2,839 research outputs found
AGN Obscuration and the Unified Model
Unification Models of Active Galactic Nuclei postulate that all the observed
differences between Type 1 and Type 2 objects are due to orientation effects
with respect to the line-of-sight to the observer. The key ingredient of these
models is the obscuring medium, historically envisaged as a toroidal structure
on a parsec scale. However, many results obtained in the last few years are
clearly showing the need for a more complex geometrical distribution of the
absorbing media. In this paper we review the various pieces of evidence for
obscuring media on different scales, from the vicinity of the black hole to the
host galaxy, in order to picture an updated unification scenario explaining the
complex observed phenomenology. We conclude by mentioning some of the open
issues.Comment: 14 pages, 8 figures, review article accepted for publication on the
special issue of Advances in Astronomy "Seeking for the Leading Actor on the
Cosmic Stage: Galaxies versus Supermassive Black Holes
Interplay between heartbeat oscillations and wind outflow in microquasar IGR J17091-3624
During the bright outburst in 2011, the black hole candidate IGR J17091-3624
exhibited strong quasi-periodic flare-like events (on timescales of tens of
seconds) in some characteristic states, the so-called heartbeat state. From the
theoretical point of view, these oscillations may be modeled by the process of
accretion disk instability, driven by the dominant radiation pressure and
enhanced heating of the plasma. Although the mean accretion rate in this source
is probably below the Eddington limit, the oscillations will still have large
amplitudes. As the observations show, the source can exhibit strong wind
outflow during the soft state. This wind may help to partially or even
completely stabilize the heartbeat. Using our hydrodynamical code GLADIS, we
modeled the evolution of an accretion disk responsible for X-ray emission of
the source. We accounted for a variable wind outflow from the disk surface. We
examined the data archive from the Chandra and XMM-Newton satellites to find
the observed limitations on the wind physical properties, such as its velocity
and ionization state. We also investigated the long-term evolution of this
source, which lasted over about 600 days of observations, using the data
collected by the Swift and RXTE satellites. During this long period, the
oscillations pattern and the observable wind properties changed systematically.
We found that this source probably exhibits observable outbursts of appropriate
timescales and amplitudes as a result of the disk instability. Our model
requires a substantial wind component to explain the proper variability
pattern, and even complete suppression of flares in some states. The wind
mass-loss rate extracted from the data agrees quantitatively well with our
scenario.Comment: 12 pages, 8 figures. Published in Astronomy and Astrophysic
MoCA: A Monte Carlo code for Comptonisation in Astrophysics. I. Description of the code and first results
We present a new Monte Carlo code for Comptonisation in Astrophysics (MoCA).
To our knowledge MoCA is the first code that uses a single photon approach in a
full special relativity scenario, and including also Klein-Nishina effects as
well as polarisation. In this paper we describe in detail how the code works,
and show first results from the case of extended coronae in accreting sources
Comptonising the accretion disc thermal emission. We explored both a slab and a
spherical geometry, to make comparison with public analytical codes more easy.
Our spectra are in good agreement with those from analytical codes for
low/moderate optical depths, but differ significantly, as expected, for optical
depths larger than a few. Klein-Nishina effects become relevant above 100 keV
depending on the optical thickness and thermal energy of the corona. We also
calculated the polarisation properties for the two geometries, which show that
X-ray polarimetry is a very useful tool to discriminate between them.Comment: 16 pages, 20 figure
Properties of the Konishi multiplet in N=4 SYM theory
We study perturbative and non-perturbative properties of the Konishi
multiplet in N=4 SYM theory in D=4 dimensions. We compute two-, three- and
four-point Green functions with single and multiple insertions of the lowest
component of the multiplet, and of the lowest component of the supercurrent
multiplet. These computations require a proper definition of the renormalized
operator and lead to an independent derivation of its anomalous dimension. The
O(g^2) value found in this way is in agreement with previous results. We also
find that instanton contributions to the above correlators vanish. From our
results we are able to identify some of the lowest dimensional gauge-invariant
composite operators contributing to the OPE of the correlation functions we
have computed. We thus confirm the existence of an operator belonging to the
representation 20', which has vanishing anomalous dimension at order g^2 and
g^4 in perturbation theory as well as at the non-perturbative level, despite
the fact that it does not obey any of the known shortening conditions.Comment: 23 pages, latex, no figure
Instantons in supersymmetric Yang-Mills and D-instantons in IIB superstring theory
The one-instanton contributions to various correlation functions of
supercurrents in four-dimensional N=4 supersymmetric SU(2) Yang-Mills theory
are evaluated to the lowest order in perturbation theory.Expressions of the
same form are obtained from the leading effects of a single D-instanton
extracted from the IIB superstring effective action around the AdS5*S5
background. This is in line with the suggested AdS/Yang-Mills correspondence.
