130 research outputs found
Progress towards making a global supply of microbial extracellular vesicles,100-times cheaper than a typical biologic
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Modeling the UBVRI time delays in Mrk 335
We develop a model of time delays between the continuum bands in the Narrow
Line Seyfert 1 galaxy Mrk 335 to explain the observed delays measured in this
source. We consider two geometries: an accretion disk with fully ionized warm
absorber of considerable optical depth, located close to the symmetry axis, and
an accretion disk with a hot corona. Both media lead to significant disk
irradiation but the disk/corona geometry gives lower values of the time delays.
Only the disk/corona models give results consistent with measurements of
Sergeev et al., and a low value of the disk inclination is favored. The
presence of an optically thick, fully ionized outflow is ruled out at the
2-sigma level.Comment: MNRAS (in press
Can we measure the accretion efficiency of Active Galactic Nuclei?
The accretion efficiency for individual black holes is very difficult to
determine accurately. There are many factors that can influence each step of
the calculation, such as the dust and host galaxy contribution to the observed
luminosity, the black hole mass and more importantly, the uncertainties on the
bolometric luminosity measurement. Ideally, we would measure the AGN emission
at every wavelength, remove the host galaxy and dust, reconstruct the AGN
spectral energy distribution and integrate to determine the intrinsic emission
and the accretion rate. In reality, this is not possible due to observational
limitations and our own galaxy line of sight obscuration. We have then to infer
the bolometric luminosity from spectral measurements made in discontinuous
wavebands and at different epochs. In this paper we tackle this issue by
exploring different methods to determine the bolometric luminosity. We first
explore the trend of accretion efficiency with black hole mass (efficiency
proportional to M^{\sim 0.5}) found in recent work by Davis & Laor and discuss
why this is most likely an artefact of the parameter space covered by their PG
quasar sample. We then target small samples of AGN at different redshifts,
luminosities and black hole masses to investigate the possible methods to
calculate the accretion efficiency. For these sources we are able to determine
the mass accretion rate and, with some assumptions, the accretion efficiency
distributions. Even though we select the sources for which we are able to
determine the parameters more accurately, there are still factors affecting the
measurements that are hard to constrain. We suggest methods to overcome these
problems based on contemporaneous multi-wavelength data measurements and
specifically targeted observations for AGN in different black hole mass ranges.Comment: 16 pages, 20 figures, accepted for publication in MNRA
On the X-ray spectra of luminous, inhomogeneous accretion flows
We discuss the expected X-ray spectral and variability properties of black
hole accretion discs at high luminosity, under the hypothesis that radiation
pressure dominated discs are subject to violent clumping instabilities and, as
a result, have a highly inhomogeneous two-phase structure. After deriving the
full accretion disc solutions explicitly in terms of the parameters of the
model, we study their radiative properties both with a simple two-zones model,
treatable analytically, and with radiative transfer simulations which account
simultaneously for energy balance and Comptonisation in the hot phase, together
with reflection, reprocessing, ionization and thermal balance in the cold
phase. We show that, if not only the density, but also the heating rate within
these flows is inhomogeneous, then complex reflection-dominated spectra can be
obtained for a high enough covering fraction of the cold phase. In general,
large reflection components in the observed X-ray spectra should be associated
with strong soft excesses, resulting from the combined emission of ionized
atomic emission lines. The variability properties of such systems are such
that, even when contributing to a large fraction of the hard X-ray spectrum,
the reflection component is less variable than the power-law like emission
originating from the hot Comptonising phase, in agreement with what is observed
in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls
within the family of those trying to explain the complex X-ray spectra of
bright AGN with ionized reflection, but presents an alternative, specific,
physically motivated, geometrical setup for the complex multi-phase structure
of the inner regions of near-Eddington accretion flows.Comment: 15 pages, 9 figures. MNRAS, in pres
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
The flare model for X-ray variability of NGC 4258
We study the variability mechanism of active galactic nuclei (AGN) within the
framework of the flare model. To this end we examine the case of Seyfert/LINER
galaxy NGC 4258, which is observed at high inclination angle and exhibits rapid
fluctuations of the X-ray light curve. We construct a model light curve based
on the assumption of magnetic flares localized in the equatorial plane and
orbiting with Keplerian speed at each given radius. We calculate the level of
variability as a function of the inclination of an observer, taking into
account all effects of general relativity near a rotating supermassive black
hole. The variability level is a monotonic function of the source inclination.
