1,392 research outputs found
A Radio Polarimetric Study of the Galactic Center Threads
Multi-frequency, polarimetric VLA observations of the non-thermal filaments
(NTF's), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern
Threads are presented at 20, 6, 3.6 and 2 cm, with high enough spatial
resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image
reveals a wealth of new detail in the radio sources lying within the inner 60
pc of the Galaxy. The Southern Thread has a prominent split along its length,
similar to splitting at the ends of previously studied NTF's. With resolutions
as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and
little substructure internal to the filament. The spectral index of the
Northern Thread has been determined over a broad range of frequencies. Its flux
density falls with frequency, alpha=-0.5 between 90 and 6 cm, and becomes much
steeper (alpha=-2.0) between 6 and 2 cm. The spectral index does not vary
significantly along the length of the Northern Thread, which implies either
that the diffusion timescale for the emitting electrons is less than their
synchrotron lifetime, or that the emitting electrons are reaccelerated
continuously at multiple positions along the filament. Because of the lack of
spectral index variation, we have not located the source of relativistic
electrons. Polarization observations at 6 and 3.6 cm confirm the non-thermal
nature of the emission from the Northern Thread. The fractional polarization in
the Northern Thread reaches 70% in some regions, although the polarized
emission is patchy. Large rotation measures (RM > 2000 rad/m2) have been
observed with irregular variations across the filament.The intrinsic magnetic
field in the Northern Thread is predominantly aligned along its long axis.Comment: 19 pages, incl. 24 figs; to appear in the Astrophysical Journa
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
Inflation-Produced Magnetic Fields in Nonlinear Electrodynamics
We study the generation of primeval magnetic fields during inflation era in
nonlinear theories of electrodynamics. Although the intensity of the produced
fields strongly depends on characteristics of inflation and on the form of
electromagnetic Lagrangian, our results do not exclude the possibility that
these fields could be astrophysically interesting.Comment: 6 page
Making the Best of Polymers with SulfurâNitrogen Bonds: From Sources to Innovative Materials
Polymers with sulfurânitrogen bonds have been underestimated for a long time, although the intrinsic characteristics of these polymers offer a myriad of superior properties (e.g., degradation, flame retardancy, filmâforming ability, good solubility in polar solvents, and high refractivity with small chromatic dispersions, among other things) compared to their carbon analogues. The remarkable characteristics of these polymers result from the unique chemical properties of the sulfurânitrogen bond (e.g., its polar character and the multiple valence states of sulfur), and thus open excellent perspectives for the development of innovative (bio)materials. Accordingly, this review describes the most common chemical approaches toward the efficient synthesis of these ubiquitous polymers possessing diverse sulfurânitrogen bonds, and furthermore highlights their applications in multiple fields, ranging from biomedicine to energy storage, with the aim of providing an informative perspective on challenges facing the synthesis of sulfurânitrogen polymers with desirable properties
ROSAT Evidence for Intrinsic Oxygen Absorption in Cooling Flow Galaxies and Groups
Using spatially resolved, deprojected ROSAT PSPC spectra of 10 of the
brightest cooling flow galaxies and groups with low Galactic column densities
we have detected intrinsic absorption over energies ~0.4-0.8 keV in half of the
sample. Since no intrinsic absorption is indicated for energies below ~0.4 keV,
the most reasonable model for the absorber is collisionally ionized gas at
temperatures T=10^{5-6} K with most of the absorption arising from ionized
states of oxygen but with a significant contribution from carbon and nitrogen.
The soft X-ray emission of this warm gas can explain the sub-Galactic column
densities of cold gas inferred within the central regions of most of the
systems. Attributing the absorption to ionized gas reconciles the large columns
of cold H and He inferred from EINSTEIN and ASCA with the lack of such columns
inferred from ROSAT. Within the central ~10-20 kpc, where the constraints are
most secure, the estimated mass of the ionized absorber is consistent with most
(perhaps all) of the matter deposited by a cooling flow over the lifetime of
the flow. Since the warm absorber produces no significant H or He absorption
the large absorber masses are consistent with the negligible atomic and
molecular H inferred from HI and CO observations of cooling flows. It is also
found that if T > ~2x10^5 K then the optical and UV emission implied by the
warm gas does not violate published constraints. Finally, we discuss how the
prediction of warm ionized gas as the product of mass drop-out in these and
other cooling flows can be verified with new CHANDRA and XMM observations.
(Abridged)Comment: 17 pages (5 figures), Accepted for publication in ApJ, expanded
discussion of multiphase spectral models, theoretical implications of warm
gas in cooling flows, and the statistical significance of the oxygen
absorptio
A small X-ray corona of the narrow-angle tail radio galaxy NGC 1265 soaring through the Perseus cluster
A deep Chandra observation of NGC 1265 (3C 83.1B), the prototype for the
narrow-angled-tailed (NAT) radio galaxy, reveals a small cool X-ray thermal
corona (~ 0.6 keV) embedded in the hot ICM of the Perseus cluster (~ 6.7 keV).
