269 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
Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution
Magnetic connections to the plunging region can exert stresses on the inner
edge of an accretion disk around a black hole. We recompute the relativistic
corrections to the thin-disk dynamics equations when these stresses take the
form of a time-steady torque on the inner edge of the disk. The additional
dissipation associated with these stresses is concentrated relatively close
outside the marginally stable orbit, scaling as r to the -7/2 at large radius.
As a result of these additional stresses: spin-up of the central black hole is
retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs
at a/M=0.94; the disk spectrum is extended toward higher frequencies; line
profiles (such as Fe K-alpha) are broadened if the line emissivity scales with
local flux; limb-brightening, especially at the higher frequencies, is
enhanced; and the returning radiation fraction is substantially increased, up
to 58%. This last effect creates possible explanations for both synchronized
continuum fluctuations in AGN, and polarization rises shortward of the Lyman
edge in quasars. We show that no matter what additional stresses occur, when
a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound
on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical
Journa
The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results
We present the final results of a ROSAT PSPC program to study the soft X-ray
emission properties of a complete sample of low quasars. The main results
are: 1. There is no evidence for significant soft excess emission or excess
foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2
keV continuum of quasars agrees remarkably well with an extrapolation of the
mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating
that there is no steep soft component below 0.2 keV. 3. The occurrence of warm
absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN.
4. The strongest correlation found is between the spectral slope, alpha_x, and
the Hb FWHM. This remarkably strong correlation may result from a dependence of
alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears
to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where
the direct X-ray source is obscured, and only scattered X-rays are observed. 6.
Thin accretion disk models cannot reproduce the observed optical to soft X-ray
spectral shape. An as yet unknown physical mechanism maintains a strong
correlation between the optical and soft X-ray emission. 7. The well known
difference in alpha_x between radio-loud and radio-quiet quasars may be due
only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray
columns implies that He in the diffuse H II component of the Galactic ISM is
ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol.
447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8
figures available at http://physics.technion.ac.il/~laor/rosat/paper.p
Co-evolution of Galaxies and Central Black Holes: Observational Evidence on the Trigger of AGN Feedback
A comprehensive analysis of extended emission line region (EELR) around
quasars is presented. New Subaru/Suprime-Cam observation is combined with
literature search, resulting in a compilation of 81 EELR measurements for
type-1 and type-2 quasars with associated active galactic nucleus (AGN) and
host galaxy properties. It is found that EELR phenomenon shows clear
correlation with Eddington ratio, which links EELR to the constituents of the
principal component 1 (PC 1), or eigenvector 1, of the AGN emission
correlations. We also find that EELR is preferentially associated with
gas-rich, massive blue galaxies. It supports the idea that the primary
determinant of EELR creation is the gas availability and that the gas may be
brought in by galaxy merger triggering the current star formation as well as
AGN activity, and also gives an explanation for the fact that most luminous
EELR is found around radio-loud sources with low Eddington ratio. By combining
all the observations, it is suggested that EELR quasars occupy the massive blue
corner of the green valley, the AGN realm, on the galaxy color - stellar mass
diagram. Once a galaxy is pushed to this corner, activated AGN would create
EELR by the energy injection into the interstellar gas and eventually blow it
away, leading to star-formation quenching. The results presented here provide a
piece of evidence for the presence of such AGN feedback process, which may be
playing a leading role in the co-evolution of galaxies and central
super-massive black holes.Comment: Accepted for publication in the Astrophysical Journa
The emission line spectrum of the UV deficient quasar Ton 34: evidence of shock excitation?
Emission lines in quasars are believed to originate from a photoionized
plasma. There are, however, some emission features which appear to be
collisionally excited, such as the FeII multiplet bands. Shortward of Ly_alpha,
there also are a few permitted lines of species from low to intermediate
ionization. Ton 34 (z=1.928) exhibits the steepest far-UV continuum decline
known (Fnu propto nu^{-5.3}) shortward of 1050A. This object also emits
unusually strong low to intermediate excitation permitted lines shortward of
the Lyman limit. Using archive spectra of Ton 34 from HST, IUE and Palomar, we
measure the fluxes of all the lines present in the spectra and compare their
relative intensities with those observed in composite quasar spectra. Our
analysis reveals unusual strengths with respect to Ly_alpha of the following
low to intermediate excitation permitted lines: OII+OIII (835A), NIII+OIII
(686-703A) and NIII+NIV (765A). We compare the observed line spectrum with both
photoionization and shock models. Photoionization cannot reproduce the
strengths of these far-UV lines. Shocks with Vs ~ 100 km/s turn out to be
extremely efficient emitters of these lines and are favored as excitation
mechanism.Comment: 5 figures, accepted for publication in A&
Masses, Accretion Rates and Inclinations of AGNs
We assume that the gravitational instability of standard thin accretion disks
leads to the Broad Line Regions (BLRs), the B band luminosity comes from
standard thin disk and the motion of BLRs is virial. The central black hole
masses, the accretion rates and the disk inclinations to the line of sight for
17 Seyfert 1 galaxies and 17 Palomar-Green (PG) quasars have been calculated.
Our results are sensitive to parameter of the standard disk.
With the same values of (), calculated central black hole
masses for 17 Seyfert 1 galaxies are consistent with that from Kaspi et al.
(2000) while that for 17 PG quasars are larger than that from Kaspi et al.
