251 research outputs found
Polarization change due to fast winds from accretion disks
A fraction of the radiation produced by an accretion disk may be Thomson
scattered by a wind flowing away from the disk. Employing a simple
plane-parallel model of the wind, we calculate the polarization of the
scattered radiation and find that its sign depends on the wind velocity,
beta=v/c. In the case, 0.12 < beta < 0.78, the polarization is parallel to the
disk normal, i.e., it is orthogonal to the standard Chandrasekhar's
polarization expected from accretion disks. The velocity of an
electron-positron wind is likely to saturate near the equilibrium value beta_*
of order 0.5 for which the accelerating radiation pressure is balanced by the
Compton drag. Then the change of polarization by the wind is most pronounced.
This may help to reconcile the standard accretion disk model with the optical
polarimetric observations of non-blazar AGNs.Comment: accepted for publication in ApJ Letter
Is the Broad-Line Region Clumped or Smooth? Constraints from the H alpha Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy
The origin and configuration of the gas which emits broad lines in Type I
active galactic nuclei is not established yet. The lack of small-scale
structure in the broad emission-line profiles is consistent with a smooth gas
flow, or a clumped flow with many small clouds. An attractive possibility for
the origin of many small clouds is the atmospheres of bloated stars, an origin
which also provides a natural mechanism for the cloud confinement. Earlier
studies of the broad-line profiles have already put strong lower limits on the
minimum number of such stars, but these limits are sensitive to the assumed
width of the lines produced by each cloud. Here we revisit this problem using
high-resolution Keck spectra of the H alpha line in NGC 4395, which has the
smallest known broad-line region (~10^14 cm). Only a handful of the required
bloated stars (each having r~10^14 cm) could fit into the broad-line region of
NGC 4395, yet the observed smoothness of the H alpha line implies a lower limit
of ~10^4-10^5 on the number of discrete clouds. This rules out conclusively the
bloated-stars scenario, regardless of any plausible line-broadening mechanisms.
The upper limit on the size of the clouds is ~10^12 cm, which is comparable to
the size implied by photoionization models. This strongly suggests that gas in
the broad-line region is structured as a smooth rather than a clumped flow,
most likely in a rotationally dominated thick disk-like configuration. However,
it remains to be clarified why such a smooth, gravity-dominated flow generates
double-peaked emission lines only in a small fraction of active galactic
nuclei.Comment: 12 pages, including 3 figures, accepted for publication in The
Astrophysical Journa
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
Q2237+0305 in X-rays: spectra and variability with XMM-Newton
X-ray observations of gravitationally lensed quasars may allow us to probe
the inner structure of the central engine of a quasar. Observations of
Q2237+0305 (Einstein Cross) in X-rays may be used to constrain the inner
structure of the X-ray emitting source. Here we analyze the XMM-Newton
observation of the quasar in the gravitational lens system Q2237+0305 taken
during 2002. Combined spectra of the four images of the quasar in this system
were extracted and modelled with a power-law model. Statistical analysis was
used to test the variability of the total flux. The total X-ray flux from all
the images of this quadruple gravitational lens system is 6 x 10^{-13}
erg/cm2/s in the range 0.2-10 keV, showing no significant X-ray spectral
variability during almost 42 ks of the observation time. Fitting of the cleaned
source spectrum yields a photon power-law index of Gamma=1.82+0.07/-0.08. The
X-ray lightcurves obtained after background subtraction are compatible with the
hypothesis of a stationary flux from the source.Comment: 6 pages, 7 figures, 4 tables, accepted by A&
Disclosing the Radio Loudness Distribution Dichotomy in Quasars: An Unbiased Monte Carlo Approach Applied to the SDSS-FIRST Quasar Sample
We investigate the dichotomy in the radio loudness distribution of quasars by
modelling their radio emission and various selection effects using a Monte
Carlo approach. The existence of two physically distinct quasar populations,
the radio-loud and radio-quiet quasars, is controversial and over the last
decade a bimodal distribution of radio loudness of quasars has been both
affirmed and disputed. We model the quasar radio luminosity distribution with
simple unimodal and bimodal distribution functions. The resulting simulated
samples are compared to a fiducial sample of 8,300 quasars drawn from the SDSS
DR7 Quasar Catalog and combined with radio observations from the FIRST survey.