The corona is asymmetric with a sharp edge (~ 2.2'', or 0.8 kpc from the
nucleus) to the south and an extension to the north (at least ~ 8'' from the
nucleus), which is interpreted as the action of ram pressure while solely the
static ICM confinement is unable to explain. We estimate that the corona is
moving with a velocity of ~ 2.4 - 4.2 times the local sound speed to the south.
The presence of the sharp edge for this small corona indicates that the
transport processes are largely suppressed by the magnetic field there. The
magnetic field around the corona also suppresses heat conduction by at least a
factor of ~ 60 across the corona boundary. We conclude that it is unrealistic
to study the interaction of the small X-ray coronae with the hot ICM without
the consideration of the roles that magnetic field plays, a factor not included
in current simulations. An absorbed (N_H=1.5-3x10^22 cm^-2) nucleus is also
detected, which is not usual for FR I radio galaxies. Weak X-ray emission from
three inner radio knots in the jets is also detected. Indentations at the east
and west of the corona indicate interaction between the jets and the X-ray
corona. Narrow jets carry great amounts of energy out of the central AGN and
release the energy outside the corona, preserving the tiny and vulnerable
corona. This case reveals that the inner kpc core of the corona of massive
galaxies can survive both high-speed stripping and powerful AGN feedback. Thus,
the cooling of the X-ray coronae potentially provides fuel to the central SMBH
in rich environments where the amount of the galactic cold gas is at a minimum.Comment: revised version, 11 pages, 4 figures, emulateapj5.sty, accepted by
ApJ, for the version with high-resolution figures
(http://hea-www.harvard.edu/~msun/n1265.ps
Magnetogenesis from Cosmic String Loops
Large-scale coherent magnetic fields are observed in galaxies and clusters,
but their ultimate origin remains a mystery. We reconsider the prospects for
primordial magnetogenesis by a cosmic string network. We show that the magnetic
flux produced by long strings has been overestimated in the past, and give
improved estimates. We also compute the fields created by the loop population,
and find that it gives the dominant contribution to the total magnetic field
strength on present-day galactic scales. We present numerical results obtained
by evolving semi-analytic models of string networks (including both one-scale
and velocity-dependent one-scale models) in a Lambda-CDM cosmology, including
the forces and torques on loops from Hubble redshifting, dynamical friction,
and gravitational wave emission. Our predictions include the magnetic field
strength as a function of correlation length, as well as the volume covered by
magnetic fields. We conclude that string networks could account for magnetic
fields on galactic scales, but only if coupled with an efficient dynamo
amplification mechanism.Comment: 10 figures; v3: small typos corrected to match published version.
MagnetiCS, the code described in paper, is available at
http://markcwyman.com/ and
http://www.damtp.cam.ac.uk/user/dhw22/code/index.htm
An Empirical Model for the Radio Emission from Pulsars
A model for slow radio pulsars is proposed which involves the entire
magnetosphere in the production of the observed radio emission. It is argued
that observations of pulsar profiles suggest that a feedback mechanism exists
between the star surface and the null charge surface, requiring particle flow
in both directions. In their flow to and from the surface the particles execute
an azimuthal drift around the magnetic pole, thereby creating a ring of
discrete `emission nodes' close to the surface. Motion of the nodes is observed
as the well-known subpulse `drift', but is interpreted here as a small residual
component of the real particle drift. The nodes can therefore move in either
direction, or even remain stationary. A precise fit is found for the pulsar
PSR0943+10. Azimuthal interactions between different regions of the
magnetosphere depend on the angle between the magnetic and rotation axes and
influence the conal type, as observed. The requirement of intermittent weak
pair-production in an outergap suggests a natural evolutionary link between
radio and gamma-ray pulsars.Comment: 17 pages 8 figure
Magnetic Field Limitations on Advection Dominated Flows
Recent papers discussing advection dominated accretion flows (ADAF) as a
solution for astrophysical accretion problems should be treated with some
caution because of their uncertain physical basis. The suggestions underlying
ADAF involve ignoring the magnetic field reconnection in heating of the plasma
flow, assuming electron heating due only to binary Coulomb collisions with
ions. Here, we analyze the physical processes in optically thin accretion flows
at low accretion rates including the influence of an equipartition turbulent
magnetic field. For these conditions there is continuous destruction of
magnetic flux by reconnection.
The reconnection is expected to significantly heat the electrons which can
efficiently emit magnetobremstrahlung radiation. Because of this electron
emission, the radiative efficiency of the ADAF is not small. We suggest that
the small luminosities of nearby galactic black holes is due to outflows rather
than ADAF accretion.Comment: 7 pages, 3 figures, Submitted to Ap
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