11,943 research outputs found
The X-ray Power Spectral Density Function and Black Hole Mass Estimate for the Seyfert AGN IC 4329a
We present the X-ray broadband power spectral density function (PSD) of the
X-ray-luminous Seyfert IC 4329a, constructed from light curves obtained via
Rossi X-ray Timing Explorer monitoring and an XMM-Newton observation. Modeling
the 3-10 keV PSD using a broken power-law PSD shape, a break in power-law slope
is significantly detected at a temporal frequency of 2.5(+2.5,-1.7) * 10^-6 Hz,
which corresponds to a PSD break time scale T_b of 4.6(+10.1,-2.3) days. Using
the relation between T_b, black hole mass M_BH, and bolometric luminosity as
quantified by McHardy and coworkers, we infer a black hole mass estimate of
M_BH = 1.3(+1.0,-0.3) * 10^8 solar masses and an accretion rate relative to
Eddington of 0.21(+0.06,-0.10) for this source. Our estimate of M_BH is
consistent with other estimates, including that derived by the relation between
M_BH and stellar velocity dispersion. We also present PSDs for the 10-20 and
20-40 keV bands; they lack sufficient temporal frequency coverage to reveal a
significant break, but are consistent with the same PSD shape and break
frequency as in the 3-10 keV band.Comment: Accepted for publication in ApJ. 11 pages, 6 figures (5 color
The X-ray Power Spectral Density Function of the Seyfert Active Galactic Nucleus NGC 7469
We present the broadband X-ray power spectral density function (PSD) of the
X-ray-luminous Seyfert 1.2 NGC 7469, measured from Rossi X-ray Timing Explorer
monitoring data and two XMM-Newton observations. We find significant evidence
for a turnover in the 2-10 keV PSD at a temporal frequency of 2.0(+3.0,-0.8)e-6
Hz or 1.0(+3.0,-0.6)e-6 Hz, depending on the exact form of the break
(sharply-broken or slowly-bending power-law, respectively). The ``surrogate''
Monte Carlo method of Press et al. (1992) was used to map out the probability
distributions of PSD model parameters and obtain reliable uncertainties (68 per
cent confidence limits quoted here). The corresponding break time scale of 5.8
(+/- 3.5) days or 11.6(+17.5,-8.7) days, respectively, is consistent with the
empirical relation between PSD break time scale, black hole mass and bolometric
luminosity of McHardy et al. Compared to the 2-10 keV PSD, the 10-20 keV PSD
has a much flatter shape at high temporal frequencies, and no PSD break is
significantly detected, suggesting an energy-dependent evolution not unlike
that exhibited by several Galactic black hole systems.Comment: Accepted for publication in The Astrophysical Journal. 17 pages, 10
figures (3 color
X-ray variability of AGNs in the soft and the hard X-ray bands
We investigate the X-ray variability characteristics of hard X-ray selected
AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data.
