The hard X-ray spectrum of NGC 1365: scattered light, not black hole spin. (arXiv:1303.4309v1 [astro-ph.HE])

The hard X-ray spectrum of NGC 1365: scattered light, not black hole spin. (arXiv:1303.4309v1 [astro-ph.HE]):
Active Galactic Nuclei (AGN) show excess X-ray emission above 10 keV compared
with extrapolation of spectra from lower energies. Risaliti et al. have
recently attempted to model the hard X-ray excess in the type 1.8 AGN NGC 1365,
concluding that the hard excess most likely arises from Compton-scattered
reflection of X-rays from an inner accretion disk close to the black hole.
Their analysis disfavored a model in which the hard excess arises from a high
column density of circumnuclear gas partially covering a primary X-ray source,
despite such components being required in the NGC 1365 data below 10 keV. Using
a Monte Carlo radiative transfer approach, we demonstrate that this conclusion
is invalidated by (i) use of slab absorption models, which have unrealistic
transmission spectra for partial covering gas, (ii) neglect of the effect of
Compton scattering on transmitted spectra and (iii) inadequate modeling of the
expected spectrum of scattered X-rays. The scattered spectrum is geometry
dependent and, for high global covering factors, may dominate above 10 keV. We
further show that, in models of circumnuclear gas, the suppression of the
observed hard X-ray flux by reprocessing may be no larger than required by the
`light bending' model invoked for inner disk reflection, and the expected
emission line strengths lie within the observed range. We conclude that the
time-invariant `red wing' in X-ray spectra is probably caused by continuum
transmitted through and scattered from circumnuclear gas, not by highly
redshifted line emission, and that measurement of black hole spin is not
possible.

A Monte Carlo Code for Relativistic Radiation Transport Around Kerr Black Holes. (arXiv:1302.3214v1 [astro-ph.HE])

A Monte Carlo Code for Relativistic Radiation Transport Around Kerr Black Holes. (arXiv:1302.3214v1 [astro-ph.HE]):
We present a new code for radiation transport around Kerr black holes,
including arbitrary emission and absorption terms, as well as electron
scattering and polarization. The code is particularly useful for analyzing
accretion flows made up of optically thick disks and optically thin coronae. We
give a detailed description of the methods employed in the code, and also
present results from a number of numerical tests to assess its accuracy and
convergence.

Discovery of Fe K{\alpha} X-ray reverberation around the black holes in MCG-5-23-16 and NGC 7314. (arXiv:1302.1761v1 [astro-ph.HE])

Discovery of Fe K{\alpha} X-ray reverberation around the black holes in MCG-5-23-16 and NGC 7314. (arXiv:1302.1761v1 [astro-ph.HE]):
Several X-ray observations have recently revealed the presence of
reverberation time delays between spectral components in AGN. Most of the
observed lags are between the power-law Comptonization component, seen
directly, and the soft excess produced by reflection in the vicinity of the
black hole. NGC 4151 was the first object to show these lags in the iron K
band. Here, we report the discovery of reverberation lags in the Fe K band in
two other sources: MCG-5-23-16 and NGC 7314. In both objects, the 6-7 keV band,
where the Fe K line peaks, lags the bands at lower and higher energies with a
time delay of ~ 1 kilo-seconds. These lags are unlikely to be due to the narrow
Fe K line. They are fully consistent with reverberation of the
relativistically-broadened iron K line. The measured lags, their time-scale and
spectral modeling, indicate that most of the radiation is emitted at ~ 5 and 24
gravitational radii for MCG-5-23-16 and NGC 7314 respectively.

Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc])

Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc]):
If astrophysical black hole candidates are the Kerr black holes predicted by
General Relativity, the value of their spin parameter must be subject to the
{\it theoretical bound} $|a_*| \le 1$. In this work, we consider the
possibility that these objects are either non-Kerr black holes in an
alternative theory of gravity or exotic compact objects in General Relativity.
Such a possibility is not in contradiction with current data and it can be
tested with future observational facilities. We study the accretion process
when their accretion disk is geometrically thick with a simple version of the
Polish doughnut model. The picture of the accretion process may be
qualitatively different from the one around a Kerr black hole. The inner edge
of the accretion disk may not have the typical cusp on the equatorial plane any
more, but there may be two cusps, respectively above and below the equatorial
plane. We discuss the evolution of the spin parameter as a consequence of the
accretion process and we estimate the maximum value of the spin parameter of
these objects as a function of their deformation. Lastly, we compare our
results with the current estimates of the mean radiative efficiency of AGNs. We
find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the
super-massive black hole candidates at the centers of galaxies, which we argue
to be almost independent of the exact nature of these objects.

Fast variability as a probe of the smallest regions around accreting black holes. (arXiv:1301.0627v1 [astro-ph.HE])

Fast variability as a probe of the smallest regions around accreting black holes. (arXiv:1301.0627v1 [astro-ph.HE]):
We extract the spectra of the fastest variability (above 10 Hz) from the
black hole XTE J1550-564 during a transition from hard to soft state on the
rise to outburst. We confirm previous results that the rapid variability
contains no significant disc component despite this being strongly present in
the total spectrum of the softer observations. We model ionised reflection
significantly better than previous work, and show that this is also suppressed
in the rapid variability spectrum compared to the total emission. This is
consistent with the fast variability having its origin in a hot inner flow
close to the black hole rather than in the accretion disc or in a corona above
it. However, the rapid variability spectrum is not simply the same as the total
Comptonised emission. It is always significantly harder, by an amount which
increases as the spectrum softens during the outburst. This adds to evidence
from time lags that the Comptonisation region is inhomogeneous, with harder
spectra produced closest to the black hole, the same region which produces the
fastest variability.

Structure of neutron stars in R-squared gravity. (arXiv:1301.5189v1 [astro-ph.CO])

Structure of neutron stars in R-squared gravity. (arXiv:1301.5189v1 [astro-ph.CO]):
The effects implied for the structure of compact objects by the modification
of General Relativity produced by the generalization of the Lagrangian density
to the form f(R)=R+\alpha R^2, where R is the Ricci curvature scalar, have been
recently explored. It seems likely that this squared-gravity may allow heavier
Neutron Stars (NSs) than GR. In addition, these objects can be useful to
constrain free parameters of modified-gravity theories. The differences between
alternative gravity theories is enhanced in the strong gravitational regime. In
this regime, because of the complexity of the field equations, perturbative
methods become a good choice to treat the problem. Following previous works in
the field, we performed a numerical integration of the structure equations that
describe NSs in f(R)-gravity, recovering their mass-radius relations, but
focusing on particular features that arise from this approach in the profiles
of the NS interior.

We show that these profiles run in correlation with the second-order
derivative of the analytic approximation to the Equation of State (EoS), which
leads to regions where the enclosed mass decreases with the radius in a
counter-intuitive way. We reproduce all computations with a simple polytropic
EoS to separate zeroth-order modified gravity effects.

The Structure of the X-ray and Optical Emitting Regions of the Lensed Quasar Q 2237+0305. (arXiv:1301.5009v1 [astro-ph.CO])

The Structure of the X-ray and Optical Emitting Regions of the Lensed Quasar Q 2237+0305. (arXiv:1301.5009v1 [astro-ph.CO]):
We use gravitational microlensing to determine the size of the X-ray and
optical emission regions of the quadruple lens system Q 2237+0305. The optical
half-light radius, log(R_{1/2,V}/cm)=16.41\pm0.18 (at lambda_{rest}=2018 \AA),
is significantly larger than the observed soft,
log(R_{1/2,soft}/cm)=15.76^{+0.41}_{-0.34} (1.1-3.5 keV in the rest frame), and
hard, log(R_{1/2,hard}/cm)=15.46^{+0.34}_{-0.29} (3.5-21.5 keV in the rest
frame), band X-ray emission. There is a weak evidence that the hard component
is more compact than the soft, with log(R_{1/2,soft}/R_{1/2,hard}) \sim
0.30^{+0.53}_{-0.45}. This wavelength-dependent structure agrees with recent
results found in other lens systems using microlensing techniques, and favors
geometries in which the corona is concentrated near the inner edge of the
accretion disk. While the available measurements are limited, the size of the
X-ray emission region appears to be roughly proportional to the mass of the
central black hole.

Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags. (arXiv:1301.1924v1 [astro-ph.HE])

Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags. (arXiv:1301.1924v1 [astro-ph.HE]):
IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton
observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV)
and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies
(i.e. hard lag), while the opposite (i.e. soft lag) is observed at high
frequencies. In this paper, we study the lag during flaring and quiescent
periods. We find that the frequency and absolute amplitude of the soft lag is
different during high-flux and low-flux periods. During the low flux intervals,
the soft lag is detected at higher frequencies and with smaller amplitude.
Assuming that the soft lag is associated with the light travel time between
primary and reprocessed emission, this behaviour suggests that the X-ray source
is more compact during low-flux intervals, and irradiates smaller radii of the
accretion disc (likely because of light bending effects). We continue with an
investigation of the lag dependence on energy, and find that isolating the
low-flux periods reveals a strong lag signature at the Fe K line energy,
similar to results found using 1.3 Ms of data on another well known Narrow-Line
Seyfert I galaxy, 1H0707-495.

Testing the space-time geometry around black hole candidates with the available radio and X-ray data. (arXiv:1301.0361v1 [gr-qc])

Testing the space-time geometry around black hole candidates with the available radio and X-ray data. (arXiv:1301.0361v1 [gr-qc]):
Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but the actual nature of these objects has
still to be proven. The Kerr black hole hypothesis can be tested by observing
strong gravity features and check if they are in agreement with the predictions
of General Relativity. In particular, the study of the properties of the
electromagnetic radiation emitted by the gas of the accretion disk can provide
information on the geometry of the space-time around these objects and
constrain possible deviations from the Kerr background.

Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE])

Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarized Radiative Modeling. (arXiv:1212.4149v1 [astro-ph.HE]):
We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra
from the accretion flow and fitting the observed spectrum. The dynamical flow
model is based on three-dimensional general relativistic magneto hydrodynamic
simulations. Fully self-consistent radiative transfer of polarized
cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of
Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of
inclination angle theta=42-75deg, mass accretion rate
Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at
6M. We discuss multiple caveats in dynamical modeling, which must be resolved
to make further progress.

The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE])

The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE]):
The X-ray emission from active galactic nuclei (AGN) is highly variable.
Measurements of time lags (characterised by lag spectra) between variability in
the light curves in energy bands corresponding to directly observed continuum
emission from the corona around the black hole and to X-rays reflected from the
accretion disc adds a further dimension to studies of the structure and
energetics of these systems. We seek to understand these measurements in terms
of the physical parameters of the X-ray source (its location, extent, etc.)
through the calculation of theoretical lag spectra for a range of source
parameters in general relativistic ray tracing simulations, combined with
knowledge of the observed variability of the X-ray emission from AGN. Due to
the proximity of the emission to the central black hole, Shapiro delays are
important and the effects of general relativity should be considered when
interpreting the lags as the light travel time between the source and
reflector. We show that it is important to consider dilution of the lag by the
contribution of both the primary and reflected spectral components to the
observed energy bands. We find that the observed lag spectrum of the narrow
line Seyfert 1 galaxy 1H 0707-495 implies an X-ray source extending radially
outwards to around 35rg and at a height of around 2rg above the plane of the
accretion disc, consistent with the constraints obtained independently by
considering the emissivity profile of the accretion disc. By investigating the
influence of the propagation of X-ray luminosity fluctuations through the
source region we find it is possible to reproduce the shape of the low
frequency part of the lag spectrum (where the hard 'primary' band lags behind
the soft 'reflected' band) as the effect of luminosity fluctuations originating
in the centre of the X-ray source, close to the black hole, and propagating
outwards.

