103 research outputs found
A jet model for Galactic black-hole X-ray sources: Some constraining correlations
Some recent observational results impose significant constraints on all the
models that have been proposed to explain the Galactic black-hole X-ray sources
in the hard state. In particular, it has been found that during the hard state
of Cyg X-1 the power-law photon number spectral index is correlated with the
average time lag between hard and soft X-rays. Furthermore, the peak
frequencies of the four Lorentzians that fit the observed power spectra are
correlated with both the photon index and the time lag. We performed Monte
Carlo simulations of Compton upscattering of soft, accretion-disk photons in
the jet and computed the time lag between hard and soft photons and the
power-law index of the resulting photon number spectra. We demonstrate that our
jet model naturally explains the above correlations, with no additional
requirements and no additional parameters
X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes
We study Compton upscattering of low-frequency photons in a converging flow
of thermal plasma. The photons escape diffusively and electron scattering is
the dominant source of opacity. We solve numerically and approximately
analytically the equation of radiative transfer in the case of spherical,
steady state accretion into black holes. Unlike previous work on this subject,
we consider the inner boundary at a finite radius and this has a significant
effect on the emergent spectrum. It is shown that the bulk motion of the
converging flow is more efficient in upscattering photons than thermal
Comptonization, provided that the electron temperature in the flow is of order
a few keV or less. In this case, the spectrum observed at infinity consists of
a soft component coming from those input photons which escaped after a few
scatterings without any significant energy change and of a power law which
extends to high energies and is made of those photons which underwent
significant upscattering. The luminosity of the power law is relatively small
compared to that of the soft component. The more reflective the inner boundary
is, the flatter the power-law spectrum becomes. The spectral energy power-law
index for black-hole accretion is always higher than 1 and it is approximately
1.5 for high accretion rates. This result tempts us to say that bulk motion
Comptonization might be the mechanism behind the power-law spectra seen in
black-hole X-ray sources.Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in Ap
The structure of the magnetic field in the massive star-forming region W75N
A debated topic in star formation theory is the role of magnetic fields
during the protostellar phase of high-mass stars. It is still unclear how
magnetic fields influence the formation and dynamics of massive disks and
outflows. Most current information on magnetic fields close to high-mass
protostars comes from polarized maser emissions, which allows us to investigate
the magnetic field on small scales by using very long-baseline interferometry.
The massive star-forming region W75N contains three radio continuum sources
(VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated
masers, while a large-scale molecular bipolar outflow is also present. Very
recently, polarization observations of the 6.7 GHz methanol masers at
milliarsecond resolution have been able to probe the strength and structure of
the magnetic field over more than 2000 AU around VLA1. The magnetic field is
parallel to the outflow, suggesting that VLA1 is its powering source. The
observations of water masers at 22 GHz can give more information about the gas
dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long
Baseline Array was used to measure the linear polarization and the
Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We
detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which
indicate two different physical environments around the two sources, where VLA1
is in a more evolved state. The linear polarization of the masers confirms the
tightly ordered magnetic field around VLA1, which is aligned with the
large-scale molecular outflow, and also reveals an ordered magnetic field
around VLA2, which is not parallel to the outflow. [abridged]Comment: 11 pages, 5 figures, 2 Tables, accepted by Astronomy & Astrophysic
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
A search for 4750- and 4765-MHz OH masers in Southern Star Forming Regions
We have used the Australia Telescope Compact Array (ATCA) to make a sensitive
(5- 100 mJy) search for maser emission from the 4765-MHz
F=10 transition of OH. Fifty five star formation regions
were searched and maser emission with a peak flux density in excess of 100 mJy
was detected toward fourteen sites, with ten of these being new discoveries. In
addition we observed the 4750-MHz F=11 transition towards a
sample of star formation regions known to contain 1720-MHz OH masers, detecting
marginal maser emission from G348.550-0.979. If confirmed this would be only
the second maser discovered from this transition. The occurrence of 4765-MHz OH
maser emission accompanying 1720-MHz OH masers in a small number of well
studied star formation regions has lead to a general perception in the
literature that the two transitions favour similar physical conditions. Our
search has found that the presence of the excited-state 6035-MHz OH transition
is a much better predictor of 4765-MHz OH maser emission from the same region
than is 1720-MHz OH maser emission. Combining our results with those of
previous high resolution observations of other OH transitions we have examined
the published theoretical models of OH masers and find that none of them
predict any conditions in which the 1665-, 6035- and 4765-MHz transitions are
simultaneously inverted.