The uncertainties involved in the individual dwell measurements of ASM are
critically examined and a method is developed to combine a large number of
dwells with appropriate error propagation to derive long duration flux
measurements (greater than 10 days). We also provide a general prescription to
estimate the errors in variability derived from rms values from unequally
spaced data. Though the derived variability for individual sources are not of
very high significance, we find that, in general, the soft X-ray variability is
higher than those in hard X-rays and the variability strengths decrease with
energy for the diverse classes of AGN. We also examine the strength of
variability as a function of the break time scale in the power density spectrum
(derived from the estimated mass and bolometric luminosity of the sources) and
find that the data are consistent with the idea of higher variability at time
scales longer than the break time scale.Comment: 17 pages, 15 Postscript figures, 3 tables, accepted for publication
in Ap
Model of Transcriptional Activation by MarA in Escherichia coli
We have developed a mathematical model of transcriptional activation by MarA
in Escherichia coli, and used the model to analyze measurements of
MarA-dependent activity of the marRAB, sodA, and micF promoters in mar-rob-
cells. The model rationalizes an unexpected poor correlation between the
mid-point of in vivo promoter activity profiles and in vitro equilibrium
constants for MarA binding to promoter sequences. Analysis of the promoter
activity data using the model yielded the following predictions regarding
activation mechanisms: (1) MarA activation of the marRAB, sodA, and micF
promoters involves a net acceleration of the kinetics of transitions after RNA
polymerase binding, up to and including promoter escape and message elongation;
(2) RNA polymerase binds to these promoters with nearly unit occupancy in the
absence of MarA, making recruitment of polymerase an insignificant factor in
activation of these promoters; and (3) instead of recruitment, activation of
the micF promoter might involve a repulsion of polymerase combined with a large
acceleration of the kinetics of polymerase activity. These predictions are
consistent with published chromatin immunoprecipitation assays of interactions
between polymerase and the E. coli chromosome. A lack of recruitment in
transcriptional activation represents an exception to the textbook description
of activation of bacterial sigma-70 promoters. However, use of accelerated
polymerase kinetics instead of recruitment might confer a competitive advantage
to E. coli by decreasing latency in gene regulation.Comment: 30 pages, 2 figure
A study of the dynamics of rotating space stations with elastically connected counterweight and attached flexible appendages. Volume 1: Theory
The formulation of a mathematical model for predicting the dynamic behavior of rotating flexible space station configurations was conducted. The overall objectives of the study were: (1) to develop the theoretical techniques for determining the behavior of a realistically modeled rotating space station, (2) to provide a versatile computer program for the numerical analysis, and (3) to present practical concepts for experimental verification of the analytical results. The mathematical model and its associated computer program are described
Low-Luminosity AGN as analogues of Galactic Black Holes in the low/hard state: Evidence from X-ray timing of NGC 4258
We present a broadband power spectral density function (PSD) measured from
extensive RXTE monitoring data of the low-luminosity AGN NGC 4258, which has an
accurate, maser-determined black hole mass of 3.9+/-0.1 * 10^7 solar masses. We
constrain the PSD break timescale to be greater than 4.5 d at >90% confidence,
which appears to rule out the possibility that NGC 4258 is an analogue of black
hole X-ray binaries (BHXRBs) in the high/soft state. In this sense, the PSD of
NGC 4258 is different to those of some more-luminous Seyferts, which appear
similar to the PSDs of high/soft state X-ray binaries. This result supports
previous analogies between LLAGN and X-ray binaries in the low/hard state based
on spectral energy distributions, indicating that the AGN/BHXRB analogy is
valid across a broad range of accretion rates.Comment: Accepted for publication in The Astrophysical Journal Letters; 6
pages; 3 figure
Evidence for a Truncated Accretion Disc in the Low Luminosity Seyfert Galaxy, NGC 7213?
We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low
luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is
well fitted by a single powerlaw of photon index Gamma = 1.75 and from
consideration of the Fermi flux limit we constrain the high energy cutoff to be
350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39
keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected
in the Suzaku spectrum, further confirming the results of previous observations
with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and
Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter
appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and
Fe_(xxvi) emission can result from a highly photo-ionised plasma of column
density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g.,
originating from an optically-thick accretion disc or a Compton-thick torus,
appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray
source, consistent with previous findings. Indeed the absence of either neutral
or ionised Compton reflection coupled with the lack of any relativistic Fe K
signatures in the spectrum suggests that an inner, optically-thick accretion
disc is absent in this source. Instead, the accretion disc could be truncated
with the inner regions perhaps replaced by a Compton-thin Radiatively
Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could
both originate in ionised material perhaps at the transition region between the
hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) -
10^(4) gravitational radii from the black hole. The origin for the unresolved
neutral Fe K_alpha emission is then likely to be further out, perhaps
originating in the optical BLR or a Compton-thin pc-scale torus.Comment: 15 pages, 11 figures, accepted for publication by MNRA
- …