247 research outputs found
ASCA observations of type-2 Seyfert Galaxies. III. Orientation and X-ray Absorption
We discuss the spectral properties of a sample of type-2 Seyfert galaxies
based upon the analysis of \asca data. In this paper we consider the sources
for which the X-ray spectra appear to be dominated by the nuclear continuum,
transmitted through a large column of absorbing material. We find that both
Seyfert-2 galaxies and NELGs show iron K line profiles indicative of
reprocessing of nuclear X-rays in a face-on accretion disk. Such line profiles
are also observed in Seyfert-1 galaxies. This result is contrary to unification
models, which would predict the inner regions of Seyfert-2 galaxies to be
observed edge-on. This raises some questions as to the orientation of the
circumnuclear absorber. If the observed differences between Seyfert type-1 and
type-2 galaxies, and NELGs are not due to differences in the orientation of the
absorbing material, then we suggest that differences in dust composition and
grain size, and in the density of the circumnuclear gas could be of primary
importance.Comment: 28 pages, 3 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres
We show that Compton scattering of soft photons with energies near 100 eV in
thermally stratified black-hole accretion plasmas with temperatures in the
range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV.
This could produce the hardening observed in the X-ray spectra of black holes,
which is generally attributed to reflection or partial covering of the incident
continuum source by cold optically thick matter. In addition, we show that the
presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting
high energy tails similar to those observed from Galactic black-hole
candidates.Comment: 11 pages, uuencoded gziped postscript, ApJ Letters in pres
The X-ray spectra and spectral variability of intermediate type Seyfert galaxies: ASCA observations of NGC 4388 and ESO 103-G35
The X-ray spectra of two intermediate type Seyfert galaxies are investigated
using ASCA observations separated by more than a year. Both NGC 4388 and ESO
103-G35 exhibit strong, narrow Fe K alpha line emission and absorption by cold
neutral gas with a column density ~ 10^23 cm^-2, characteristic of the X-ray
spectra of type 2 Seyfert galaxies. The power law continuum flux has changed by
a factor of 2 over a time-scale of ~ 2 years for both objects, declining in the
case of NGC 4388 and rising in ESO 103-G35. No variation was observed in the
equivalent width of the Fe K alpha line in the spectra of NGC 4388, implying
that the line flux declined with the continuum. We find that the strength of
the line cannot be accounted for by fluorescence in line-of-sight material with
the measured column density unless a `leaky-absorber' model of the type favored
for IRAS 04575-7537 is employed. The equivalent width of the Fe K alpha
emission line is seen to decrease between the observations of ESO 103-G35 while
the continuum flux increased. The 1996 observation of ESO 103-G35 can also be
fitted with an absorption edge at 7.4 0.2 keV due to partially ionized
iron, and when an ionized absorber model is fitted to the data it is found that
the equivalent column of neutral hydrogen rises to 3.5 x 10^23 cm^-2. The Fe K
alpha line flux can be accounted by fluorescence in this material alone and
this model is also a good representation of the 1988 and 1991 Ginga
observations. There is then no requirement for a reflection component in the
ASCA spectra of ESO 103-G35 or NGC 4388.Comment: 45 pages, 5 tables, 11 figures. Accepted for publication in the
Astrophysical Journa
Anti-cytokine therapy in fibrosing alveolitis: where are we now?
Idiopathic pulmonary fibrosis (IPF) is a condition that has a poor prognosis, with a median survival of 4-5 years irrespective of treatment. Ziesche et al (N Engl J Med 1999, 341: 1264-1269) describe an open randomised trial of 18 patients with IPF, unresponsive to corticosteroid treatment at high dose. Nine patients were treated with continued corticosteroid and nine with prednisolone plus interferon-γ 1b (IFN-γ). Significant benefits in physiological parameters are reported in the IFN-γ-treated group. An analysis of lung tissue by reverse-transcriptase-mediated polymerase chain reaction showed corresponding decreases in the transcription of transforming growth factor-β1 and connective tissue growth factor. This is the first report of treatment showing efficacy in this disease, albeit in a very preliminary study, but the data should be viewed with caution. This study is discussed in the context of other published studies of treatment for IPF and the scientific rationale on which it was based
XMM-Newton observation of the Lockman Hole; I. The X-ray Data
We report on the first deep X-ray survey with the XMM-Newton observatory
during the performance verification phase. The field of the Lockman Hole, one
of the best studied sky areas over a very wide range of wavelengths, has been
observed. A total of ~100 ksec good exposure time has been accumulated.
