1,742 research outputs found
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
A Cutoff in the X-ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516
During 1997 March-July, XTE observed the bright, strongly variable Seyfert 1
galaxy NGC 3516 once every ~12.8 hr for 4.5 months and nearly continuously
(with interruptions due to SAA passage but not Earth occultation) for a 4.2 day
period in the middle. These were followed by ongoing monitoring once every ~4.3
days. These data are used to construct the first well-determined X-ray
fluctuation power density spectrum (PDS) of an active galaxy to span more than
4 decades of usable temporal frequency. The PDS shows no signs of any strict or
quasi-periodicity, but does show a progressive flattening of the power-law
slope from -1.74 at short time scales to -0.73 at longer time scales. This is
the clearest observation to date of the long-predicted cutoff in the PDS. The
characteristic variability time scale corresponding to this cutoff temporal
frequency is 1 month. Although it is unclear how this time scale may be
interpreted in terms of a physical size or process, there are several promising
candidate models. The PDS appears similar to those seen for Galactic black hole
candidates such as Cyg X-1, suggesting that these two classes of objects with
very different luminosities and putative black hole masses (differing by more
than a factor of 10^5) may have similar X-ray generation processes and
structures.Comment: 21 pages, incl. 5 figures, AASTe
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
An investigation of the origin of soft X-ray excess emission from Narrow-line Seyfert 1 galaxies Akn564 and Mrk1044
We investigate the origin of the soft X-ray excess emission from narrow-line
Seyfert 1 galaxies Akn564 and Mrk1044 using XMM-Newton observations. We find
clear evidence for time delays between the soft and hard X-ray emission from
Akn564 based on a 100ks long observation. The variations in the 4-10keV band
lag behind that in the 0.2-0.5keV band by 1768+/-122s. The full band power
density spectrum (PDS) of Akn~564 has a break at ~1.2e-3Hz with power-law
indices of ~1 and ~3 below and above the break. The hard (3-10keV) band PDS is
stronger and flatter than that in the soft (0.2-0.5keV) band. Based on a short
observation of Mrk1044, we find no correlation between the 0.2-0.3keV and
5-10keV bands at zero lag. These observations imply that the soft excess is not
the reprocessed hard X-ray emission. The high resolution spectrum of Akn564
obtained with the RGS shows evidence for a highly ionized and another weakly
ionized warm absorber medium. The smeared wind and blurred ionized reflection
models do not describe the pn data adequately. The spectrum is consistent with
a complex model consisting of optically thick Comptonization in a cool plasma
for the soft excess and a steep power-law, modified by two warm absorber media
as inferred from the RGS data and the foreground Galactic absorption. The
smeared wind and optically thick Comptonization models both describe the
spectrum of Mrk1044 satisfactorily, but the ionized reflection model requires
extreme parameters. The data suggest two component corona -- a cool, optically
thick corona for the soft excess and a hot corona for the power-law component.
The existence of a break in the soft band PDS suggests a compact cool corona
that can either be an ionized surface of the inner disk or an inner optically
thick region coupled to a truncated disk.Comment: Accepted for publication in ApJ, 19 page
X-ray Fluctuation Power Spectral Densities of Seyfert 1 Galaxies
By combining complementary monitoring observations spanning long, medium and
short time scales, we have constructed power spectral densities (PSDs) of six
Seyfert~1 galaxies. These PSDs span 4 orders of magnitude in temporal
frequency, sampling variations on time scales ranging from tens of minutes to
over a year. In at least four cases, the PSD shows a "break," a significant
departure from a power law, typically on time scales of order a few days. This
is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass
compact systems with breaks on time scales of seconds. NGC 3783 shows tentative
evidence for a doubly-broken power law, a feature that until now has only been
seen in the (much better-defined) PSDs of low-state XRBs. It is also
interesting that (when one previously-observed object is added to make a small
sample of seven), an apparently significant correlation is seen between the
break time scale and the putative black hole mass , while none
is seen between break time scale and luminosity. The data are consistent with
the linear relation T = M_{\rm BH}/10^{6.5} \Msun; extrapolation over 6--7
orders of magnitude is in reasonable agreement with XRBs. All of this
strengthens the case for a physical similarity between Seyfert~1s and XRBs.Comment: 27 pages, 13 figures. Accepted for publication in ApJ. Typo correcte
Quantifying Rapid Variability in Accreting Compact Objects
I discuss some practical aspects of the analysis of millisecond time
variability X-ray data obtained from accreting neutron stars and black holes.
First I give an account of the statistical methods that are at present commonly
applied in this field. These are mostly based on Fourier techniques. To a large
extent these methods work well: they give astronomers the answers they need.
Then I discuss a number of statistical questions that astronomers don't really
know how to solve properly and that statisticians may have ideas about. These
questions have to do with the highest and the lowest frequency ranges
accessible in the Fourier analysis: how do you determine the shortest time
scale present in the variability, how do you measure steep low-frequency noise.
The point is stressed that in order for any method that resolves these issues
to become popular, it is necessary to retain the capabilities the current
methods already have in quantifying the complex, concurrent variability
processes characteristic of accreting neutron stars and black holes.Comment: To be published in the Proceedings of "Statistical Challenges in
Modern Astronomy II", University Park PA, USA, June 199
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