2,526 research outputs found
Inside the Bondi radius of M87
Chandra X-ray observations of the nearby brightest cluster galaxy M87 resolve
the hot gas structure across the Bondi accretion radius of the central
supermassive black hole, a measurement possible in only a handful of systems
but complicated by the bright nucleus and jet emission. By stacking only short
frame-time observations to limit pileup, and after subtracting the nuclear PSF,
we analysed the X-ray gas properties within the Bondi radius at 0.12-0.22 kpc
(1.5-2.8 arcsec), depending on the black hole mass. Within 2 kpc radius, we
detect two significant temperature components, which are consistent with
constant values of 2 keV and 0.9 keV down to 0.15 kpc radius. No evidence was
found for the expected temperature increase within ~0.25 kpc due to the
influence of the SMBH. Within the Bondi radius, the density profile is
consistent with . The lack of a temperature increase inside
the Bondi radius suggests that the hot gas structure is not dictated by the
SMBH's potential and, together with the shallow density profile, shows that the
classical Bondi rate may not reflect the accretion rate onto the SMBH. If this
density profile extends in towards the SMBH, the mass accretion rate onto the
SMBH could be at least two orders of magnitude less than the Bondi rate, which
agrees with Faraday rotation measurements for M87. We discuss the evidence for
outflow from the hot gas and the cold gas disk and for cold feedback, where gas
cooling rapidly from the hot atmosphere could feed the cirumnuclear disk and
fuel the SMBH. At 0.2 kpc radius, the cooler X-ray temperature component
represents ~20% of the total X-ray gas mass and, by losing angular momentum to
the hot gas component, could provide a fuel source of cold clouds within the
Bondi radius.Comment: 14 pages, 8 figures, accepted by MNRA
An extreme rotation measure in the high-redshift radio galaxy PKS B0529-549
We present the results of a radio polarimetric study of the high-redshift
radio galaxy PKS B0529-549 (z=2.575), based on high-resolution 12 mm and 3 cm
images obtained with the Australia Telescope Compact Array (ATCA). The source
is found to have a rest-frame Faraday rotation measure of -9600 rad m^{-2}, the
largest seen thus far in the environment of a z > 2 radio galaxy. In addition,
the rest-frame Faraday dispersion in the screen responsible for the rotation is
calculated to be 5800 rad m^{-2}, implying rotation measures as large as -15400
rad m^{-2}. Using supporting near-IR imaging from the Very Large Telescope
(VLT), we suggest that the rotation measure originates in the Ly-alpha halo
surrounding the host galaxy, and estimate the magnetic field strength to be ~10
microGauss. We also present a new optical spectrum of PKS B0529-549 obtained
with the New Technology Telescope (NTT), and propose that the emission-line
ratios are best described by a photoionization model. Furthermore, the host
galaxy is found to exhibit both hot dust emission at 8.0 microns and
significant internal visual extinction (~1.6 mag), as inferred from Spitzer
Space Telescope near/mid-IR imaging.Comment: 12 pages, 6 figures, accepted for publication in MNRA
SATPLOT for Analysis of SECCHI Heliospheric Imager Data
Determining trajectories of solar transients such as coronal mass ejections is not always easy. White light images from SECCHI's (Sun Earth Connection Coronal and Heliospheric Investigation) heliospheric imagers are difficult to interpret because they represent a line-of-sight projection of optically thin solar wind structures. A structure's image by itself gives no information about its angle of propagation relative to the Sunspacecraft line, and an image may show a superposition of several structures, all propagating at different angles. Analyzing SECCHI heliospheric imager data using plots of elongation (angle from the Sun) versus time at fixed position angle (aka Jplots ) has proved extremely useful in understanding the observed solar wind structures. This technique has been used to study CME (coronal mass ejection) propagation, CIRs (corotating interaction regions), and blobs. SATPLOT software was developed to create and analyze such elongation versus time plots. The tool uses a library of cylindrical maps of the data for each spacecraft s panoramic field-of-view. Each map includes data from three SECCHI white-light telescopes (the COR2 coronagraph and both heliospheric imagers) at one time for one spacecraft. The maps are created using a Plate Carree projection, optimized for creating the elongation versus time plots. The tool can be used to analyze the observed tracks of features seen in the maps, and the tracks are then used to extract information, for example, on the angle of propagation of the feature
The Equation of State of Neutron-Star Matter in Strong Magnetic Fields
We study the effects of very strong magnetic fields on the equation of state
(EOS) in multicomponent, interacting matter by developing a covariant
description for the inclusion of the anomalous magnetic moments of nucleons.
