2,045 research outputs found
X-ray Variability Characteristics of the Seyfert 1 Galaxy NGC 3783
We have characterized the energy-dependent X-ray variability properties of
the Seyfert~1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-ray Timing
Explorer data. The high-frequency fluctuation power spectral density function
(PSD) slope is consistent with flattening towards higher energies. Light curve
cross correlation functions yield no significant lags, but peak coefficients
generally decrease as energy separation of the bands increases on both short
and long timescales. We have measured the coherence between various X-ray bands
over the temporal frequency range of 6e-8 to 1e-4 Hz; this range includes the
temporal frequency of the low-frequency power spectral density function (PSD)
break tentatively detected by Markowitz et al. and includes the lowest temporal
frequency over which coherence has been measured in any AGN to date. Coherence
is generally near unity at these temporal frequencies, though it decreases
slightly as energy separation of the bands increases. Temporal
frequency-dependent phase lags are detected on short time scales; phase lags
are consistent with increasing as energy separation increases or as temporal
frequency decreases. All of these results are similar to those obtained
previously for several Seyfert galaxies and stellar-mass black hole systems.
Qualitatively, these results are consistent with the variability models of
Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly
propagating variations in the local mass accretion rate.Comment: Accepted for publication in The Astrophysical Journal, 2005, vol.
635, p. 180; version 2 has minor grammatical changes; 23 pages; uses
emulateapj
Probing BH mass and accretion through X-ray variability in the CDFS
Recent work on nearby AGNs has shown that X-ray variability is correlated
with the mass and accretion rate onto the central SMBH. Here we present the
application of the variability-luminosity relation to high redshift AGNs in the
CDFS, making use of XMM-Newton observations. We use Monte Carlo simulations in
order to properly account for bias and uncertainties introduced by the sparse
sampling and the very low statistics. Our preliminary results indicate that BH
masses span over the range from 10^5 to 10^9 solar mass while accretion rates
range from 10^-3 up to values greater than 1, in unit of Eddington accretion
rate.Comment: 2 pages, 2 figures,in press in the X-ray 2009 Conference Proceedings
(Bologna, 7-11 September 2009
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
The Energy-dependent X-ray Timing Characteristics of the Narrow Line Seyfert 1 Mkn 766
We present the energy-dependent power spectral density (PSD) and
cross-spectral properties of Mkn 766, obtained from combining data obtained
during an XMM-Newton observation spanning six revolutions in 2005 with data
obtained from an XMM-Newton long-look in 2001. The PSD shapes and rms-flux
relations are found to be consistent between the 2001 and 2005 observations,
suggesting the 2005 observation is simply a low-flux extension of the 2001
observation and permitting us to combine the two data sets. The resulting PSD
has the highest temporal frequency resolution for any AGN PSD measured to date.
Applying a broken power-law model yields break frequencies which increase in
temporal frequency with photon energy. Obtaining a good fit when assuming
energy-independent break frequencies requires the presence of a Lorentzian at
4.6+/-0.4 * 10^-4 Hz whose strength increases with photon energy, a behavior
seen in black hole X-ray binaries. The cross-spectral properties are measured;
temporal frequency-dependent soft-to-hard time lags are detected in this object
for the first time. Cross-spectral results are consistent with those for other
accreting black hole systems. The results are discussed in the context of
several variability models, including those based on inwardly-propagating
viscosity variations in the accretion disk.Comment: Accepted for publication in The Astrophysical Journal. 18 pages, 9
figures. Uses emulateapj5.st
Evidence for Nonlinear X-ray Variability from the Broad-line Radio Galaxy 3C 390.3
We present analysis of the light curve from the ROSAT HRI monitoring
observations of the broad-line radio galaxy 3C 390.3. Observed every three days
for about 9 months, this is the first well sampled X-ray light curve on these
time scales. The flares and quiescent periods in the light curve suggest that
the variability is nonlinear, and a statistical test yields a detection with >6
sigma confidence. The structure function has a steep slope ~0.7, while the
periodogram is much steeper with a slope ~2.6, with the difference partially
due to a linear trend in the data. The non-stationary character of the light
curve could be evidence that the variability power spectrum has not turned over
to low frequencies, or it could be an essential part of the nonlinear process.
Evidence for X-ray reprocessing suggests that the X-ray emission is not from
the compact radio jet, and the reduced variability before and after flares
suggests there cannot be two components contributing to the X-ray short term
variability. Thus, these results cannot be explained easily by simple models
for AGN variability, including shot noise which may be associated with flares
in disk-corona models or active regions on a rotating disk, because in those
models the events are independent and the variability is therefore linear. The
character of the variability is similar to that seen in Cygnus X-1, which has
been explained by a reservoir or self-organized criticality model. Inherently
nonlinear, this model can reproduce the reduced variability before and after
large flares and the steep PDS seen generally from AGN. The 3C 390.3 light
curve presented here is the first support for such models to explain AGN
variability on intermediate time scales from a few days to months.Comment: 10 pages using (AASTeX) aaspp4.sty and 3 Postscript figures.
Astrophysical Journal Letters, in pres
In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems
Using a novel technique, we make quantitative measurements of the spin
polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional
holes as a function of an in-plane magnetic field. As the field is increased
the system gradually becomes spin polarized, with the degree of spin
polarization depending on the orientation of the field relative to the crystal
axes. Moreover, the behavior of the system turns from metallic to insulating
\textit{before} it is fully spin polarized. The minority-spin population at the
transition is ~8*10^{9} cm^{-2}, close to the density below which the system
makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure
Low-field magnetoresistance in GaAs 2D holes
We report low-field magnetotransport data in two-dimensional hole systems in
GaAs/AlGaAs heterostructures and quantum wells, in a large density range, cm, with primary focus on
samples grown on (311)A GaAs substrates. At high densities, cm, we observe a remarkably strong positive magnetoresistance.
It appears in samples with an anisotropic in-plane mobility and predominantly
along the low-mobility direction, and is strongly dependent on the
perpendicular electric field and the resulting spin-orbit interaction induced
spin-subband population difference. A careful examination of the data reveals
that the magnetoresistance must result from a combination of factors including
the presence of two spin-subbands, a corrugated quantum well interface which
leads to the mobility anisotropy, and possibly weak anti-localization. None of
these factors can alone account for the observed positive magnetoresistance. We
also present the evolution of the data with density: the magnitude of the
positive magnetoresistance decreases with decreasing density until, at the
lowest density studied ( cm), it vanishes and is
replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in
MNRA
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