36 research outputs found
QPOs from Random X-ray Bursts around Rotating Black Holes
We continue our earlier studies of quasi-periodic oscillations (QPOs) in the
power spectra of accreting, rapidly-rotating black holes that originate from
the geometric "light echoes" of X-ray flares occurring within the black hole
ergosphere. Our present work extends our previous treatment to
three-dimensional photon emission and orbits to allow for arbitrary latitudes
in the positions of the distant observers and the X-ray sources in place of the
mainly equatorial positions and photon orbits of the earlier consideration.
Following the trajectories of a large number of photons we calculate the
response functions of a given geometry and use them to produce model light
curves which we subsequently analyze to compute their power spectra and
autocorrelation functions. In the case of an optically-thin environment,
relevant to advection-dominated accretion flows, we consistently find QPOs at
frequencies of order of ~kHz for stellar-mass black hole candidates while order
of ~mHz for typical active galactic nuclei (~10^7 Msun) for a wide range of
viewing angles (30 to 80deg) from X-ray sources predominantly concentrated
toward the equator within the ergosphere. As in our previous treatment, here
too, the QPO signal is produced by the frame-dragging of the photons by the
rapidly-rotating black hole, which results in photon "bunches" separated by
constant time-lags, the result of multiple photon orbits around the hole. Our
model predicts for various source/observer configurations the robust presence
of a new class of QPOs, which is inevitably generic to curved spacetime
structure in rotating black hole systems.Comment: 26 pages, 8 b/w figs, accepted to Ap
Constraining X-ray Coronal Size with Transverse Motion of AGN Ultra-Fast Outflows
One of the canonical physical properties of ultra-fast outflows (UFOs) seen
in a diverse population of active galactic nuclei (AGNs) is its seemingly very
broad width (i.e. km~s) , a feature often
required for X-ray spectral modeling. While unclear to date, this condition is
occasionally interpreted and justified as internal turbulence within the UFOs
for simplicity. In this work, we exploit a transverse motion of a
three-dimensional accretion-disk wind, an essential feature of non-radial
outflow morphology unique to magnetohydrodynamic (MHD) outflows. We argue that
at least part of the observed line width of UFOs may reflect the degree of
transverse velocity gradient due to Doppler broadening around a putative
compact X-ray corona in the proximity of a black hole. In this scenario, line
broadening is sensitive to the geometrical size of the corona, . We
calculate the broadening factor as a function of coronal radius and
velocity smearing factor at a given plasma position. We
demonstrate, as a case study of the quasar, PDS~456, that the spectral analysis
favors a compact coronal size of where is
gravitational radius. Such a compact corona is long speculated from both X-ray
reverberation study and the lamppost model for disk emission also consistent
with microlensing results. Combination of such a transverse broadening around a
small corona can be a direct probe of a substantial rotational motion perhaps
posing a serious challenge to radiation-driven wind viewpoint.Comment: 15 pages, 5 figures, 1 table; accepted to ApJ Letter
Toward a Unified AGN Structure
We present a unified model for the structure and appearance of accretion
powered sources across their entire luminosity range from galactic X-ray
binaries to luminous quasars, with emphasis on AGN and their phenomenology.
Central to this model is the notion of MHD winds launched from the accretion
disks that power these objects. These winds provide the matter that manifests
as blueshifted absorption features in the UV and X-ray spectra of a large
fraction of these sources; furthermore, their density distribution in the
poloidal plane determines the "appearance" (i.e. the column and velocity
structure of these absorption features) as a function of the observer
inclination angle. This work focuses on just the broadest characteristics of
these objects; nonetheless, it provides scaling laws that allow one to
reproduce within this model the properties of objects spanning a very wide
luminosity range and viewed at different inclination angles, and trace them to
a common underlying dynamical structure. Its general conclusion is that the AGN
phenomenology can be accounted for in terms of three parameters: The wind mass
flux in units of the Eddington value, , the observer's inclination
angle and the logarithmic slope between the O/UV and X-ray fluxes
. However, because of a significant correlation between
and UV luminosity, we conclude that the AGN structure depends on
only two parameters. Interestingly, the correlations implied by this model
appear to extend to and consistent with the characteristics of galactic X-ray
sources, suggesting the presence of a truly unified underlying structure for
accretion powered sources.Comment: submitted to the Astronomical Review, 32pg, 8 fig
Stratified Magnetically-Driven Accretion-Disk Winds and Their Relations to Jets
We explore the poloidal structure of two-dimensional (2D) MHD winds in
relation to their potential association with the X-ray warm absorbers (WAs) and
the highly-ionized ultra-fast outflows (UFOs) in AGN, in a single unifying
approach. We present the density , ionization parameter
, and velocity structure of such ionized winds for
typical values of their fluid-to-magnetic flux ratio, , and specific angular