Our results indicate that the SDSS-FIRST sample is best described by a radio
loudness distribution which consists of two components, with 12+/-1 % of
sources in the radio-loud component. On the other hand, the evidence for a
local minimum in the loudness distribution (bimodality) is not strong and we
find that previous claims for its existence were probably affected by the
incompleteness of the FIRST survey close to its faint limit. We also
investigate the redshift and luminosity dependence of the radio loudness
distribution and find tentative evidence that at high redshift radio-loud
quasars were rarer, on average "louder", and exhibited a smaller range in radio
loudness. In agreement with other recent work, we conclude that the SDSS-FIRST
sample strongly suggests that the radio loudness distribution of quasars is not
a universal function, and that more complex models than presented here are
needed to fully explain available observations.Comment: Accepted for publication in ApJ; 13 pages, 10 figure
Thermal disc emission from a rotating black hole: X-ray polarization signatures
Thermal emission from the accretion disc around a black hole can be
polarized, due to Thomson scattering in a disc atmosphere. In Newtonian space,
the polarization angle must be either parallel or perpendicular to the
projection of the disc axis on the sky. As first pointed out by Stark and
Connors in 1977, General Relativity effects strongly modify the polarization
properties of the thermal radiation as observed at infinity. Among these
effects, the rotation of the polarization angle with energy is particularly
useful as a diagnostic tool.
In this paper, we extend the Stark and Connors calculations by including the
spectral hardening factor, several values of the optical depth of the
scattering atmosphere and rendering the results to the expected performances of
planned X-ray polarimeters. In particular, to assess the perspectives for the
next generation of X-ray polarimeters, we consider the expected sensitivity of
the detectors onboard the planned POLARIX and IXO missions. We assume the two
cases of a Schwarzschild and an extreme Kerr black hole with a standard thin
disc and a scattering atmosphere. We compute the expected polarization degree
and the angle as functions of the energy as they could be measured for
different inclinations of the observer, optical thickness of the atmosphere and
different values of the black hole spin. We assume the thermal emission
dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105
in the thermal state, we calculate the observed polarization.Comment: 8 pages, 7 figures, accepted by MNRA
Effects of Kerr Spacetime on Spectral Features from X-Ray Illuminated Accretion Discs
We performed detailed calculations of the relativistic effects acting on both
the reflection continuum and the iron line from accretion discs around rotating
black holes. Fully relativistic transfer of both illuminating and reprocessed
photons has been considered in Kerr spacetime. We calculated overall spectra,
line profiles and integral quantities, and present their dependences on the
black hole angular momentum.
We show that the observed EW of the lines is substantially enlarged when the
black hole rotates rapidly and/or the source of illumination is near above the
hole. Therefore, such calculations provide a way to distinguish among different
models of the central source.Comment: 9 pages, latex, 13 figures, 3 Tables; accepted for publication in
MNRA
The relativistic shift of narrow spectral features from black-hole accretion discs
Transient spectral features have been discovered in the X-ray spectra of
Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several
interpretations were proposed for the origin of these features. We examined a
model of Doppler boosted blue horns of the iron line originating from a spot in
a black hole accretion disc, taking into account different approximations of
general relativistic light rays and the resulting shift of energy of photons.
We provide a practical formula for the blue horn energy of an intrinsically
narrow line and assess its accuracy by comparing the approximation against an
exact value, predicted under the assumption of a planar accretion disc. The
most accurate approximation provides excellent agreement with the spot orbital
radius down to the marginally stable orbit of a non-rotating black hole.Comment: Accepted for publication in A&A; 8 pages, 5 figure
The estimation of black-hole masses in distant radio galaxies
We have estimated the masses of the central supermassive black holes of 2442
radio galaxies froma catalog compiled using data from the NED, SDSS, and CATS
databases. Mass estimates based on optical photometry and radio data are
compared. Relationships between the mass of the central black hole
and the redshift are constructed for both wavelength ranges. The
distribution of the galaxies in these diagrams and systematic effects
influencing estimation of the black-hole parameters are discussed.
Upperenvelope cubic regression fits are obtained using the maximum estimates of
the black-hole masses. The optical and radio upper envelopes show similar
behavior, and have very similar peaks in position, , and
amplitude, = 9.4. This is consistent with a model in which the
growth of the supermassive black holes is self-regulating, with this redshift
corresponding to the epoch when the accretion-flow phase begins to end and the
nuclear activity falls off.Comment: 8 pages, 6 figure
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
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