651 research outputs found
A self-consistent test of Comptonization models using a long BeppoSAX observation of NGC 5548
We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548. The data are well represented by a plane
parallel corona with an inclination angle of 30, a soft photon
temperature of 5 eV and a hot plasma temperature and optical depth of 360 keV and 0.1, respectively. If energy balance
applies, such values suggest that a more ``photon-starved'' geometry (e.g. a
hemispheric region) is necessary. The spectral softening detected during a
flare, appears to be associated to a decrease of the heating-to-cooling ratio,
indicating a geometric and/or energetic modification of the disk plus corona
system. The hot plasma temperature derived with the models above is
significantly higher than that obtained fitting the same data with a power law
plus high energy cut off model for the continuum. This is due to the fact that
in anisotropic geometries Comptonization spectra show "intrinsic" curvature
which moves the fitted high energy cut-off to higher energies.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"X-ray Astronomy '99", Bologna, Italy, September 199
HEXIT-SAT: a mission concept for X-ray grazing incidence telescopes from 0.5 to 70 keV
While the energy density of the Cosmic X-ray Background (CXB) provides a
statistical estimate of the super massive black hole (SMBH) growth and mass
density in the Universe, the lack, so far, of focusing instrument in the 20-60
keV (where the CXB energy density peaks), frustrates our effort to obtain a
comprehensive picture of the SMBH evolutionary properties. HEXIT-SAT (High
Energy X-ray Imaging Telescope SATellite) is a mission concept capable of
exploring the hard X-ray sky with focusing/imaging instrumentation, to obtain
an unbiased census of accreting SMBH up to the redshifts where galaxy formation
peaks, and on extremely wide luminosity ranges. This will represent a leap
forward comparable to that achieved in the soft X-rays by the Einstein
Observatory in the late 70'. In addition to accreting SMBH, and very much like
the Einstein Observatory, this mission would also have the capabilities of
investigating almost any type of the celestial X-ray sources. HEXIT-SAT is
based on high throughput (>400 cm2 @ 30 keV; >1200 cm2 @ 1 keV), high quality
(15 arcsec Half Power Diameter) multi-layer optics, coupled with focal plane
detectors with high efficiency in the full 0.5-70keV range. Building on the
BeppoSAX experience, a low-Earth, equatorial orbit, will assure a low and
stable particle background, and thus an extremely good sensitivity for faint
hard X-ray sources. At the flux limits of 1/10 microCrab (10-30 keV) and 1/3
microCrab (20-40 keV) (reachable in one Msec observation) we should detect ~100
and ~40 sources in the 15 arcmin FWHM Field of View respectively, thus
resolving >80% and ~65% of the CXB where its energy density peaks.Comment: to appear in Proceeedings of SPIE Vol. 5488, UV to Gamma Ray Space
Telescope System
A Long Observation of NGC 5548 by BeppoSAX: the High Energy Cut-off, Intrinsic Spectral Variability and a Truly Warm Absorber
NGC 5548 was observed by BeppoSAX in a single long (8 day) observation from
0.2 to 200 keV. We find (1) the spectral variation of the source is produced by
a change of the intrinsic power law slope; (2) a high energy cut-off at keV with a hint of change of with flux; (3) OVII and
OVIII absorption K edges, and a possible blended OVII-OVIII K
emission feature at keV, inconsistent with a purely
photoionized gas in equilibrium. We propose that the temperature of the
absorbing and emitting gas is K so that both collisional ionization
and photoionization contribute.Comment: 23 pages, 8 figures. Accepted for publication in The Astrophysical
Journa
Evolution of the X-ray spectrum in the flare model of Active Galactic Nuclei
Nayakshin & Kazanas (2002) have considered the time-dependent illumination of
an accretion disc in Active Galactic Nuclei, in the lamppost model. We extend
their study to the flare model, which postulates the release of a large X-ray
flux above a small region of the accretion disc. A fundamental difference with
the lamppost model is that the region of the disc below the flare is not
illuminated before the onset of the flare.
