86 research outputs found
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
Optical Multicolor WBVR-Observations of the X-Ray Star V1341 Cyg = Cyg X-2 in 1986-1992
We present the results of observations of the low-mass X-ray binary
V1341 X--2. Our observations include a total of
2375 individual measurements in four bands on 478 nights in 1986-1992. We tied
the comparison and check stars used for the binary to the catalog using
their magnitudes. The uncertainty of this procedure was 3 in the
and bands and 8%-10% for the and bands. In quiescence, the
amplitude of the periodic component in the binary's brightness variations
is within ( in ); this is due
to the ellipsoidal shape of the optical component, which is distorted with
gravitational forces from the X-ray component. Some of the system's active
states (long flares) may be due to instabilities in the accretion disk, and
possibly to instabilities of gas flows and other accretion structures. The
binary possesses a low-luminosity accretion disk. The light curves reveal no
indications of an eclipse near the phases of the upper and lower conjunctions
in quiescence or in active states during the observed intervals. We conclude
that the optical star in the close binary V1341
X-2 is a red giant rather than a blue straggler. We studied the long-term
variability of the binary during the seven years covered by our observations.
The optical observations presented in this study are compared to X-ray data
from the Ginga observatory for the same time intervals.Comment: 35 pages, 8 figure
One Hundred Years of Observations of the Be Star HDE 245770 (the X-ray Binary A0535+26/V725 Tau): The End of an Active Phase
UBV observations of the X-ray binary system A0535+26/V725 Tau at the Crimean
Station of the Sternberg Astronomical Institute in 1980-1998 are presented.
Based on our and published data, we analyze the photometric history of the star
from 1898.Comment: Translated from Pis'ma Astronomicheskii Zhurnal, Vol. 26, No. 1,
2000, pp. 13-2
The Structure and Motions of the 3C~120 Radio Jet on Scales of 0.6 to 300 Parsecs
Results are presented from long term VLBI monitoring of the parsec-scale
radio jet in 3C120, primarily at 5 and 1.7 GHz. Numerous superluminal features
are followed, some over large fractional distances from the core. The speeds of
the features are not all the same, but vary by less than a factor of 2. The
superluminal features extend to core distances of over 0.2 arcseconds and the
region of slowing has not been found. The 1.7 GHz images show evidence for
stationary features in addition to the superluminal features. The structure of
the jet in the vicinity of the most likely stationary feature is suggestive of
a helical pattern seen in projection. The deprojected wavelength of the pattern
is very long relative to the jet radius, unlike the situation in sources such
as M87. If the 3C120 jet does contain a slowly-moving, helical structure, then
theory suggests that the jet resides in a relatively cool medium, not in a
relativistically hot cocoon or lobe.Comment: 35 pages with 8 postscript figures. Accepted for publication in the
Astrophysical Journa
Resolved Mid-IR Emission in the Narrow Line Region of NGC 4151
We present subarcsecond resolution mid infrared images of NGC 4151 at 10.8
micron and 18.2 micron. These images were taken with the University of Florida
mid-IR camera/spectrometer OSCIR at the Gemini North 8-m telescope. We resolve
emission at both 10.8 micron and 18.2 micron extending ~ 3.5" across at a P.A.
of ~ 60 degrees. This coincides with the the narrow line region of NGC 4151 as
observed in [OIII] by the Hubble Space Telescope. The most likely explanation
for this extended mid-IR emission is dust in the narrow line region heated by a
central engine. We find no extended emission associated with the proposed torus
and place an upper limit on its mid-IR size of less than or equal to ~ 35 pc.Comment: accepted for publication in the Astrophysical Journal, 19 pages
including 5 figure
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
WFPC2 Observations of Compact Star Cluster Nuclei in Low Luminosity Spiral Galaxies
We have used the Wide Field Planetary Camera 2 aboard the Hubble Space
Telescope to image the compact star cluster nuclei of the nearby, late-type,
low-luminosity spiral galaxies NGC 4395, NGC 4242, and ESO 359-029. We also
analyze archival WFPC2 observations of the compact star cluster nucleus of M33.
A comparative analysis of the structural and photometric properties of these
four nuclei is presented. All of the nuclei are very compact, with luminosity
densities increasing at small radii to the resolution limit of our data. NGC
4395 contains a Seyfert 1 nucleus with a distinct bipolar structure and bright
associated filaments which are likely due to [OIII] emission. The M33 nucleus
has a complex structure, with elongated isophotes and possible signatures of
weak activity, including a jet-like component. The other two nuclei are not
known to be active, but share similar physical size scales and luminosities to
the M33 and NGC 4395 nuclei. The circumnuclear environments of all four of our
program galaxies are extremely diffuse, have only low-to-moderate star
formation, and appear to be devoid of large quantities of dust. The central
gravitational potentials of the galaxies are also quite shallow, making the
origin of these types of `naked' nuclei problematic.Comment: to appear in the July 1999 Astronomical Journal; 38 pages (Latex), 5
tables (postscript), 21 figures (gif); postscript versions of the figures may
be obtained via anonymous ftp at
ftp://ftp.cv.nrao.edu/NRAO-staff/lmatthew/lanl-nucle
The photometric and spectral investigation of CI Camelopardalis, an X-ray transient and B[e] star
We combined the results of UBVR photometry of CI Cam taken at Sternberg
Astronomical Institute in 1998--2001, and moderate resolution spectroscopy
taken at Special Astrophysical Observatory during the same time period.
