21 research outputs found
Modeling the optical/UV polarization while flying around the tilted outflows of NGC 1068
Recent modeling of multi-waveband spectroscopic and maser observations
suggests that the ionized outflows in the nuclear region of the archetypal
Seyfert-2 galaxy NGC 1068 are inclined with respect to the vertical axis of the
obscuring torus. Based on this suggestion, we build a complex reprocessing
model of NGC 1068 for the optical/UV band. We apply the radiative transfer code
STOKES to compute polarization spectra and images. The effects of electron and
dust scattering and the radiative coupling occurring in the inner regions of
the multi-component object are taken into account and evaluated at different
polar and azimuthal viewing angles. The observed type-1/type-2 polarization
dichotomy of active galactic nuclei is reproduced. At the assumed observer's
inclination toward NGC 1068, the polarization is dominated by scattering in the
polar outflows and therefore it indicates their tilting angle with respect to
the torus axis. While a detailed analysis of our model results is still in
progress, we briefly discuss how they relate to existing polarization
observations of NGC 1068.Comment: To appear in the proceedings of the meeting "The Central Kiloparsec
in Galactic Nuclei" held in Bad Honnef (Germany) from August 29th to
September 2nd 201
Modeling the Polarization of Dusty Scattering Cones in Active Galactic Nuclei
We have used the STOKES radiative transfer code, to model polarization induced by dust scattering in the polar regions of Active Galactic Nuclei (AGN). We discuss the wavelength-dependence of the spectral intensity and polarization over the optical/UV range at different viewing angles for two different types of dust: a Galactic dust model, and a dust model inferred from extinction properties of AGN
DISCOVERY OF POLARIZATION REVERBERATION IN NGC 4151
Observations of the optical polarization of NGC 4151 in 1997–2003 show variations of an order of magnitude in the polarized flux while the polarization position angle remains constant. The amplitude of variability of the polarized flux is comparable to the amplitude of variability of the total U-band flux, except that the polarized flux follows the total flux with a lag of 8±3 days. The time lag and the constancy of the position angle strongly favor a scattering origin for the variable polarization rather than a non-thermal synchrotron origin. The orientation of the position angle of the polarized flux (parallel to the radio axis) and the size of the lag imply that the polarization arises from electron scattering in a flattened region within the low-ionization component of the broad-line-region. Polarization from dust scattering in the equatorial torus is ruled out as the source of the lag in polarized flux because it would produce a larger lag and polarization perpendicular to the radio axis. We note a long-term change in the percentage polarization at similar total flux levels and we attribute this to a change in the number of scatterers on a timescale of years
Discovery of Polarization Reverberation in NGC 4151
Observations of the optical polarization of NGC 4151 in 1997-2003 show
variations of an order of magnitude in the polarized flux while the
polarization position angle remains constant. The amplitude of variability of
the polarized flux is comparable to the amplitude of variability of the total
U-band flux, except that the polarized flux follows the total flux with a lag
of 8 +/- 3 days. The time lag and the constancy of the position angle strongly
favor a scattering origin for the variable polarization rather than a
non-thermal synchrotron origin. The orientation of the position angle of the
polarized flux (parallel to the radio axis) and the size of the lag imply that
the polarization arises from electron scattering in a flattened region within
the low-ionization component of the broad-line-region. Polarization from dust
scattering in the equatorial torus is ruled out as the source of the lag in
polarized flux because it would produce a larger lag and, unless the
half-opening angle of the torus is more than 53 degrees, the polarization would
be perpendicular to the radio axis. We note a long-term change in the
percentage of polarization at similar total flux levels and this could be due
either to changing non-axisymmetry in the optical continuum emission, or a
change in the number of scatterers on a timescale of years.Comment: 5 pages, 2 figures. Astrophysical Journal in press. Some
clarifications, additional discussion and references adde
Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNs
Velocity-resolved reverberation mapping suggests that the broad-line regions
(BLRs) of AGNs can have significant net inflow. We use the STOKES radiative
transfer code to show that electron and Rayleigh scattering off the BLR and
torus naturally explains the blueshifted profiles of high-ionization lines and
the ionization dependence of the blueshifts. This result is insensitive to the