A few test models show that the spectrum which follows immediately the
increase in continuum flux should display the characteristics of a highly
illuminated but dense gas, i.e. very intense X-ray emission lines and
ionization edges in the soft X-ray range. The behaviour of the iron line is
different in the case of a "moderate" and a ``strong'' flare: for a moderate
flare, the spectrum displays a neutral component of the Fe K line at
6.4 keV, gradually leading to more highly ionized lines. For a strong flare,
the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and
have an equivalent width of several hundreds of eV. We find that the observed
correlations between , , and the X-ray flux, are well accounted by a
combination of flares having not achieved pressure equilibrium, strongly
suggesting that the observed spectrum is dominated by regions in non-pressure
equilibrium, typical of the onset of the flares. Finally a flare being confined
to a small region of the disc, the spectral lines should be narrow (except for
a weak Compton broadening), Doppler shifted, and moving.Comment: 14 pages, 13 figures, accepted in A & A, english corrected versio
Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies
A sample of nine bright Seyfert 1 and NELG type galaxies, observed with
BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the
issue of the spectral contributions of Compton reflection and iron line
fluorescence from circumnuclear gas. The empirical description adopted for the
direct continuum is the commonly used power law with an exponential cut-off.
The most direct test of the theoretical predictions, namely that the equivalent
width of the line, Walpha, and the strength R of the reflection relative to the
direct continuum are closely related to each other, gives a substantially
positive result, that is their mean ratio is very close to expectation, and
only a modest spread in the iron abundance seems implied. The existence of a
steep correlation between R and the slope Gamma of the power law is not
confirmed. A weak evidence is found that the existence of a very shallow trend
to increase on average with Gamma cannot be altogether excluded in both R and
Walpha, but needs to be tested with a larger sample. The energy Ef in the
exponential cut-off spans a range from about 80 to more than 300 keV. A
possible correlation is found, with Ef increasing on average with Gamma: if
ignored, for instance by keeping Ef at a fixed value in a sample study, it
could be cause of artificial steepening in a correlation between R and Gamma.Comment: accepted for publication in A&
The broad band spectrum and variability of NGC 4151 observed by BeppoSAX
We present an analysis of all SAX observations of NGC 4151. This source was observed 5 times from 1996 to 2001 with durations ranging from a day to four days. The intrinsic continuum, is absorbed at low energies by a complex system: a cold patchy absorber plus a warm uniform screen photoionized by the central continuum. We find that this dual absorber is the main driver of the observed variability, up to a factor of eight, at 3 keV. In particular the covering fraction of the cold absorber changes on time scales of the order of a day, supporting its association with the Broad Line Region. The column density of the warm gas varies on a longer time scale (months to year). Some of the small amplitude spectral variability above 10 keV can be explained with an intrinsic variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming an extended origin. Its spectrum is reproduced by a combination of a thermal component and a power law with the same slope as the intrinsic continuum but with an intensity a few per cent. A Compton reflection component is significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with intensity decreasing on time scale of year, and it desappears in 2000 and 2001. The long time scale of variations argues for an association with an optically thick torus at a distance of few light years. An iron line was detected in all spectra. Its energy is consistent with fluorescence by cold iron. We find that the line is variable. Its behaviour is reproduced by a variable component proportional to the level of the reflection flux plus a constant component. The flux of the latter is consistent with the extended line emission observed by Chandra. We conclude that the first component is likely arising from the torus and the second is produced in the extended Narrow Line Region
Testing Comptonizing coronae on a long BeppoSAX observation of the Seyfert 1 galaxy NGC 5548
We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548, allowing for different geometries of the
scattering region, different temperatures of the input soft photon field and
different viewing angles. We find that the BeppoSAX data are well represented
by a plane parallel or hemispherical corona viewed at an inclination angle of
30. For both geometries the best fit temperature of the soft photons
is close to 15 eV. The corresponding best fit values of the hot
plasma temperature and optical depth are 250--260 keV and
0.16--0.37 for the slab and hemisphere respectively. These values
are substantially different from those derived fitting the data with a
power-law + cut off approximation to the Comptonization component (kT_{\rm
e}\lta 60 keV, 2.4). This is due to the fact that accurate
Comptonization spectra in anisotropic geometries show "intrinsic" curvature
which reduces the necessity of a high energy cut-off. The Comptonization
parameter derived for the slab model {is} larger than predicted for a two phase
plane parallel corona in energy balance, suggesting that a more
``photon-starved'' geometry is necessary. The spectral softening detected
during a flare which occurred in the central part of the observation
corresponds to a decrease of the Comptonization parameter, probably associated
with an increase of the soft photon luminosity, the {hard} photon luminosity
remaining constant.Comment: 36 pages, 9 figures, accepted by Ap
Investigation of the causal relationships between human IgG N-glycosylation and twelve common diseases associated with changes in the IgG N-glycome
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