315 research outputs found
Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66
In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized
by high peak luminosities in the X-ray and optical bands. The optical outbursts
were qualitatively explained as X-ray reprocessing in a gas cloud surrounding
the binary system. Since then, further important information about A0538-66
have been obtained, and sophisticated photoionization codes have been developed
to calculate the radiation emerging from a gas nebula illuminated by a central
X-ray source. In the light of the new information and tools available, we
studied again the enhanced optical emission displayed by A0538-66 to understand
the mechanisms responsible for these unique events among the class of Be/XRBs.
We performed about 10^5 simulations of a gas envelope photoionized by an X-ray
source. We assumed for the shape of the gas cloud either a sphere or a
circumstellar disc observed edge-on. We studied the effects of varying the main
properties of the envelope and the influence of different input X-ray spectra
on the optical/UV emission emerging from the photoionized cloud. We compared
the computed spectra with the IUE spectrum and photometric UBV measurements
obtained during the outburst of 29 April 1981. We also explored the role played
by the X-ray heating of the surface of the donor star irradiated by the X-ray
emission of the neutron star (NS). We found that reprocessing in a spherical
cloud with a shallow radial density distribution can reproduce the optical/UV
emission. To our knowledge, this configuration has never been observed either
in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the
case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray
heating of the surface of the donor star irradiated by the NS is
non-negligible, due to the particular orbital parameters of this system that
bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract
abridged to meet arXiv requirement
Disk emission and atmospheric absorption lines in black hole candidate 4U 1630-472
We re-analyzed SUZAKU data of the black hole candidate 4U 1630-472 being in
the high/soft state. We show that the continuum X-ray spectrum of 4U 1630-472
with iron absorption lines can be satisfactorily modeled by the spectrum from
an accretion disk atmosphere. Absorption lines of highly ionized iron
originating in hot accretion disk atmosphere can be an alternative or
complementary explanation to the wind model usually favored for these type of
sources. We model continuum and line spectra using a single model. Absorption
lines of highly ionized iron can origin in upper parts of the disk atmosphere
which is intrinsically hot due to high disk temperature. Iron line profiles
computed with natural, thermal and pressure broadenings match very well
observations. We showed that the accretion disk atmosphere can effectively
produce iron absorption lines observed in 4U 1630-472 spectrum. Absorption line
arising in accretion disk atmosphere is the important part of the observed line
profile, even if there are also other mechanisms responsible for the absorption
features. Nevertheless, the wind theory can be an artifact of the fitting
procedure, when the continuum and lines are fitted as separate model
components.Comment: 10 pages, 4 figures, accepted by Astronomy and Astrophysic
The intermediate line region in active galactic nuclei
We show that the recently observed suppression of the gap between the broad
line region (BLR) and the narrow line region (NLR) in some AGN can be fully
explained by an increase of the gas density in the emitting region. Our model
predicts the formation of the intermediate line region (ILR) that is observed
in some Seyfert galaxies by the detection of emission lines with intermediate
velocity full width half maximum (FWHM) 700 - 1200 km s. These
lines are believed to be originating from an ILR located somewhere between the
BLR and NLR. As it was previously proved, the apparent gap is assumed to be
caused by the presence of dust beyond the sublimation radius. Our computations
with the use of {\sc cloudy} photoionization code, show that the differences in
the shape of spectral energy distribution (SED) from the central region of AGN,
do not diminish the apparent gap in the line emission in those objects. A
strong discontinuity in the line emission vs radius exists for all lines at the
dust sublimation radius. However, increasing the gas density to
10 cm at the sublimation radius provides the continuous line
emission vs radius and fully explains the recently observed lack of apparent
gap in some AGN. We show that such a high density is consistent with the
density of upper layers of an accretion disk atmosphere. Therefore, the upper
layers of the disk atmosphere can give rise to the formation of observed
emission line clouds.Comment: 9 pages, 6 figures, accepted for publication in Ap
Intermediate-line Emission in AGNs: The Effect of Prescription of the Gas Density
The requirement of intermediate line component in the recently observed
spectra of several AGNs points to possibility of the existence of a physically
separate region between broad line region (BLR) and narrow line region (NLR).