Accretion disks around black holes in modified strong gravity. (arXiv:1212.2640v1 [astro-ph.CO])

Accretion disks around black holes in modified strong gravity. (arXiv:1212.2640v1 [astro-ph.CO]):
Stellar-mass black holes offer what is perhaps the best scenario to test
theories of gravity in the strong-field regime. In particular, f(R) theories,
which have been widely discuss in a cosmological context, can be constrained
through realistic astrophysical models of phenomena around black holes. We aim
at building radiative models of thin accretion disks for both Schwarzschild and
Kerr black holes in f(R) gravity. We study particle motion in
f(R)-Schwarzschild and Kerr space-times. We present the spectral energy
distribution of the accretion disk around constant Ricci scalar f(R) black
holes, and constrain specific f(R) prescriptions using features of these
systems. A precise determination of both the spin and accretion rate onto black
holes along with X-ray observations of their thermal spectrum might allow to
identify deviations of gravity from General Relativity. We use recent data on
the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters
of a class of f(R) models.

The imminent detection of gravitational waves from massive black-hole binaries with pulsar timing arrays. (arXiv:1211.4590v1 [astro-ph.CO])

The imminent detection of gravitational waves from massive black-hole binaries with pulsar timing arrays. (arXiv:1211.4590v1 [astro-ph.CO]):
Recent observations of massive galaxies indicate that they double in mass and
quintuple in size between redshift z = 1 and the present, despite undergoing
very little star formation, suggesting that galaxy mergers drive the evolution.
Since these galaxies will contain supermassive black holes, this suggests a
larger black hole merger rate, and therefore a larger gravitational-wave
signal, than previously expected. We calculate the merger-driven evolution of
the mass function, and find that merger rates are 10 to 30 times higher and
gravitational waves are 3 to 5 times stronger than previously estimated, so
that the gravitational-wave signal may already be detectable with existing data
from pulsar timing arrays. We also provide an explanation for the disagreement
with past estimates that were based on dark matter halo simulations.

Effects of Kerr Strong Gravity on Quasar X-ray Microlensing. (arXiv:1211.6487v1 [astro-ph.HE])

Effects of Kerr Strong Gravity on Quasar X-ray Microlensing. (arXiv:1211.6487v1 [astro-ph.HE]):
Recent quasar microlensing observations have constrained the sizes of X-ray
emission regions to be within about 10 gravitational radii of the central
supermassive black hole. Therefore, the X-ray emission from lensed quasars is
first strongly lensed by the black hole before it is lensed by the foreground
galaxy and star fields. We present a scheme that combines the initial strong
lensing of a Kerr black hole with standard linearized microlensing by
intervening stars. We find that X-ray microlensed light curves incorporating
Kerr strong gravity can differ significantly from standard curves. The
amplitude of the fluctuations in the light curves can increase or decrease by
~0.65-0.75 mag by including Kerr strong gravity. Larger inclination angles give
larger amplitude fluctuations in the microlensing light curves. Consequently,
current X-ray microlensing observations might have under or overestimated the
sizes of the X-ray emission regions. We estimate this bias using a simple
metric based on the amplitude of magnitude fluctuations. The half light radius
of the X-ray emission region can be underestimated up to ~50% or overestimated
up to ~20%. Underestimates are found in most situations we have investigated.
The only exception is for a disk with large spin, radially flat emission
profile, and observed nearly face on, where an overestimate is found. Thus,
more accurate microlensing size constraints should be obtainable by including
Kerr lensing. The caustic crossing time can differ by months after including
Kerr strong gravity. A simultaneous monitoring of gravitational lensed quasars
in both X-ray and optical bands with densely sampled X-ray light curves might
reveal this feature. We conclude that it should be possible to constrain
important parameters such as inclination angles and black hole spins from
combined Kerr and microlensing effects.

