60 research outputs found
Extragalactic Radio Sources and the WMAP Cold Spot
We detect a dip of 20-45% in the surface brightness and number counts of NVSS
sources smoothed to a few degrees at the location of the WMAP cold spot. The
dip has structure on scales of approximately 1-10 degrees. Together with
independent all-sky wavelet analyses, our results suggest that the dip in
extragalactic brightness and number counts and the WMAP cold spot are
physically related, i.e., that the coincidence is neither a statistical anomaly
nor a WMAP foreground correction problem. If the cold spot does originate from
structures at modest redshifts, as we suggest, then there is no remaining need
for non-Gaussian processes at the last scattering surface of the CMB to explain
the cold spot. The late integrated Sachs-Wolfe effect, already seen
statistically for NVSS source counts, can now be seen to operate on a single
region. To create the magnitude and angular size of the WMAP cold spot requires
a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This
is far outside the current expectations of the concordance cosmology, and adds
to the anomalies seen in the CMB.Comment: revised version, ApJ, in pres
SKA Deep Polarization and Cosmic Magnetism
Deep surveys with the SKA1-MID array offer for the first time the opportunity
to systematically explore the polarization properties of the microJy source
population. Our knowledge of the polarized sky approaching these levels is
still very limited. In total intensity the population will be dominated by
star-forming and normal galaxies to intermediate redshifts (), and
low-luminosity AGN to high redshift. The polarized emission from these objects
is a powerful probe of their intrinsic magnetic fields and of their magnetic
environments. For redshift of order 1 and above the broad bandwidth of the
mid-bands span the Faraday thick and thin regimes allowing study of the
intrinsic polarization properties of these objects as well as depolarization
from embedded and foreground plasmas. The deep field polarization images will
provide Rotation Measures data with very high solid angle density allowing a
sensitive statistical analysis of the angular variation of RM on critical
arc-minute scales from a magnetic component of Large Scale Structure of the
Universe.Comment: 9 pages, 3 figures; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)11
Kinematics of X-ray Emitting Components in Cassiopeia A
We present high-resolution X-ray proper motion measurements of Cassiopeia A
using Chandra observations from 2000 and 2002. We separate the emission into
four spectrally distinct classes: Si-dominated, Fe-dominated,
low-energy-enhanced, and continuum-dominated. These classes also represent
distinct spatial and kinematic components. The Si- and Fe-dominated classes are
ejecta and have a mean expansion rate of 0.2%/yr. This is the same as for the
forward shock filaments but less than the 0.3%/yr characteristic of optical
ejecta. The low-energy-enhanced spectral class possibly illuminates a clumpy
circumstellar component and has a mean expansion rate of 0.05%/yr. The
continuum-dominated emission likely represents the forward shock and consists
of diffuse circumstellar material which is seen as a circular ring around the
periphery of the remnant as well as projected across the center.Comment: 15 pages, 3 figures, accepted to Ap
Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula
We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula
obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and
High-resolution Spitzer IRS spectra of selected positions within the nebula,
and a near-infrared ground-based image made in the light of [Fe II]1.644
micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0
micron, resembles the general morphology of visible H-alpha and near-IR [Fe II]
line emission, while the 3.6 and 4.5 micron images are dominated by continuum
synchrotron emission. The 24 micron and 70 micron images show enhanced emission
that may be due to line emission or the presence of a small amount of warm dust
in the nebula on the order of less than 1% of a solar mass. The ratio of the
3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power
law index ranging from approximately 0.3 to 0.8 across the nebula. Combining
this information with optical and X-ray synchrotron images, we derive a
broadband spectrum that reflects the superposition of the flatter spectrum jet
and torus with the steeper diffuse nebula, and suggestions of the expected
pileup of relativistic electrons just before the exponential cutoff in the
X-ray. The pulsar, and the associated equatorial toroid and polar jet
structures seen in Chandra and HST images (Hester et al. 2002) can be
identified in all of the IRAC images. We present the IR photometry of the
pulsar. The forbidden lines identified in the high resolution IR spectra are
all double due to Doppler shifts from the front and back of the expanding
nebula and give an expansion velocity of approximately 1264 km/s.Comment: 21 pages, 4 tables, 16 figure
Canonical Particle Acceleration in FRI Radio Galaxies
Matched resolution multi-frequency VLA observations of four radio galaxies
are used to derive the asymptotic low energy slope of the relativistic electron
distribution. Where available, low energy slopes are also determined for other
sources in the literature. They provide information on the acceleration physics
independent of radiative and other losses, which confuse measurements of the
synchrotron spectra in most radio, optical and X-ray studies. We find a narrow
range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the
currently small sample of lower luminosity sources classified as FRI (not
classical doubles). This distribution is close to, but apparently inconsistent
with, the test particle limit of n(E)=const*E^-2.0 expected from strong
diffusive shock acceleration in the non-relativistic limit. Relativistic shocks
or those modified by the back-pressure of efficiently accelerated cosmic rays
are two alternatives to produce somewhat steeper spectra. We note for further
study the possiblity of acceleration through shocks, turbulence or shear in the
flaring/brightening regions in FRI jets as they move away from the nucleus.