momentum, , for which wind solutions become super-\Alfvenic. We explore the
geometrical shape of winds for different values of these parameters and
delineate the values that produce the widest and narrowest opening angles of
these winds, quantities necessary in the determination of the statistics of AGN
obscuration. We find that winds with smaller show a poloidal geometry of
narrower opening angles with their \Alfven\ surface at lower inclination angles
and therefore they produce the highest line of sight (LoS) velocities for
observers at higher latitudes with the respect to the disk plane. We further
note a physical and spatial correlation between the X-ray WAs and UFOs that
form along the same LoS to the observer but at different radii, , and
distinct values of , and consistent with the latest spectroscopic
data of radio-quiet Seyfert galaxies. We also show that, at least in the case
of 3C 111, the winds' pressure is sufficient to contain the relativistic plasma
responsible for its radio emission. Stratified MHD disk-winds could therefore
serve as a unique means to understand and unify the diverse AGN outflows.Comment: version 2 (modified), 27 pages, 5 figures, accepted to Ap
Magnetically-Driven Accretion-Disk Winds and Ultra-Fast Outflows in PG1211+143
We present a study of X-ray ionization of magnetohydrodynamic (MHD)
accretion-disk winds in an effort to constrain the physics underlying the
highly-ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often
detected in various sub-classes of Seyfert active galactic nuclei (AGNs). Our
primary focus is to show that magnetically-driven outflows are indeed
physically plausible candidates for the observed outflows accounting for the
AGN absorption properties of the present X-ray spectroscopic observations.
Employing a stratified MHD wind launched across the entire AGN accretion disk,
we calculate its X-ray ionization and the ensuing X-ray absorption line
spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds
to model the absorption features in an {\it XMM-Newton}/EPIC spectrum of the
narrow-line Seyfert, \pg. We find, through identifying the detected features
with Fe K transitions, that the absorber has a characteristic
ionization parameter of ^{-1} and a
column density on the order of cm, outflowing at a
characteristic velocity of (where is the speed of
light). The best-fit model favors its radial location at
( is the black hole innermost stable circular orbit), with an inner wind
truncation radius at . The overall K-shell feature in
the data is suggested to be dominated by \fexxv\ with very little contribution
from \fexxvi\ and weakly-ionized iron, which is in a good agreement with a
series of earlier analysis of the UFOs in various AGNs including \pg.Comment: v.3 as of 5/6/15 with eliminating extra figs: accepted to ApJ, 28
pages, figs.1-6 (color), 3 table
Magnetic Origin of Black Hole Winds Across the Mass Scale
Black hole accretion disks appear to produce invariably plasma outflows that
result in blue-shifted absorption features in their spectra. The X-ray
absorption-line properties of these outflows are quite diverse, ranging in
velocity from non-relativistic ( km/sec) to sub-relativistic ( where is the speed of light) and a similarly broad range in the
ionization states of the wind plasma. We report here that semi-analytic,
self-similar magnetohydrodynamic (MHD) wind models that have successfully
accounted for the X-ray absorber properties of supermassive black holes, also
fit well the high-resolution X-ray spectrum of the accreting stellar-mass black
hole, GRO J1655-40. This provides an explicit theoretical argument of their MHD
origin (aligned with earlier observational claims) and supports the notion of a
universal magnetic structure of the observed winds across all known black hole
sizes.Comment: published in 2017 March 6 Nature Astronomy, 23 pages, 4 figures, 4
supplementary figure
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin exists
and therefore only a lower limit of the spin value can be given.Comment: accepted by MNRAS for publication; now proof corrected Versio
Accretion Disk Illumination in Schwarzschild and Kerr Geometries: Fitting Formulae
We describe the methodology and compute the illumination of geometrically
thin accretion disks around black holes of arbitrary spin parameter exposed
to the radiation of a point-like, isotropic source at arbitrary height above
the disk on its symmetry axis. We then provide analytic fitting formulae for
the illumination as a function of the source height and the black hole
angular momentum . We find that for a source on the disk symmetry axis and
, the main effect of the parameter is allowing the disk to extend
to smaller radii (approaching as ) and thus allow the
illumination of regions of much higher rotational velocity and redshift. We
also compute the illumination profiles for anisotropic emission associated with
the motion of the source relative to the accretion disk and present the
fractions of photons absorbed by the black hole, intercepted by the disk or
escaping to infinity for both isotropic and anisotropic emission for
and . As the anisotropy (of a source approaching the disk) increases
the illumination profile reduces (approximately) to a single power-law, whose
index, , because of absorption of the beamed photons by the black hole,
saturates to a value no higher than . Finally, we compute the
fluorescence Fe line profiles associated with the specific illumination and
compare them among various cases.Comment: 26 pages, 21 b/w figures, accepted for publication in the
Astrophysical Journal as of 4/16/200