559 research outputs found
Evidence for Rapid Iron K_alpha Line Flux Variability in MCG--6-30-15
This paper employs direct spectral fitting of individual orbital data in
order to measure rapid X-ray iron K_alpha line and continuum spectral slope
variations in Seyfert 1 galaxies with unprecedented temporal resolution.
Application of this technique to a long RXTE observation of MCG--6-30-15
indicates that the line flux does vary on short (~1d) timescales, but that
these variations are not correlated with changes in the continuum flux or
slope. These rapid variations indicate that the line does indeed originate
close to the black hole, confirming predictions based on its very broad
profile. However, the lack of a correlation with the continuum presents
problems for models in which the line variations are driven by those in the
continuum, modified only by light-travel time effects. Instead, it may be that
the line responds according to a physical process with a different time scale,
such as ionization instabilities in the disk, or perhaps that the geometry and
physical picture is more complex than implied by the simplest disk-corona
models.
These data also indicate that the slope of the underlying power-law continuum
(Gamma) shows strong variability and is tightly correlated with the continuum
flux in the sense that the spectrum steepens as the source brightens. All of
these results have been checked with extensive simulations, which also
indicated that a spurious correlation between Gamma and Compton reflection
fraction (R) will result if these quantities are measured from the same
spectra. This casts serious doubts on previous claims of such a Gamma-R
correlation.Comment: Accepted for publication in Ap
Rest-frame stacking of 2XMM catalog sources : Properties of the Fe Kalpha line
The aim of this work is to characterize the average Fe K emission properties
of active galactic nuclei (AGNs) in the source rest-frame. We selected a sample
of 248 AGNs from the 2XMM catalog, covering a wide redshift range 0 < z < 5 and
with the EPIC-PN net 2-10 keV rest-frame counts >=200 and power law photon
indices in the range 1.5-2.2. We employed two fully independent rest-frame
stacking procedures to compute the mean Fe K profile. The counting statistics
for the integrated spectrum is comparable to the one available for the best
studied local Seyferts. To identify the artifacts possibly introduced by the
stacking procedure, we have carried out simulations. We report that the average
Fe K line profile in our sample is best represented by a combination of a
narrow and a broad line. The equivalent widths of the narrow and broad
(parametrized with a diskline) components are ~30 eV and ~100 eV, respectively.
We also discuss the results of more complex fits and the implications of the
adopted continuum modeling on the broad line parameters and its detection
significance.Comment: 13 pages, 13 figures, Accepted for publication in A&
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
On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure
We use the method of Press, Rybicki & Hewitt (1992) to search for time lags
and time leads between different energy bands of the RXTE data for MCG-6-30-15.
We tailor our search in order to probe any reverberation signatures of the
fluorescent iron Kalpha line that is thought to arise from the inner regions of
the black hole accretion disk. In essence, an optimal reconstruction algorithm
is applied to the continuum band (2-4keV) light curve which smoothes out noise
and interpolates across the data gaps. The reconstructed continuum band light
curve can then be folded through trial transfer functions in an attempt to find
lags or leads between the continuum band and the iron line band (5-7keV). We
find reduced fractional variability in the line band. The spectral analysis of
Lee et al. (1999) reveals this to be due to a combination of an apparently
constant iron line flux (at least on timescales of few x 10^4s), and flux
correlated changes in the photon index. We also find no evidence for iron line
reverberation and exclude reverberation delays in the range 0.5-50ksec. This
extends the conclusions of Lee et al. and suggests that the iron line flux
remains constant on timescales as short as 0.5ksec. The large black hole mass
(>10^8Msun) naively suggested by the constancy of the iron line flux is
rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a
mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk
may produce the puzzling spectral variability. Finally, it is found that the
8-15keV band lags the 2-4keV band by 50-100s. This result is used to place
constraints on the size and geometry of the Comptonizing medium responsible for
the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap
High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres
We show that Compton scattering of soft photons with energies near 100 eV in
thermally stratified black-hole accretion plasmas with temperatures in the
range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV.
This could produce the hardening observed in the X-ray spectra of black holes,
which is generally attributed to reflection or partial covering of the incident
continuum source by cold optically thick matter. In addition, we show that the
presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting
high energy tails similar to those observed from Galactic black-hole
candidates.Comment: 11 pages, uuencoded gziped postscript, ApJ Letters in pres
X-ray Line Emitting Objects in XMM-Newton Observations: the Tip of the Iceberg
We present preliminary results from a novel search for X-ray Line Emitting
Objects (XLEOs) in XMM-Newton images. Three sources have been detected in a
test-run analysis of 13 XMM-Newton observations. The three objects found are
most likely extremely absorbed AGN characterized by a column density
NH~10^24cm^-2. Their redshift has been directly determined from the X-ray data,
by interpreting the detected emission line as the 6.4 keV Fe line. The measured
equivalent width of the X-ray line is, in all three cases, several keV. This
pilot study demonstrates the success of our search method and implies that a
large sample of XLEOs can be obtained from the public XMM-Newton data archive.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
The long time-scale X-ray variability of the radio-quiet quasar PG 0804+761
We present the results from a study of the timing properties and the energy
spectrum of the radio-quiet quasar PG0804+761, based on monitoring RXTE PCA
observations that lasted for a year. This is a systematic study of the X-ray
variations on time scales of weeks/months of the most luminous radio-quiet
quasar studied so far. We detect significant variations in the 2-10 keV band of
an average amplitude of ~15%. The excess variance of the light curve is smaller
than that of Seyfert galaxies, entirely consistent with the relationship
between variability amplitude and luminosity defined from the Seyfert data
alone. The power spectrum of the source follows a power-like form of slope ~
-1. However, when we extend the power spectrum estimation at higher frequencies
using archival ASCA data, we find strong evidence for an intrinsic steepening
to a slope of ~ -2 at around ~ 1x10^(-6) Hz. This "break frequency" corresponds
to a time scale of ~ 10 days. The time-average energy spectrum is well fitted
by a power law model with Gamma ~ 2. We also find evidence for an iron line at
\~ 6.4 keV (rest frame) with EW 110 eV, similar to what is observed in Seyfert
galaxies. The flux variations are not associated with any spectral variation.
This is the only major difference that we find when we compare the variability
properties of the source with those of Seyfert galaxies. Our results support
the hypothesis that the same X-ray emission and variability mechanism operates
in both Seyfert galaxies and quasars.Comment: 8 pages, 6 figures, accepted for publication in MNRA
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