The relation between Yang--Mills instantons and D-instantons is further
confirmed by the explicit form of the classical D-instanton solution in the
AdS5*S5 background and its associated supermultiplet of zero modes.
Speculations are made concerning instanton effects in the large-N_c limit of
the SU(N_c) Yang-Mills theory.Comment: 41 pages, LaTeX. Typos corrected and minor clarifications adde
The nature of the soft X-ray emission in obscured AGN
The origin of the soft X-ray emission in obscured AGN is still largely
unknown. However, important progresses have been made thanks to the high energy
and spatial resolution of XMM-Newton and Chandra. We review here the latest
results on this issue, focusing on the physical properties of the material
responsible for the soft X-ray emission and its relation to the circumnuclear
environment, putting them in the general context of our understanding of the
AGN structure and its feedback to the host galaxy.Comment: 8 pages, 12 figures, invited review talk at the Workshop "The
multicoloured landscape of compact objects and their explosive origin",
Cefalu' (Sicily), 11-24 June 2006, to be published by AI
The soft X-ray polarization in obscured AGN
The soft X-ray emission in obscured active galactic nuclei (AGN) is dominated
by emission lines, produced in a gas photoionized by the nuclear continuum and
likely spatially coincident with the optical narrow line region (NLR). However,
a fraction of the observed soft X-ray flux appears like a featureless power law
continuum. If the continuum underlying the soft X-ray emission lines is due to
Thomson scattering of the nuclear radiation, it should be very highly
polarized. We calculated the expected amount of polarization assuming a simple
conical geometry for the NLR, combining these results with the observed
fraction of the reflected continuum in bright obscured AGN.Comment: 6 pages, 3 figures, to appear in 'X-ray Polarimetry: A New Window in
Astrophysics', edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferr
The XMM-Newton long look of NGC 1365: lack of a high/soft state in its ultraluminous X-ray sources
Based on our long (~ 300 ks) 2007 XMM-Newton observation of the Seyfert
galaxy NGC 1365, we report here on the spectral and timing behaviour of two
ultraluminous X-ray sources, which had previously reached isotropic X-ray
luminosities L_X ~ 4 x 10^{40} erg/s (0.3-10 keV band). In 2007, they were in a
lower state (L_X ~ 5 x 10^{39} erg/s, and L_X ~ 1.5 x 10^{39} erg/s for X1 and
X2, respectively). Their X-ray spectra were dominated by power-laws with photon
indices Gamma ~ 1.8 and Gamma ~ 1.2, respectively. Thus, their spectra were
similar to those at their outburst peaks. Both sources have been seen to vary
by a factor of 20 in luminosity over the years, but their spectra are always
dominated by a hard power-law; unlike most stellar-mass BHs, they have never
been found in a canonical high/soft state dominated by a standard disk. The
lack of a canonical high/soft state seems to be a common feature of ULXs. We
speculate that the different kind of donor star and/or a persistently
super-Eddington accretion rate during their outbursts may prevent accretion
flows in ULXs from settling into steady standard disks.Comment: 9 pages, accepted by Ap
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