Probing General Relativity with Accreting Black Holes. (arXiv:1211.2146v1 [astro-ph.HE])

Probing General Relativity with Accreting Black Holes. (arXiv:1211.2146v1 [astro-ph.HE]):
Most of the X-ray emission from luminous accreting black holes emerges from
within 20 gravitational radii. The effective emission radius is several times
smaller if the black hole is rapidly spinning. General Relativistic effects can
then be very important. Large spacetime curvature causes strong lightbending
and large gravitational redshifts. The hard X-ray, power-law-emitting corona
irradiates the accretion disc generating an X-ray reflection component. Atomic
features in the reflection spectrum allow gravitational redshifts to be
measured. Time delays between observed variations in the power-law and the
reflection spectrum (reverberation) enable the physical scale of the reflecting
region to be determined. The relative strength of the reflection and power-law
continuum depends on light bending. All of these observed effects enable the
immediate environment of the black hole where the effects of General Relativity
are on display to be probed and explored.

Testing the space-time geometry around black hole candidates with the analysis of the broad K$\alpha$ iron line. (arXiv:1211.2513v1 [gr-qc])

Testing the space-time geometry around black hole candidates with the analysis of the broad K$\alpha$ iron line. (arXiv:1211.2513v1 [gr-qc]):
Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but there is not yet a clear evidence that the
geometry of the space-time around these objects is really described by the Kerr
metric. In order to confirm the Kerr black hole hypothesis, we have to observe
strong gravity features and check they are in agreement with the ones predicted
by General Relativity. In this paper, I study what kind of information can be
extracted by analyzing the broad K$\alpha$ iron line, which is often seen in
the X-ray spectrum of both stellar-mass and super-massive black hole candidates
and whose shape is supposed to be strongly affected by the space-time geometry.
I extend previous studies in the literature. It turns out that there is a
strong degeneracy between the spin parameter and the deformation parameter;
that is, the line emitted around a Kerr black hole with a certain spin can be
very similar to the one coming from the space-time around a non-Kerr object
with a quite different spin. As in this paper I include the effect of the
disk's inclination angle, which is also a fit parameter, this degeneracy is
much stronger than the one found in previous studies. Despite that, the
analysis of the broad K$\alpha$ iron line is potentially more powerful than the
continuum-fitting method, as it can put a bound on possible deviations from the
Kerr geometry independently of the value of the spin parameter and without
additional measurements.

Inclination-Dependent AGN Flux Profiles From Strong Lensing of the Kerr Space-Time. (arXiv:1211.2510v1 [astro-ph.HE])

Inclination-Dependent AGN Flux Profiles From Strong Lensing of the Kerr Space-Time. (arXiv:1211.2510v1 [astro-ph.HE]):
Recent quasar microlensing observations have constrained the X-ray emission
sizes of quasars to be about 10 gravitational radii, one order of magnitude
smaller than the optical emission sizes. Using a new ray-tracing code for the
Kerr space-time, we find that the observed X-ray flux is strongly influenced by
the gravity field of the central black hole, even for observers at moderate
inclination angles. We calculate inclination-dependent flux profiles of active
galactic nuclei in the optical and X-ray bands by combining the Kerr lensing
and projection effects for future references. We further study the dependence
of the X-ray-to-optical flux ratio on the inclination angle caused by
differential lensing distortion of the X-ray and optical emission, assuming
several corona geometries. The strong lensing X-ray-to-optical magnification
ratio can change by a factor of ~10 for normal quasars in some cases, and
another factor of ~10 for broad absorption line quasars (BALs) and obscured
quasars. Comparing our results with the observed distributions in normal and
broad absorption line quasars, we find that the inclination angle dependence of
the magnification ratios can change the X-ray-to-optical flux ratio
distributions significantly. In particular, the mean value of the spectrum
slope parameter $\alpha_{ox},$ $0.3838\log F_{2 keV}/F_{2500 {\AA}}$, can
differ by ~0.1-0.2 between normal and broad absorption line quasars, depending
on corona geometries, suggesting larger intrinsic absorptions in BALs.