Combining the images of the European Photon Imaging Camera (EPIC) detectors we
reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1},
respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of
10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S)
relation in the three bands is compared with previous results. In particular in
the 5-10 keV band these observations present the deepest X-ray survey ever,
about a factor 20 more sensitive than the previous BeppoSAX observations. Using
X-ray spectral diagnostics and the set of previously known, spectroscopically
identified ROSAT sources in the field, the new sources can be classified.
XMM-Newton detects a significant number (~40%) of X-ray sources with hard,
probably intrinsically absorbed X-ray spectra, confirming a prediction of the
population synthesis models for the X-ray background.Comment: 6 pages, based on the new A&A style file (included), to be published
in A&A 365 (2001, special XMM issue), one page contains color image
NGC 7582: The Prototype Narrow-Line X-ray Galaxy
NGC 7582 is a candidate prototype of the Narrow Line X-ray Galaxies (NLXGs)
found in deep X-ray surveys. An ASCA observation shows the hard (> 3 keV) X-ray
continuum of NGC 7582 drops 40% in ~6 ks, implying an AGN, while the soft band
(< 3 keV) does not drop in concert with the hard continuum, requiring a
separate component. The X-ray spectrum of NGC 7582 also shows a clear 0.5-2 keV
soft (kT = 0.8 (+0.9,-0.3) keV or Gamma = 2.4 +/- 0.6; L(X) = 6 x 10**40 ergs
s**-1) low--energy component, in addition to a heavily absorbed [N(H) = (6 +/-
2)\times 10**22 cm**-2 ] and variable 2-10 keV power law [Gamma = 0.7
(+0.3,-0.4); L(X) = (1.7-2.3) x 10**42 ergs s**-1]. This is one of the flattest
2-10 keV slopes in any AGN observed with ASCA. (The ROSAT HRI image of NGC 7582
further suggests extent to the SE.)
These observations make it clear that the hard X-ray emission of NGC 7582,
the most "narrow-line" of the NLXGs, is associated with an AGN. The strong
suggestion is that all NLXGs are obscured AGNs, as hypothesized to explain the
X-ray background spectral paradox. The separate soft X-ray component makes NGC
7582 (and by extension other NLXGs) detectable as a ROSAT source.Comment: text: Latex2e 10 pages, including 1 table, and 2 postscript figures
via psfi
A ROSAT HRI survey of bright nearby galaxies
We use the extensive public archive of ROSAT High Resolution Imager (HRI)
observations to carry out a statistical investigation of the X-ray properties
of nearby galaxies. Specifically we focus on the sample of 486 bright (B_T <
12.5) northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the
context of their exploration of the optical spectroscopic properties of nearby
galactic nuclei. Over 20% of HFS galaxies are encompassed in ROSAT HRI fields
of reasonable (> 10ks) exposure. The X-ray sources detected within the optical
extent of each galaxy are categorised as either nuclear or non-nuclear
depending on whether the source is positioned within or outside of a 25
arcsecond radius circle centred on the optical nucleus. A nuclear X-ray source
is detected in over 70% of the galaxies harbouring either a Seyfert or LINER
nucleus compared to a detection rate of only ~40% in less active systems. The
correlation of the H alpha luminosity with nuclear X-ray luminosity previously
observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the
regime of ultra-low luminosity (L(x)~10^38 - 10^40 erg/s) active galactic
nuclei (AGN). The inferred accretion rates for this sample of low-luminosity
AGN are significantly sub-Eddington. In total 142 non-nuclear sources were
detected. In combination with published data for M31 this leads to a luminosity
distribution (normalised to an optical blue luminosity of L(B) = 10^10
L(solar)) for the discrete X-ray source population in spiral galaxies of the
form dN/dL38 = 1.0 +/- 0.2 L38^-1.8, where L38 is the X-ray luminosity in units
of 10^38 erg/s. The implied L(x)/L(B) ratio is ~1.1 x 10^39 erg/s/(10^10