Jets on pc scales and the collimated jets and hot spots of FRII (classical
double) sources would be governed by different acceleration sites and
mechanisms; they appear to show a much wider range of spectra than for FRI
sources.Comment: 16 figures, including 5 color. Accepted to Astrophysical Journa
Spitzer IRAC Images and Sample Spectra of Cassiopeia A's Explosion
We present Spitzer IRAC images, along with representative 5.27 to 38.5 micron
IRS spectra of the Cassiopeia A supernova remnant. We find that various IRAC
channels are each most sensitive to a different spectral and physical
component. Channel 1 (3.6 micron) matches radio synchrotron images. Where
Channel 1 is strong with respect to the other channels, the longer-wavelength
spectra show a broad continuum gently peaking around 26 micron, with weak or no
lines. We suggest that this is due to un-enriched progenitor circumstellar dust
behind the outer shock, processed by shock photons and electrons. Where Channel
4 (8 micron) is bright relative to the other IRAC channels, the long-wavelength
spectra show a strong, 2-3 micron-wide peak at 21 micron, likely due to
silicates and proto-silicates, as well as strong ionic lines of [Ar II], [Ar
III], [S IV] and [Ne II]. In these locations, the dust and ionic emission
originate from the explosion's O-burning layers. The regions where Channels 2
(4.5 micron) and 3 (5.6 micron) are strongest relative to Channel 4 show a
spectrum that rises gradually to 21 micron, and then flattens or rises more
slowly to longer wavelengths, along with higher ratios of [Ne II] to [Ar II].
Dust and ionic emission in these locations arise primarily from the C- and Ne-
burning layers. These findings are consistent with asymmetries in the explosion
producing variations in the velocity structure in different directions, but
preserving the nucleosynthetic layers. At each location, the dust and ionic
lines in the mid-infrared, and the hotter and more highly ionized optical and
X-ray emission are then dominated by the layer currently encountering the
reverse shock in that direction.Comment: 28 pages, 10 figures, accepted in to the Astrophysical Journal. For
full-resolution images, please see
http://webusers.astro.umn.edu/~jennis/iracpaper.html. Revised to correct an
error in reference
Ergs: The Evolution of Shell Supernova Remnants
This paper reports on a workshop hosted by the University of Minnesota, March
23-26, 1997. It addressed fundamental dynamical issues associated with the
evolution of shell supernova remnants and the relationships between supernova
remnants and their environments. The workshop considered, in addition to
classical shell SNRs, dynamical issues involving X-ray filled composite
remnants and pulsar driven shells, such as that in the Crab Nebula.
Approximately 75 participants with wide ranging interests attended the
workshop. An even larger community helped through extensive on-line debates
prior to the meeting. Each of the several sessions, organized mostly around
chronological labels, also addressed some underlying, general physical themes:
How are SNR dynamics and structures modified by the character of the CSM and
the ISM and vice versa? How are magnetic fields generated in SNRs and how do
magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and
ions) produced in SNRs and how does their presence influence or reveal SNR
dynamics? How does SNR blast energy partition into various components over time
and what controls conversion between components? In lieu of a proceedings
volume, we present here a synopsis of the workshop in the form of brief
summaries of the workshop sessions. The sharpest impressions from the workshop
were the crucial and under-appreciated roles that environments have on SNR
appearance and dynamics and the critical need for broad-based studies to
understand these beautiful, but enigmatic objects. \\Comment: 54 pages text, no figures, Latex (aasms4.sty). submitted to the PAS
MeerKAT uncovers the physics of an odd radio circle
Odd radio circles (ORCs) are recently-discovered faint diffuse circles of radio emission, of unknown cause, surrounding galaxies at moderate redshift (z âŒ0.2-0.6). Here, we present detailed new MeerKAT radio images at 1284 MHz of the first ORC, originally discovered with the Australian Square Kilometre Array Pathfinder, with higher resolution (6 arcsec) and sensitivity (âŒ2.4 ÎŒJy/beam). In addition to the new images, which reveal a complex internal structure consisting of multiple arcs, we also present polarization and spectral index maps. Based on these new data, we consider potential mechanisms that may generate the ORCs
MAGNETIC FIELD DISORDER AND FARADAY EFFECTS ON THE POLARIZATION OF EXTRAGALACTIC RADIO SOURCES
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