57,324 research outputs found
X-ray reflection in accreting stellar-mass black hole systems
The X-ray spectra of accreting stellar-mass black hole systems exhibit
spectral features due to reflection, especially broad iron K alpha emission
lines. We investigate the reflection by the accretion disc that can be expected
in the high/soft state of such a system. First, we perform a self-consistent
calculation of the reflection that results from illumination of a hot, inner
portion of the disc with its atmosphere in hydrostatic equilibrium. Then we
present reflection spectra for a range of illumination strengths and disc
temperatures under the assumption of a constant-density atmosphere. Reflection
by a hot accretion disc differs in important ways from that of a much cooler
disc, such as that expected in an active galactic nucleus.Comment: 5 pages with 9 figures, accepted for publication in MNRA
On the interpretation of the multicolour disc model for black hole candidates
We present a critical analysis of the usual interpretation of the multicolour
disc model parameters for black hole candidates in terms of the inner radius
and temperature of the accretion disc. Using a self-consistent model for the
radiative transfer and the vertical temperature structure in a Shakura-Sunyaev
disc, we simulate the observed disc spectra, taking into account doppler
blurring and gravitational redshift, and fit them with multicolour models. We
show not only that such a model systematically underestimates the value of the
inner disc radius, but that when the accretion rate and/or the energy
dissipated in the corona are allowed to change the inner edge of the disc, as
inferred from the multicolour model, appears to move even when it is in fact
fixed at the innermost stable orbit.Comment: 4 pages including 2 figures, accepted for publication in MNRA
Iron line profiles including emission from within the innermost stable orbit of a black hole accretion disc
Reynolds & Begelman (1997) have recently proposed a model in which the broad
and extremely redshifted iron line seen during a deep minimum of the light
curve of the Seyfert 1 galaxy MCG-6-30-15 originates from matter spiralling
into a Schwarzschild black hole, contrary to previous claims that the black
hole may be spinning rapidly (Iwasawa et al 1996; Dabrowski et al 1997). Here
we calculate in detail the X-ray spectrum produced by their model using the
full reflected continuum emission, including absorption features. This
calculation takes into account the doppler and relativistic effects. For the
range of parameters we consider, we find that the spectrum should show a large
photoelectric absorption edge of iron, which is not seen in the data. The
absorption edge is a consequence of the line emitting matter within the
innermost stable orbit being highly ionized, and is largely independent of the
parameters chosen for their model. If we restrict our attention to the 3-10 keV
band we may effectively remove this absorption edge by fitting a steeper power
law, but this results in a significant underprediction of the 0.4-0.5 keV flux.
We conclude that the data on MCG-6-30-15 are more consistent with the Kerr than
the Schwarzschild model.Comment: 5 pages with 5 postscript figures. Accepted for publication in MNRA
Vacuum oscillations of quasi degenerate solar neutrinos
The atmospheric neutrino oscillations and the vacuum oscillation solution of
the solar neutrino problem can be consistently described by a doubly or triply
degenerate neutrino spectrum as long as the high level of degeneracy required
is not spoiled by radiative corrections. We show that this is the case for
neutrino mass matrices generated by symmetries. This imposes a strong
constraint on the mixing angles and requires the mixing should be close to
bi-maximal. We briefly discuss the relevance of our results for the
measurability of the neutrino spectrum.Comment: 6 pages. Final version, more clear presentatio
H{\alpha} Imaging of Nearby Seyfert Host Galaxies
We used narrowband interference filters with the CCD imaging camera on the
Nickel 1.0 meter telescope at Lick Observatory to observe 31 nearby (z < 0.03)
Seyfert galaxies in the 12 {\mu}m Active Galaxy Sample. We obtained pure
emission line images of each galaxy in order to separate H{\alpha} emission
from the nucleus from that of the host galaxy. The extended H{\alpha} emission
is expected to be powered by newly formed hot stars, and correlates well with
other indicators of current star formation in these galaxies: 7.7 {\mu}m PAH,
far-infrared, and radio luminosity. Relative to what would be expected from
recent star formation, there is a 0.8 dex excess of radio emission in our
Seyfert galaxies. The nuclear H{\alpha} luminosity is dominated by the AGN, and
is correlated with the hard X-ray luminosity. There is an upward offset of 1
dex in this correlation for the Seyfert 1s due to a strong contribution from
the Broad Line Region. We found a correlation between star formation rate and
AGN luminosity. In spite of selection effects, we concluded that the absence of
bright Seyfert nuclei in galaxies with low SFRs is real, albeit only weakly
significant. We used our measured spatial distributions of H{\alpha} emission
to determine what these Seyfert galaxies would look like when observed through
fixed apertures at high redshifts. Although all would be detectable emission
line galaxies at any redshift, most would appear dominated by HII region
emission. Only the most luminous AGN would still be identified at z~0.3.Comment: 13 pages, 11 figures, accepted for publication in Ap
X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation Mapping
One of the principal scientific objectives of the upcoming Constellation-X
mission is to attempt to map the inner regions of accretion disks around black
holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence
line. This area of the disk is likely radiation pressure dominated and subject
to various dynamical instabilities. Here, we show that density inhomogeneities
in the disk atmosphere resulting from the photon bubble instability (PBI) can
cause rapid changes in the X-ray reflection features, even when the
illuminating flux is constant. Using a simulation of the development of the
PBI, we find that, for the disk parameters chosen, the Fe K and O VIII Ly\alpha
lines vary on timescales as short as a few hundredths of an orbital time. In
response to the changes in accretion disk structure, the Fe K equivalent width
(EW) shows variations as large as ~100 eV. The magnitude and direction
(positive or negative) of the changes depends on the ionization state of the
atmosphere. The largest changes are found when the disk is moderately ionized.
The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid,
low-amplitude changes. This effect provides a natural explanation for some
observed instances of short timescale Fe K variability which was uncorrelated
with the continuum (e.g., Mrk 841). New predictions for Fe K reverberation
mapping should be made which include the effects of this accretion disk driven
line variability and a variable ionization state. Reflection spectra averaged
over the evolution of the instability are well fit by constant density models
in the 2-10 keV region.Comment: 20 pages, 3 figures. Accepted by Ap
X-ray reflection spectra from ionized slabs
X-ray reflection spectra are an important component in the X-ray spectra of
many active galactic nuclei and Galactic black hole candidates. It is likely
that reflection takes place from highly ionized surfaces of the accretion disc
in some cases. This can lead to strong Comptonization of the emergent iron, and
other, absorption and emission features. We present such reflection spectra
here, computed in a self-consistent manner with the method described by Ross
and Fabian. In particular we emphasise the range where the ionization parameter
(the flux to density ratio) \xi is around and above 10^4. Such spectra may be
relevant to the observed spectral features found in black hole candidates such
as Cygnus X-1 in the low/hard state.Comment: 7 pages with 5 postscript figures. Accepted for publication in MNRA
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