L(solar)). The nature of the substantial number of ``super-luminous''
non-nuclear objects detected in the survey is discussed.Comment: 20 pages, 7 figures, accepted for publication in MNRAS. Also
available from http://www.star.le.ac.uk/~tro/papers/xhfs.p
Origin of Magnetic Fields in the Universe Due to Nonminimal Gravitational-Electromagnetic Coupling
Basically the only existing theories for the creation of a magnetic field B
in the Universe are the creation of a seed field of order 10^{-20} G in spiral
galaxy which is subsequently supposedly amplified up to the observed 10^{-6} -
10^{-5} G by a dynamo process or a seed intergalactic field of magnitude
10^{-12} - 10^{-10} G which is amplified by collapse and differential rotation.
No satisfactory dynamo theory, however, exists today. We show that a 10^{-6} -
10^{-5} G magnetic field in spiral galaxies is directly obtained from a
nonminimal gravitational-electromagnetic coupling, without the need of
significant dynamo amplification.Comment: 5 pages, Latex (Revtex), no figures, To appear in PR
X-ray Absorption and Reflection in Active Galactic Nuclei
X-ray spectroscopy offers an opportunity to study the complex mixture of
emitting and absorbing components in the circumnuclear regions of active
galactic nuclei, and to learn about the accretion process that fuels AGN and
the feedback of material to their host galaxies. We describe the spectral
signatures that may be studied and review the X-ray spectra and spectral
variability of active galaxies, concentrating on progress from recent Chandra,
XMM-Newton and Suzaku data for local type 1 AGN. We describe the evidence for
absorption covering a wide range of column densities, ionization and dynamics,
and discuss the growing evidence for partial-covering absorption from data at
energies > 10 keV. Such absorption can also explain the observed X-ray spectral
curvature and variability in AGN at lower energies and is likely an important
factor in shaping the observed properties of this class of source.
Consideration of self-consistent models for local AGN indicates that X-ray
spectra likely comprise a combination of absorption and reflection effects from
material originating within a few light days of the black hole as well as on
larger scales. It is likely that AGN X-ray spectra may be strongly affected by
the presence of disk-wind outflows that are expected in systems with high
accretion rates, and we describe models that attempt to predict the effects of
radiative transfer through such winds, and discuss the prospects for new data
to test and address these ideas.Comment: Accepted for publication in the Astronomy and Astrophysics Review. 58
pages, 9 figures. V2 has fixed an error in footnote
ASCA observations of Seyfert 1 galaxies: I. Data Analysis, Imaging and Timing
We present the first in a series of papers describing the X-ray properties of
a sample of 18 Seyfert 1 galaxies, using data obtained by \asca. The imaging
data reveal a number of serendipitous hard X-ray sources in some source fields,
but none contribute significantly to the hard X-ray flux of the AGN. All but
one of the Seyferts show evidence for variability on timescales of
minutes-hours, with the amplitude anti-correlated with the source luminosity,
confirming previous results. In at least 8 sources, there is evidence that the
variability amplitude below 2 keV is greater than that in the hard X-ray band,
perhaps indicating variable components other than the power-law in the soft
band. Ultra-rapid variability, implying significant power at frequencies ~Hz is detected in at least 5 sources, but is difficult to detect in
most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and
MCG-6-30-15 there is also an indication that the high--frequency power--spectra
are variable in shape and/or intensity. There is similar evidence in NGC 4151,
but on longer time scales.Comment: 30 pages, 5 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/~nandra/pubs/paper1/abstract.htm
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