Erratum abstract:
Dodson & Ellingsen (2002) included several observations with significant
pointing errors, invalidating the upper limits found in these directions. These
have now been reobserved or recalculated. A new table of upper limits has been
generated, and two more masers that would have been seen have been found.Comment: Included an Erratum with Max as another author. This erratum was
rejected by MNRAS (Feb 04) as it contained too much data. Resubmitted as a
paper (Jun 04). Rejected (Sep 04) it had too little data. Resubmitted as
reduced erratum (Apr 05). Still waitin
Effects of Disks on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of a quasar by a spiral galaxy should often be
accompanied by damped Lyman-alpha absorption and dust extinction due to the
intervening gaseous disk. In nearly edge-on configurations, the surface mass
density of the gas and stars in the disk could by itself split the quasar image
and contribute significantly to the overall lensing cross section. We calculate
the lensing probability of a disk+halo mass model for spiral galaxies,
including cosmic evolution of the lens parameters. A considerable fraction of
the lens systems contains two images with sub-arcsecond separation, straddling
a nearly edge-on disk. Because of that, extinction by dust together with
observational selection effects (involving a minimum separation and a maximum
flux ratio for the lensed images), suppress the detection efficiency of spiral
lenses in optical wavebands by at least an order of magnitude. The missing
lenses could be recovered in radio surveys. In modifying the statistics of
damped Lyman-alpha absorbers, the effect of extinction dominates over the
magnification bias due to lensing.Comment: 19 pages, 12 figures; submitted to Ap
Six supersoft X-ray binaries: system parameters and twin-jet outflows
A comparison is made between the properties of CAL 83, CAL 87, RX
J0513.9-6951, 1E 0035.4-7230 (SMC 13), RX J0019.8+2156, and RX J0925.7-4758,
all supersoft X-ray binaries. Spectra with the same resolution and wavelength
coverage of these systems are compared and contrasted. Some new photometry is
also presented. The equivalent widths of the principal emission lines of H and
He II differ by more than an order of magnitude among these sources, although
those of the highest ionization lines (e.g. O VI) are very similar. In
individual systems, the velocity curves derived from various ions often differ
in phasing and amplitude, but those whose phasing is consistent with the light
curves (implying the lines are formed near the compact star) give masses of
and for the degenerate and mass-losing
stars, respectively. This finding is in conflict with currently prevailing
theoretical models for supersoft binaries. The three highest luminosity sources
show evidence of "jet" outflows, with velocities of .
In CAL 83 the shape of the He II 4686\AA profile continues to show evidence
that these jets may precess with a period of days.Comment: 27 pages including 5 tables, plus 6 figures. To appear in Ap
RoboPol: First season rotations of optical polarization plane in blazars
We present first results on polarization swings in optical emission of
blazars obtained by RoboPol, a monitoring program of an unbiased sample of
gamma-ray bright blazars specially designed for effective detection of such
events. A possible connection of polarization swing events with periods of high
activity in gamma rays is investigated using the dataset obtained during the
first season of operation. It was found that the brightest gamma-ray flares
tend to be located closer in time to rotation events, which may be an
indication of two separate mechanisms responsible for the rotations. Blazars
with detected rotations have significantly larger amplitude and faster
variations of polarization angle in optical than blazars without rotations. Our
simulations show that the full set of observed rotations is not a likely
outcome (probability ) of a random walk of the
polarization vector simulated by a multicell model. Furthermore, it is highly
unlikely () that none of our rotations is physically
connected with an increase in gamma-ray activity.Comment: 16 pages, 9 figure
DART-RAY: a 3D ray-tracing radiative transfer code for calculating the propagation of light in dusty galaxies
We present DART-Ray, a new ray-tracing 3D dust radiative transfer (RT) code designed specifically to calculate radiation field energy density (RFED) distributions within dusty galaxy models with arbitrary geometries. In this paper, we introduce the basic algorithm implemented in . DART-Ray which is based on a pre-calculation of a lower limit for the RFED distribution. This pre-calculation allows us to estimate the extent of regions around the radiation sources within which these sources contribute significantly to the RFED. In this way, ray-tracing calculations can be restricted to take place only within these regions, thus substantially reducing the computational time compared to a complete ray-tracing RT calculation. Anisotropic scattering is included in the code and handled in a similar fashion. Furthermore, the code utilizes a Cartesian adaptive spatial grid and an iterative method has been implemented to optimize the angular densities of the rays originated from each emitting cell. In order to verify the accuracy of the RT calculations performed by DART-Ray, we present results of comparisons with solutions obtained using the dusty 1D RT code for a dust shell illuminated by a central point source and existing 2D RT calculations of disc galaxies with diffusely distributed stellar emission and dust opacity. Finally, we show the application of the code on a spiral galaxy model with logarithmic spiral arms in order to measure the effect of the spiral pattern on the attenuation and RFED. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
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