Photometry as well as fluxes of Balmer emissions and of some Fe II emission
lines of CI Cam in quiet state reveal a cyclic variation with the period of
. The variation like this may be due to an orbital motion in a
wide pair with a giant star companion that exhibits the reflection effect on
its side faced to a compact companion.
The V-band photometry also confirms the pre-outburst 11.7 day period found by
Miroshnichenko earlier, but with a lower amplitude of 3 per cent. The
possibility of identity of this photometric period with the period of jet's
rotation in the VLA radio map of the object CI Cam was investigated. The radio
map modelling reveals the inclination of the jet rotation axis to the line of
sight, , the angle between the rotation axis and the direction of
ejection of the jet, , and jet's spatial velocity of
0.23--0.26c.
Equivalent widths and fluxes of various spectral lines show different
amplitudes of changes during the outburst, and essentially distinct behaviour
in quiescence. Five types of such behaviour were revealed, that indicates the
strong stratification of a gas and dust envelope round the system . The time
lag of strengthening of 50--250 in the forbidden line of nitrogen [N II]
was found relatively to the X-ray outburst maximum.Comment: 15 pages, 8 figures, 3 tables, Astron. Zh., 2002, (in press), vol.79,
number
The Symbiotic Neutron Star Binary GX 1+4/V2116 Ophiuchi
We present multiwavelength observations of this S-type symbiotic LMXB which
consists of a 2-min X-ray pulsar accreting from an M6 III giant. This is the
only symbiotic system definitely known to contain a neutron star. The steady
interstellar extinction toward the binary (Av=5) contrasts the variable N_H
inferred from X-ray measurements, most likely evidence for a stellar wind. The
mass donor is probably near the tip of the first-ascent red giant branch, in
which case the system is 3-6 kpc distant and has an X-ray luminosity of 10^37
erg/s. It is also possible, though less likely, that the donor star is just
beginning its ascent of the asymptotic giant branch, in which case the system
is 12-15 kpc distant and has an X-ray luminosity of 10^38 erg/s. However, our
measured Av argues against such a large distance. We show that the dense (10^9
cm^-3) emission-line nebula enshrouding the binary is powered by UV radiation
from an accretion disk. The emission-line spectrum constrains the temperature
and inner radius of the disk (and thus the pulsar's magnetic field strength),
and we discuss this in the context of the accretion torque reversals observed
in the pulsar. We also show that the binary period must be >100 d and is most
likely >260 d, making GX 1+4 the only known LMXB with Porb>10 d. If the mass
donor fills its Roche lobe, the mass transfer must be highly super-Eddington,
requiring much mass loss from the binary. We discuss the alternative that the
disk forms from the slow, dense stellar wind expected from the red giant.Comment: 46 pages including 7 PS figures. Accepted for publication in Ap
Multiwavelength observations of short time-scale variability in NGC 4151. I. Ultraviolet observations
We present the results of an intensive ultraviolet monitoring campaign on the
Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale
variability over a broad range in wavelength. The nucleus of NGC 4151 was
observed continuously with the {\it International Ultraviolet Explorer} (IUE)
for 9.3 days, yielding a pair of LWP and SWP spectra every 70 minutes,
and during four-hour periods for 4 days prior to and 5 days after the
continuous monitoring period. The sampling frequency of the observations is an
order of magnitude higher than that of any previous UV monitoring campaign on a
Seyfert galaxy.
The continuum fluxes in bands from 1275 \AA\ to 2688 \AA\ went through four
significant and well-defined ``events'' of duration 2 -- 3 days during the
continuous monitoring period. We find that the amplitudes of the continuum
variations decrease with increasing wavelength, which extends a general trend
for this and other Seyfert galaxies to smaller time scales (i.e., a few days).
The continuum variations in all of the UV bands are {\it simultaneous} to
within an accuracy of about 0.15 days, providing a strict constraint on
continuum models. The emission-line light curves show only one major event
during the continuous monitoring (a slow rise followed by a shallow dip), and
do not correlate well with continuum light curves over the (short) duration of
the campaign, because the time scale for continuum variations is apparently
smaller than the response times of the emission lines.Comment: 39 pages, LaTeX, including 7 PostScript figures; To appear in the ApJ
(October 20, 1996) Vol. 47
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