1,706 research outputs found
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
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
Evaluating forecast uncertainty due to errors in estimated coefficients: empirical comparison of alternative methods
This paper is concerned with the contribution to forecast errors of errors in the estimated structural coefficients of a macro-econometric model (simultaneous equations). Its main purpose is to perform, on several "real-world" models, an empirical comparison of alternative techniques available in the literature for this purpose.Forecast errors; coefficient estimation errors; Monte Carlo; simultaneous equation models
The variance of forecast errors in econometric models: application to a nonlinear model of the Italian economy
When econometric models are used as forecasting tools, forecast errors can be decomposed into several components, one of which is due to estimation errors, while another one is due to the stochastic nature of the variables to be predicted. Conditional on model's specification and on the predetermined variables, it is possible to compute a standard error of forecasts one-step-ahead.Forecast errors; standard errors; nonlinear econometric model; Italian economy
A simulation approach to some dynamic properties of econometric models
Numerical simulation methods can overcome the difficulties and limitations of analytical methods, when analyzing dynamic properties of econometric models.Multipliers; econometric models; asymptotic standard errors; simulation
The iron Kalpha Compton Shoulder in transmitted and reflected spectra
We calculate the Equivalent Widht of the Core and the centroid energy and
relative flux of the 1st order Compton Shoulder of the iron Kalpha emission
line from neutral matter. The calculations are performed with Monte Carlo
simulations. We explore a large range of column densities for both transmitted
and reflected spectra, and study the dependence on the iron abundance. The
Compton Shoulder is now becoming observable in many objects thanks to the
improved sensitivity and/or energy resolution of XMM-Newton and Chandra
satellites, and the present work aims to provide a tool to derive informations
on the geometry and element abundances of the line emitting matter from Compton
Shoulder measurements.Comment: 4 pages; accepted for publication in MNRA
The circumnuclear X-ray reflectors in NGC 1068 and the Circinus Galaxy
The ASCA and BeppoSAX spectra of the Circinus Galaxy and NGC 1068 are
analysed and compared with photoionization models based on cloudy. In the case
of Circinus, a single, mildly ionized reflector can account for the line
spectrum, while in NGC 1068 at least three different reflectors (with different
ionization states) are needed. We suggest that the reflector in Circinus, and
the low ionized one in NGC1068 are the inner and visible part of the material
responsible for the X-ray absorption. With this assumption, we estimate for the
inner radius of the absorber a value of 0.2 pc for Circinus and of a few
parsecs for NGC 1068.Comment: accepted for publication in MNRA
Active galaxy 4U 1344-60: did the relativistic line disappear?
X-ray bright active galactic nuclei represent a unique astrophysical
laboratory for studying accretion physics around super-massive black holes. 4U
1344-60 is a bright Seyfert galaxy which revealed relativistic reflection
features in the archival XMM-Newton observation. We present the spectroscopic
results of new data obtained with the Suzaku satellite and compare them with
the previous XMM-Newton observation. The X-ray continuum of 4U 1344-60 can be
well described by a power-law component with the photon index ~ 1.7 modified by
a fully and a partially covering local absorbers. We measured a substantial
decrease of the fraction of the partially absorbed radiation from around 45% in
the XMM-Newton observation to less than 10% in the Suzaku observation while the
power-law slope remains constant within uncertainties. The iron line in the
Suzaku spectrum is relatively narrow, keV, without any
suggestion for relativistic broadening. Regarding this, we interpret the iron
line in the archival XMM-Newton spectrum as a narrow line of the same width
plus an additional red-shifted emission around 6.1 keV. No evidence of the
relativistic reflection is present in the Suzaku spectra. The detected
red-shifted iron line during the XMM-Newton observation could be a temporary
feature either due to locally enhanced emission or decreased ionisation in the
innermost accretion flow.Comment: 10 pages, 11 figures, accepted to A&
Spatially resolved Fe K spectroscopy of NGC 4945
We present the imaging and spectroscopic analysis of the combined Chandra
ACIS-S observations of the Compton-thick Seyfert 2 galaxy NGC 4945. We
performed a spatially-resolved spectroscopy of the circumnuclear environment of
the source, picturing the innermost 200 parsecs around the highly absorbed
nucleus. The additional 200 ks ACIS-S data with respect to the previous
campaign allowed us to map with even greater detail the central structure of
this source and to discover an enhanced iron emission in the innermost nuclear
region, with respect to the associated Compton reflection continuum. We
revealed that the Equivalent Width of the iron K line is spatially
variable (ranging from 0.5 to 3 keV), on scales of tens of parsecs, likely due
to the ionization state and orientation effects of the reprocessing material,
with respect to the central X-ray illuminating source. A clump of highly
ionized Fe XXV He- is also detected, 40 parsecs east to the nucleus.
When observations taken years apart are considered, the central unresolved
reflected emission is found to remain constant.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
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