2,546 research outputs found
Modelling the variable broad-band optical/UV/X-ray spectrum of PG1211+143: Implications for the ionized outflow
We present the results from a detailed analysis of the 2007 Swift monitoring
campaign of the quasar PG1211+143. We constructed broad-band, optical/UV/X-ray
spectral energy distributions over three X-ray flux intervals. We fitted them
with a model which accounts for the disc and the X-ray coronal emission and the
warm absorber (well established in this source). The three flux spectra are
well fitted by the model we considered. The disc inner temperature remains
constant at ~2 eV, while X-rays are variable both in spectral slope and
normalization. The absorber covers almost 90% of the central source. It is
outflowing with a velocity less than 2.3*10^4 km/s (3sigma upper limit), and
has a column density of ~10^23.2. Its ionization parameter varies by a factor
of 1.6, and it is in photo-ionizing equilibrium with the ionizing flux. It is
located at a distance of less than 0.35 pc from the central source and its
relative thickness, DR/R is less than 0.1. The absorber' s ionization parameter
variations can explain the larger than average amplitude of the X-ray
variations. The absence of optical/UV variations (consistent with the high
black hole mass estimate) argues against the presence of inward propagating
disc fluctuations and strong X-ray illumination of the disc (in agreement with
the low ratio of X-ray over the bolometric luminosity of ~20-35). We estimate
an upper limit for the mass outflow of ~5 solar masses per year (~2.3 times the
Eddington mass accretion rate). If the outflow rate is indeed that high, then
it must be a short-lived episode in the quasar's life time. Finally, we
estimate an upper limit for the kinetic power of the outflow of ~1.4*10^43
ergs/s. This outflow cannot deploy significant mechanical energy to the
surrounding ISM of the quasar's host galaxy, but is sufficient to heat the ISM
to 10^7 K and to produce a fast decline to the star formation rate of the
galaxy.Comment: Accepted for publication by A&
CLEAR: Covariant LEAst-square Re-fitting with applications to image restoration
In this paper, we propose a new framework to remove parts of the systematic
errors affecting popular restoration algorithms, with a special focus for image
processing tasks. Generalizing ideas that emerged for regularization,
we develop an approach re-fitting the results of standard methods towards the
input data. Total variation regularizations and non-local means are special
cases of interest. We identify important covariant information that should be
preserved by the re-fitting method, and emphasize the importance of preserving
the Jacobian (w.r.t. the observed signal) of the original estimator. Then, we
provide an approach that has a "twicing" flavor and allows re-fitting the
restored signal by adding back a local affine transformation of the residual
term. We illustrate the benefits of our method on numerical simulations for
image restoration tasks
The long-term optical spectral variability of BL Lacertae
We present the results from a study of the long-term optical spectral
variations of BL Lacertae, using the long and well-sampled B and R-band light
curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time
intervals of 1 day. The relation between spectral slope and flux (the spectrum
gets bluer as the source flux increases) is well described by a power-law
model, although there is significant scatter around the best-fitting model
line. To some extent, this is due to the spectral evolution of the source
(along well-defined loop-like structures) during low-amplitude events, which
are superimposed on the major optical flares, and evolve on time scales of a
few days. The "bluer-when-brighter" mild chromatism of the long-term variations
of the source can be explained if the flux increases/decreases faster in the B
than in the R band. The B and R-band variations are well correlated, with no
significant, measurable delays larger than a few days. On the other hand, we
find that the spectral variations lead those in the flux light curves by ~ 4
days. Our results can be explained in terms of Doppler factor variations due to
changes in the viewing angle of a curved and inhomogeneous emitting jet.Comment: 7 pages, 5 figures, accepted for publication in A&
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
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
The high frequency power spectrum of Markarian 766
An analysis is presented of the power spectrum of X-ray variability of the
bright Seyfert 1 galaxy Mrk 766 as observed by XMM-Newton. Over the 0.2-10 keV
energy range the power spectral density (PSD) is well-represented by a
power-law with a slope of alpha_low ~ 1 at low frequencies, breaking to a slope
of alpha_hi = 2.8 (-0.4/+0.2) at a frequency f_br ~ 5 x 10^-4 Hz. As has been
noted before this broken power-law PSD shape is similar to that observed in the
Galactic black hole candidate Cygnus X-1. If it is assumed that Mrk 766 shows a
power spectrum similar in form to that of Cyg X-1, and that the break timescale
scales linearly with black hole mass, then the mass of the black hole in Mrk
766 is inferred to be < 5 x 10^5 M_sun. This rather low mass would mean Mrk 766
radiates above the Eddington limit. The coherence between different energy
bands is significantly below unity implying that variations in the different
energy bands are rather poorly correlated. The low coherence can be explained
in the framework of standard Comptonisation models if the properties of the
Comptonising medium are rapidly variable or if there are several distinct
emission sites.Comment: 6 pages, 5 figures, accepted for publication in MNRAS Letter
The Effect of Spin Splitting on the Metallic Behavior of a Two-Dimensional System
Experiments on a constant-density two-dimensional hole system in a GaAs
quantum well reveal that the metallic behavior observed in the
zero-magnetic-field temperature dependence of the resistivity depends on the
symmetry of the confinement potential and the resulting spin-splitting of the
valence band
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