(2000) by almost 2 orders of magnitude. Inclinations of 17 Seyfert 1 galaxies
are about 6 times larger than that of 17 PG quasars. These inclinations, with a
mean value of for 17 Seyfert 1 galaxies that agrees well with the
result obtained by fitting the iron lines of Seyfert 1 galaxies
observed with ASCA (Nandra et al. 1997) and the result obtained by Wu & Han
(2001), provide further support for the orientation-dependent unification
scheme of active galactic nuclei. There is a relation between the FWHM of
H and the inclination, namely the inclination is smaller in AGNs with
smaller FWHM of H. The effect of inclinations in narrow line Seyfert 1
galaxies (NLS1s) should be considered when one studies the physics of NLS1s.
(abbreviated)Comment: 9 pages, 9 figures, 4 tables, accepted by A&
Is the black hole in GX 339-4 really spinning rapidly?
The wide-band Suzaku spectra of the black hole binary GX 339-4, acquired in
2007 February during the Very High state, were reanalyzed. Effects of event
pileup (significant within ~ 3' of the image center) and telemetry saturation
of the XIS data were carefully considered. The source was detected up to ~ 300$
keV, with an unabsorbed 0.5--200 keV luminosity of ~3.8 10^{38} erg/s at 8 kpc.
The spectrum can be approximated by a power-law of photon index 2.7, with a
mild soft excess and a hard X-ray hump. When using the XIS data outside 2' of
the image center, the Fe-K line appeared extremely broad, suggesting a high
black hole spin as already reported by Miller et al. (2008) based on the Suzaku
data and other CCD data. When the XIS data accumulation is further limited to
>3' to avoid event pileup, the Fe-K profile becomes narrower, and there appears
a marginally better solution that suggests the inner disk radius to be 5-14
times the gravitational radius (1-sigma), though a maximally spinning black
hole is still allowed by the data at the 90% confidence level. Consistently,
the optically-thick accretion disk is inferred to be truncated at a radius 5-32
times the gravitational radius. Thus, the Suzaku data allow an alternative
explanation without invoking a rapidly spinning black hole. This inference is
further supported by the disk radius measured previously in the High/Soft
state.Comment: 5 pages, figures, Suzaku results on GX 339-4, accepted to APJL. Nov.
11, 2009, accepted to ApJ
On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs
We investigate properties of iron fluorescent line arising as a result of
illumination of a black hole accretion disc by an X-ray source located above
the disc surface. We study in details the light-bending model of variability of
the line, extending previous work on the subject.
We indicate bending of photon trajectories to the equatorial plane, which is
a distinct property of the Kerr metric, as the most feasible effect underlying
reduced variability of the line observed in several objects. A model involving
an X-ray source with a varying radial distance, located within a few central
gravitational radii around a rapidly rotating black hole, close to the disc
surface, may explain both the elongated red wing of the line profile and the
complex variability pattern observed in MCG--6-30-15 by XMM-Newton.
We point out also that illumination by radiation which returns to the disc
(following the previous reflection) contributes significantly to formation of
the line profile in some cases. As a result of this effect, the line profile
always has a pronounced blue peak (which is not observed in the deep minimum
state in MCG--6-30-15), unless the reflecting material is absent within the
innermost 2--3 gravitational radii.Comment: 24 pages, 22 figures. Accepted for publication in MNRA
The Intrinsically X-ray Weak Quasar PHL 1811. I. X-ray Observations and Spectral Energy Distribution
This is the first of two papers reporting observations and analysis of the
unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line
quasar PHL 1811, focusing on the X-ray properties and the spectral energy
distribution. Two Chandra observations reveal a weak X-ray source with a steep
spectrum. Variability by a factor of 4 between the two observations separated
by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton
spectra are modelled in the 0.3--5 keV band by a steep power law with \Gamma =
2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20}
cm^{-2}. The spectral slopes are consistent with power law indices commonly
observed in NLS1s, and it appears that we observe the central engine X-rays
directly. Including two recent Swift ToO snapshots, a factor of ~5 variability
was observed among the five X-ray observations reported here. In contrast, the
UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST
spectrum reveal no significant UV variability. The \alpha_{ox} inferred from
the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly
steeper than observed from other quasars of the same optical luminosity. The
steep, canonical X-ray spectra, lack of absorption, and significant X-ray
variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We
also discuss an accretion disk model, and the host galaxy of PHL 1811.Comment: 45 pages, 9 figures, accepted for publication in Ap
Two Physically Distinct Populations of Low-Ionization Nuclear Emission-Line Regions
The nature of Low-ionization Nuclear Emission-line Regions (LINERs) has been
an open question for a long time. We study the properties of LINERs from
several different aspects. The LINERs are found to consist of two different
categories that can be clearly separated in the traditional BPT diagrams,
especially in the [OI]/Ha vs. [OIII]/Hb diagram. LINERs with high [O]/Ha ratios
(population I) differ from ones with low ratios (population II) in several
properties. Broad emission lines are only identified in the spectra of
population I LINERs. While only the population II LINERs show luminous infrared
emission and occurrence of core-collapse supernovae in the host. Combining
these results with the known distribution of stellar populations not only
suggests that the two populations have different line excitation mechanisms,
but also implies that they are at different evolutionary stages.Comment: 12 pages, 2 figures, accepted by ApJ Lette
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