNRAS. Seven pages, 2 figure
The Effect of Changes in the ASCA Calibration on the Fe-Kalpha Lines in Active Galaxies
The ASCA calibration has evolved considerably since launch and indeed, is
still evolving. There have been concerns in the literature that changes in the
ASCA calibration have resulted in the Fe-Kalpha lines in active galaxies (AGN)
now being systematically narrower than was originally thought. If this were
true, a large body of ASCA results would be impacted. In particular, it has
been claimed that the broad red wing (when present) of the Fe-Kalpha line has
been considerably weakened by changes in the ASCA calibration. We demonstrate
explicitly that changes in the ASCA calibration over a period of about eight
years have a negligible effect on the width, strength, or shape of the
Fe-Kalpha lines. The reduction in both width and equivalent width is only ~8%
or less. We confirm this with simulations and individual sources, as well as
sample average profiles. The average profile for type 1 AGN is still very
broad, with the red wing extending down to ~4 keV. The reason for the claimed,
apparently large, discrepancies is that in some sources the \fekalfa line is
complex, and a single-Gaussian model, being an inadequate description of the
line profile, picks up different portions of the profile with different
calibration. Single-Gaussian fits do not therefore model all of the line
emission in some sources, in which case they do not compare old and current
calibration since the models do not then describe the data.Comment: Accepted for publication in the Astrophysical Journal, Vol. 569, 10
April, 2002. 22 pages, 4 figure
Concentration- and thickness-dependent magnetic properties of NixMn100âx in epitaxially grown NixMn100âx/Ni/(Co/)Cu3Au(001)
Magnetic proximity effects in single-crystalline NixMn100x/Ni(/Co) bilayers on
Cu3Au.001/ are investigated for in-plane (IP) and out-of-plane (OoP)
magnetization by means of the longitudinal and polar magneto-optical Kerr
effect. Attention is paid to the influence on concentration- and thickness-
dependent antiferromagnetic ordering (TAFM) and blocking (Tb) temperatures as
well as the exchange bias field (Heb). For all the NixMn100x films under study
in contact with IP Ni, increasing TAFM is observed with decreasing Ni
concentration from 50 to 20%, whereas only a slight change in TAFM is observed
for the OoP case. Between 28% and 35% Ni concentration, a crossover
temperature exists below which TAFM for the IP samples is higher than for the
OoP samples and vice versa. Tb is higher for the IP case than for OoP, except
for an equi-atomic NiMn film, while Heb increases significantly for both
magnetization directions with decreasingx. These results are attributed to:
(i) a rotation of the non-collinear 3Q-like spin structure of NixMn100-x from
the more-OoP to the more-IP direction for decreasing Ni concentrationx, along
with an associated increased magnetic anisotropy, and (ii) a smaller domain
wall width within the NixMn100-x films at smaller x, leading to a smaller
thickness required to establish exchange bias at a fixed temperature
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