It rises more rapidly for larger values of the black hole spin (Kerr parameter)
and for steeper emissivity (index beta of the radial profile). We compare the
expected level of variability for the viewing angle 81.6 deg, as inferred for
NGC 4258, with the case of moderate viewing angles about 30 deg, typical for
Seyfert type-1 galaxies. Highly inclined sources such as this one are
particularly suitable to test the flare model because the effects of orbital
motion, Doppler boosting and light bending are all expected to have maximum
when the accretion disk is seen almost edge-on. The model is consistent with
the NGC 4258 variability, where the obscuring material is thought to be
localized mainly towards the equatorial plane rather than forming a
geometrically thick torus. Once the intrinsic time-scales of the flare duration
are determined to better precision, this kind of highly inclined objects with a
precisely known mass of the black hole can be used to set independent
constraints on the spin parameter.Comment: 7 pages, 3 figures; Astronomy & Astrophysics (this version includes
minor language corrections
The iron lines as a tool for magnetic field estimations in non-flat accretion flows
Observations of AGNs and microquasars by ASCA, RXTE, Chandra and XMM-Newton
indicate the existence of broad X-ray emission lines of ionized heavy elements
in their spectra. Such spectral lines were discovered also in X-ray spectra of
neutron stars and X-ray afterglows of GRBs. Recently, Zakharov et al. (MNRAS,
2003, 342, 1325) described a procedure to estimate an upper limit of the
magnetic fields in regions from which X-ray photons are emitted. The authors
simulated typical profiles of the iron line in the presence of
magnetic field and compared them with observational data in the framework of
the widely accepted accretion disk model. Here we further consider typical
Zeeman splitting in the framework of a model of non-flat accretion flows, which
is a generalization of previous consideration into non-equatorial plane motion
of particles emitting X-ray photons.
Using perspective facilities of space borne instruments (e.g. Constellation-X
mission) a better resolution of the blue peak structure of iron line
will allow to evaluate the magnetic fields with higher accuracy.Comment: 22 pages, 6 figure
Reprocessing of X-rays in AGN. I. Plane parallel geometry -- test of pressure equilibrium
We present a model of the vertical stratification and the spectra of an
irradiated medium under the assumption of constant pressure. Such a solution
has properties intermediate between constant density models and hydrostatic
equilibrium models, and it may represent a flattened configuration of gas
clumps accreting onto the central black hole. Such a medium develops a hot
skin, thicker than hydrostatic models, but thinner than constant density
models, under comparable irradiation. The range of theoretical values of the
alpha_ox index is comparable to those from hydrostatic models and both are
close to the observed values for Seyfert galaxies but lower than in quasars.
The amount of X-ray Compton reflection is consistent with the observed range.
The characteristic property of the model is a frequently multicomponent iron K
alpha line.Comment: accepted for publication in Astronomy and Astrophysic
The structure and radiation spectra of illuminated accretion disks in AGN. II. Flare/spot model of X-ray variability
We discuss a model of X-ray variability of active galactic nuclei (AGN). We
consider multiple spots which originate on the surface of an accretion disk
following intense irradiation by coronal flares. The spots move with the disk
around the central black hole and eventually decay while new spots continuously
emerge. We construct time sequences of the spectra of the spotted disk and
compute the corresponding energy-dependent fractional variability amplitude. We
explore the dependence on the disk inclination and other model parameters. AGN
seen at higher inclination with respect to the observer, such as Seyfert 2
galaxies, are expected to have fractional variability amplitude of the direct
emission by a factor of a few higher than objects seen face on, such as the
Seyfert 1s.Comment: Astronomy and Astrophysics (in press
Piecing Together the X-ray Background: Bolometric Corrections for Active Galactic Nuclei
(Abridged) The X-ray background can be used to constrain the accretion
history of Supermassive Black Holes (SMBHs) in Active Galactic Nuclei (AGN). A
knowledge of the hard X-ray bolometric correction, \kappa_{2-10keV} is a vital
input into these studies. Variations in the disk emission in the UV have not
previously been taken into account in calculating \kappa_{2-10keV}; we show
that such variations are important by constructing optical--to--X-ray SEDs for
54 AGN. In particular, we use FUSE UV and X-ray data from the literature to
constrain the disk emission as well as possible. Previous work has suggested a
dependence of \kappa_{2-10keV} on AGN luminosity, but we find significant
spread in \kappa_{2-10keV} with no simple dependence on luminosity. Populations
such as Narrow-Line Seyfert 1 nuclei (NLS1s), Radio Loud and X-ray Weak AGN may
have values of \kappa_{2-10keV} differing systematically from the rest of the
AGN population. Other sources of uncertainty include intrinsic extinction in
the optical--UV, X-ray and UV variability and uncertainties in SMBH mass
estimates. Our results suggest a more well-defined relationship between
\kappa_{2-10keV} and Eddington ratio in AGN, with a transitional region at an
Eddington ratio of ~0.1, below which the bolometric correction is typically 15
- 25, and above which it is typically 40 - 70. We consider the potential
implied parallels with the low/hard and high/soft states in Galactic Black Hole
(GBH) accretion, and present bolometric corrections for the GBH binary GX 339-4
for comparison. Our findings reinforce previous studies proposing a multi-state
description of AGN accretion analogous to that for GBH binaries. Future
calculations of the SMBH mass density may need to account for the possible
dependence of \kappa_{2-10keV} on Eddington ratio.Comment: 19 pages, 16 figures, 3 tables. Accepted for publication in MNRA
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