A code to compute the emission of thin accretion disks in non-Kerr space-times and test the nature of black hole candidates. (arXiv:1210.5679v1 [gr-qc])

A code to compute the emission of thin accretion disks in non-Kerr space-times and test the nature of black hole candidates. (arXiv:1210.5679v1 [gr-qc]):
Astrophysical black hole candidates are thought to be the Kerr black holes
predicted by General Relativity, but the actual nature of these objects has
still to be proven. The analysis of the electromagnetic radiation emitted by a
geometrically thin and optically thick accretion disk around a black hole
candidate can provide information about the geometry of the space-time around
the compact object and it can thus test the Kerr black hole hypothesis. In this
paper, I present a code based on a ray-tracing approach and capable of
computing some basic properties of thin accretion disks in space-times with
deviations from the Kerr background. The code can be used to fit current and
future X-ray data of stellar-mass black hole candidates and constrain possible
deviations from the Kerr geometry in the spin parameter-deformation parameter
plane.

A general relativistic model of accretion disks with coronae surrounding Kerr black holes. (arXiv:1210.2662v1 [astro-ph.HE])

A general relativistic model of accretion disks with coronae surrounding Kerr black holes. (arXiv:1210.2662v1 [astro-ph.HE]):
We calculate the structure of a standard accretion disk with corona
surrounding a massive Kerr black hole in general relativistic frame, in which
the corona is assumed to be heated by the reconnection of the strongly buoyant
magnetic fields generated in the cold accretion disk. The emergent spectra of
the accretion disk-corona systems are calculated by using the relativistic
ray-tracing method. We propose a new method to calculate the emergent
Comptonized spectra from the coronae. The spectra of the disk-corona systems
with a modified $\alpha$-magnetic stress show that both the hard X-ray spectral
index and the hard X-ray bolometric correction factor $L_{\rm bol}/L_{\rm
X,2-10keV}$ increase with the dimensionless mass accretion rate, which are
qualitatively consistent with the observations of active galactic nuclei
(AGNs). The fraction of the power dissipated in the corona decreases with
increasing black hole spin parameter $a$, which leads to lower electron
temperatures of the coronas for rapidly spinning black holes. The X-ray
emission from the coronas surrounding rapidly spinning black holes becomes weak
and soft. The ratio of the X-ray luminosity to the optical/UV luminosity
increases with the viewing angle, while the spectral shape in the X-ray band is
insensitive with the viewing angle. We find that the spectral index in the
infrared waveband depends on the mass accretion rate and the black hole spin
$a$, which deviates from $f_\nu\propto\nu^{1/3}$ expected by the standard thin
disk model.

Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE])

Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE]):
Strong evidence is emerging that winds can be driven from the central regions
of accretion disks in both active galactic nuclei (AGN) and Galactic black hole
binaries (GBHBs). Direct evidence for highly-ionized, Compton-thin inner-disk
winds comes from observations of blueshifted (v~0.05-0.1c) iron-K X-ray
absorption lines. However, it has been suggested that the inner regions of
black hole accretion disks can also drive Compton-thick winds --- such winds
would enshroud the inner disk, preventing us from seeing direct signatures of
the accretion disk (i.e. the photospheric thermal emission, or the
Doppler/gravitationally broadened iron K-alpha line). Here, we show that,
provided the source is sub-Eddington, the well-established wind driving
mechanisms fail to launch a Compton-thick wind from the inner disk. For the
accelerated region of the wind to be Compton-thick, the momentum carried in the
wind must exceed the available photon momentum by a factor of at least
2/lambda, where lambda is the Eddington ratio of the source, ruling out
radiative acceleration unless the source is very close to the Eddington limit.
Compton-thick winds also carry large mass-fluxes, and a consideration of the
connections between the wind and the disk show this to be incompatible with
magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out
on the basis of the large Compton-radii that typify black hole systems. In the
absence of some new acceleration mechanism, we conclude that the inner regions
of sub-Eddington accretion disks around black holes are indeed naked.