In this paper we explore the emission from intermediate line region (ILR) by
using the photoionization simulations of the gas clouds distributed radially
from the AGN center. The gas clouds span distances typical for BLR, ILR and
NLR, and the appearance of dust at the sublimation radius is fully taken into
account in our model. Single cloud structure is calculated under the assumption
of the constant pressure. We show that the slope of the power law cloud density
radial profile does not affect the existence of ILR in major types of AGN. We
found that the low ionization iron line, Fe~II, appears to be highly sensitive
for the presence of dust and therefore becomes potential tracer of dust content
in line emitting regions. We show that the use of disk-like cloud density
profile computed at the upper part of the accretion disc atmosphere reproduces
the observed properties of the line emissivities. In particular, the distance
of H line inferred from our model agrees with that obtained from the
reverberation mapping studies in Sy1 galaxy NGC 5548.Comment: 15 pages, 13 figure
SALT long-slit spectroscopy of LBQS 2113-4538: variability of the Mg II and Fe II component
The Mg II line is of extreme importance in intermediate redshift quasars
since it allows us to measure the black hole mass in these sources and to use
these sources as probes of the distribution of dark energy in the Universe, as
a complementary tool to SN Ia. Reliable use of Mg II requires a good
understanding of all the systematic effects involved in the measurement of the
line properties, including the contamination by Fe II UV emission. We performed
three spectroscopic observations of a quasar LBQS 2113-4538 (z = 0.956) with
the SALT telescope separated in time by several months and we analyze in detail
the mean spectrum and the variability in the spectral shape. We show that even
in our good-quality spectra the Mg II doublet is well fit by a single
Lorentzian shape. We tested several models of the Fe II pseudo-continuum and
showed that one of them well represents all the data. The amplitudes of both
components vary in time, but the shapes do not change significantly. The
measured line width of LBQS 2113-4538 identifies this object as a class A
quasar. The upper limit of for the contribution of the Narrow Line Region
(NLR) to Mg II may suggest that the separation of the Broad Line Region (BLR)
and NLR disappears in this class of objects.Comment: 10 pages, 8 figures, accepted to A&
The properties of active galaxies at the extreme of eigenvector 1
Eigenvector 1 (EV1) is the formal parameter which allows the introduction of
some order in the properties of the unobscured type 1 active galaxies. We aim
to understand the nature of this parameter by analyzing the most extreme
examples of quasars with the highest possible values of the corresponding
eigenvalues . We selected the appropriate sources from the Sloan
Digital Sky Survey (SDSS) and performed detailed modeling, including various
templates for the Fe II pseudo-continuum and the starlight contribution to the
spectrum. Out of 27 sources with larger than 1.3 and with the
measurement errors smaller than 20\% selected from the SDSS quasar catalog,
only six sources were confirmed to have a high value of , defined as
being above 1.3. All other sources have of approximately 1. Three
of the high objects have a very narrow H line, below 2100 km
s but three sources have broad lines, above 4500 km s, that do
not seem to form a uniform group, differing considerably in black hole mass and
Eddington ratio; they simply have a very similar EW([OIII]5007) line.
Therefore, the interpretation of the EV1 remains an open issue.Comment: Astronomy and Astrophysics (in press
Tracing dark energy with quasars
The nature of dark energy, driving the accelerated expansion of the Universe,
is one of the most important issues in modern astrophysics. In order to
understand this phenomenon, we need precise astrophysical probes of the
universal expansion spanning wide redshift ranges. Quasars have recently
emerged as such a probe, thanks to their high intrinsic luminosities and, most
importantly, our ability to measure their luminosity distances independently of
redshifts. Here we report our ongoing work on observational reverberation
mapping using the time delay of the Mg II line, performed with the South
African Large Telescope (SALT).Comment: 3 pages, 2 figures, submitted as PTA proceeding
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