geometry of the scattering region. If correct, this model resolves the
long-standing conflict between the absence of outflow implied by
velocity-resolved reverberation mapping and the need for outflow if the
blueshifting is the result of obscuration. The accretion rate implied by the
inflow is sufficient to power the AGN. We suggest that the BLR is part of the
outer accretion disk and that similar MHD processes are operating. In the
scattering model the blueshifting is proportional to the accretion rate so
high-accretion-rate AGNs will show greater high-ionization line blueshifts as
is observed. Scattering can lead to systematically too high black hole mass
estimates from the C IV line. We note many similarities between narrow-line
region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a
similar explanation. Our model explains the higher blueshifts of broad
absorption line QSOs if they are more highly inclined. Rayleigh scattering from
the BLR and torus could be more important in the UV than electron scattering
for predominantly neutral material around AGNs. The importance of Rayleigh
scattering versus electron scattering can be assessed by comparing line
profiles at different wavelengths arising from the same emission-line region.Comment: 10 pages, 7 figures, Astrophysical Journal in press. The only changes
from the previous version are to include some additional discussion of the
plausibility of supersonic inflow velocities (see section 5.2) and some
additional reference
Modeling the UV-X-spetrum
With the last generation of X-ray satellites, such as XMM-Newton and Chandra, our knowledge about the X-ray properties of Active Galactic Nuclei (AGN) has greatly improved. Detailed spectroscopy and long-term observations of hundreds of kilo-seconds have provided new insights into the mechanisms of X-ray production, its modification by intervening matter, and X-ray variability. In this thesis I present radiative transfer modeling of AGN in the Far-UV and X-ray range. The modeling considers several aspects of the observed X-ray properties: the Compton reflection/reprocessed component, X-ray variability and flares, and the effects of the warm absorber. For the X-ray reprocessing, I investigate in detail the magnetic flare model assuming solar-like magnetic reconnections above the accretion diskthesi
AGN polarization modeling with STOKES
We introduce a new, publicly available Monte Carlo radiative transfer code, STOKES, which has been developed to model polarization induced by scattering off free electrons and dust grains. It can be used in a wide range of astrophysical applications. Here, we apply it to model the polarization produced by the equatorial obscuring and scattering tori assumed to exist in active galactic nuclei (AGNs). We present optical/UV modeling of dusty tori with a curved inner shape and for two different dust types: one composition reproduces extinction properties of our Galaxy, and the other is derived from composite quasar spectra. The polarization spectra enable us to clearly distinguish between the two dust compositions
Modelling continuum optical and ultraviolet polarization of active galactic nuclei
We present results from a new Monte Carlo radiative transfer computer code, STOKES, developed to model polarization induced by scattering off free electrons and dust grains. STOKES is freely available on the web and can be used to model scattering in a wide variety of astrophysical situations. For edge-on (type-2) viewing positions, the polarization produced by a dusty torus alone is largely wavelength-independent. This is because the torus is optically thick and the change in albedo with wavelength is slight. Wavelength-independent polarization therefore does not necessarily imply electron scattering. We are able to fully explain wavelength independent type-2 polarization without the need to invoke electron scattering cones and consider the dusty torus to be main origin of the polarization in type-2 Seyferts. Polarization perpendicular to the axis of symmetry in type-2 AGNs requires torus half-opening angles of The polarization parallel to the axis of symmetry seen in many type-1 (face-on) AGNs cannot be explained by scattering from a torus alone, or by a torus plus scattering cones. For type-1 objects, the torus gives negligible polarization and electron cones give the wrong angle of polarization. The parallel polarization must arise from a flattened electron-scattering distribution inside the torus. We demonstrate how both the accretion disc itself, and a flattened nearby electron-scattering region can produce the required degree of polarization.We show how the expected polarization varies with the flattening of the regions and source coverage. The dominant parameter influencing the parallel polarization is the electron-scattering optical depth. For a highly ionized emitting and scattering accretion disk an optical depth of only a few tenths is sufficient to produce the observed polarization