For the description of neutron star matter, we employ a field-theoretical
approach which permits the study of several models which differ in their
behavior at high density. Effects of Landau quantization in ultra-strong
magnetic fields ( Gauss) lead to a reduction in the electron
chemical potential and a substantial increase in the proton fraction. We find
the generic result for Gauss that the softening of the EOS caused
by Landau quantization is overwhelmed by stiffening due to the incorporation of
the anomalous magnetic moments of the nucleons. In addition, the neutrons
become completely spin polarized. The inclusion of ultra-strong magnetic fields
leads to a dramatic increase in the proton fraction, with consequences for the
direct Urca process and neutron star cooling. The magnetization of the matter
never appears to become very large, as the value of never deviates from
unity by more than a few percent. Our findings have implications for the
structure of neutron stars in the presence of large frozen-in magnetic fields.Comment: 40 pages, 7 figures, accepted for publication in Ap
Effects of Feeding Legume Silage With Differing Tannin Levels on Lactating Dairy Cattle
Condensed tannins (CT) bind to plant proteins in the rumen, reducing protein degradation to ammonia and increasing milk production and milk protein (e.g. Waghorn, 1987). Previous research showed that the reduced soluble non-protein nitrogen (NPN) content of red clover (Trifolium pratense) silage (RCS) was related to its greater N efficiency relative to lucerne (Medicago sativa) silage (LS) (Broderick et al., 2001). Commercial cultivars of birdsfoot trefoil (Lotus corniculatus; BFT) contain modest levels of CT which reduce NPN formation in silage (Albrecht & Muck, 1991). The objective was to compare silages made from BFT with RCS and LS for milk production and N efficiency in lactating dairy cows
The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm
We present new high-resolution observations of Sagittarius A* at wavelengths
of 17.4 to 23.8 cm with the Very Large Array in A configuration with the Pie
Town Very Long Baseline Array antenna. We use the measured sizes to calibrate
the interstellar scattering law and find that the major axis size of the
scattering law is smaller by ~6% than previous estimates. Using the new
scattering law, we are able to determine the intrinsic size of Sgr A* at
wavelengths from 0.35 cm to 6 cm using existing results from the VLBA. The new
law increases the intrinsic size at 0.7 cm by ~20% and <5% at 0.35 cm. The
intrinsic size is 13^{+7}_{-3} Schwarzschild radii at 0.35 cm and is
proportional to lambda^gamma, where gamma is in the range 1.3 to 1.7.Comment: ApJL, in pres
How the Charge Can Affect the Formation of Gravastars
In recent work we physically interpreted a special gravastar solution
characterized by a zero Schwarzschild mass. In fact, in that case, none
gravastar was formed and the shell expanded, leaving behind a de Sitter or a
Minkowski spacetime, or collapsed without forming an event horizon, originating
what we called a massive non-gravitational object. This object has two
components of non zero mass but the exterior spacetime is Minkowski or de
Sitter. One of the component is a massive thin shell and the other one is de
Sitter spacetime inside. The total mass of this object is zero Schwarzschild
mass, which characterizes an exterior vacuum spacetime. Here, we extend this
study to the case where we have a charged shell. Now, the exterior is a
Reissner-Nordstr\"om spacetime and, depending on the parameter
of the equation of state of the shell, and the charge, a
gravastar structure can be formed. We have found that the presence of the
charge contributes to the stability of the gravastar, if the charge is greater
than a critical value. Otherwise, a massive non-gravitational object is formed
for small charges.Comment: 17 pages and 7 figures, several typos corrected, accepted for
publication in JCA
The Search for Signatures Of Transient Mass Loss in Active Stars
The habitability of an exoplanet depends on many factors. One such factor is
the impact of stellar eruptive events on nearby exoplanets. Currently this is
poorly constrained due to heavy reliance on solar scaling relationships and a
lack of experimental evidence. Potential impacts of Coronal Mass Ejections
(CMEs), which are a large eruption of magnetic field and plasma from a star,
are space weather and atmospheric stripping. A method for observing CMEs as
they travel though the stellar atmosphere is the type II radio burst, and the
new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15
hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star,
taken in LOFAR's beam-formed observation mode for the purposes of measuring
transient frequency-dependent low frequency radio emission. The observations
utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for
five three-hour observation periods. In this data set, there were no confirmed
type II events in this frequency range. We explore the range of parameter space
for type II bursts constrained by our observations Assuming the rate of shocks
is a lower limit to the rate at which CMEs occur, no detections in a total of
15 hours of observation places a limit of shocks/hr for YZ CMi due to the stochastic nature of the events and
limits of observational sensitivity. We propose a methodology to interpret
jointly observed flares and CMEs which will provide greater constraints to CMEs
and test the applicability of solar scaling relations
Cuspons, peakons and regular gap solitons between three dispersion curves
A general wave model with the cubic nonlinearity is introduced to describe a
situation when the linear dispersion relation has three branches, which would
intersect in the absence of linear couplings between the three waves. Actually,
the system contains two waves with a strong linear coupling between them, to
which a third wave is then coupled. This model has two gaps in its linear
spectrum. Realizations of this model can be made in terms of temporal or
spatial evolution of optical fields in, respectively, a planar waveguide or a
bulk-layered medium resembling a photonic-crystal fiber. Another physical
system described by the same model is a set of three internal wave modes in a
density-stratified fluid. A nonlinear analysis is performed for solitons which
have zero velocity in the reference frame in which the group velocity of the
third wave vanishes. Disregarding the self-phase modulation (SPM) term in the
equation for the third wave, we find two coexisting families of solitons:
regular ones, which may be regarded as a smooth deformation of the usual gap
solitons in a two-wave system, and cuspons with a singularity in the first
derivative at their center. Even in the limit when the linear coupling of the
third wave to the first two vanishes, the soliton family remains drastically
different from that in the linearly uncoupled system; in this limit, regular
solitons whose amplitude exceeds a certain critical value are replaced by
peakons. While the regular solitons, cuspons, and peakons are found in an exact
analytical form, their stability is tested numerically, which shows that they
all may be stable. If the SPM terms are retained, we find that there again
coexist two different families of generic stable soliton solutions, namely,
regular ones and peakons.Comment: a latex file with the text and 10 pdf files with figures. Physical
Review E, in pres
The `bare' strange stars might not be bare
It is proposed that the `bare' strange matter stars might not be bare, and
radio pulsars might be in fact `bare' strange stars. As strange matter stars
being intensely magnetized rotate, the induced unipolar electric fields would
be large enough to construct magnetospheres. This situation is very similar to
that discussed by many authors for rotating neutron stars. Also, the strange
stars with accretion crusts in binaries could act as X-ray pulsars or X-ray
bursters. There are some advantages if radio pulsars are `bare' strange stars.Comment: 11 pages, 1 Postscript figures, LaTeX, Chin. Phys. Lett. 1998,
Vol.15, Nov.12, p.93
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