277 research outputs found
The Submillimeter Array Polarimeter
We describe the Submillimeter Array (SMA) Polarimeter, a polarization
converter and feed multiplexer installed on the SMA. The polarimeter uses
narrow-band quarter-wave plates to generate circular polarization sensitivity
from the linearly-polarized SMA feeds. The wave plates are mounted in rotation
stages under computer control so that the polarization handedness of each
antenna is rapidly selectable. Positioning of the wave plates is found to be
highly repeatable, better than 0.2 degrees. Although only a single polarization
is detected at any time, all four cross correlations of left- and
right-circular polarization are efficiently sampled on each baseline through
coordinated switching of the antenna polarizations in Walsh function patterns.
The initial set of anti-reflection coated quartz and sapphire wave plates
allows polarimetry near 345 GHz; these plates have been have been used in
observations between 325 and 350 GHz. The frequency-dependent
cross-polarization of each antenna, largely due to the variation with frequency
of the retardation phase of the single-element wave plates, can be measured
precisely through observations of bright point sources. Such measurements
indicate that the cross-polarization of each antenna is a few percent or
smaller and stable, consistent with the expected frequency dependence and very
small alignment errors. The polarimeter is now available for general use as a
facility instrument of the SMA.Comment: To appear in Proc. SPIE 7020, 'Millimeter and Submillimeter Detectors
and Instrumentation'. Uses spie.cl
Magnetic Fields in the Formation of Sun-Like Stars
We report high-angular-resolution measurements of polarized dust emission
toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this
system the observed magnetic field morphology is in agreement with the standard
theoretical models of the formation of Sun-like stars in magnetized molecular
clouds at scales of a few hundred astronomical units; gravity has overcome
magnetic support, and the magnetic field traces a clear hourglass shape. The
magnetic field is substantially more important than turbulence in the evolution
of the system, and the initial misalignment of the magnetic and spin axes may
have been important in the formation of the binary system.Comment: 8 pages including 2 figures, accepted draft for publication in
Scienc
LoCuSS: Hydrostatic Mass Measurements of the High- Cluster Sample -- Cross-calibration of Chandra and XMM-Newton
We present a consistent analysis of Chandra and XMM-Newton observations of an
approximately mass-selected sample of 50 galaxy clusters at -- the
"LoCuSS High- Sample". We apply the same analysis methods to data from
both satellites, including newly developed analytic background models that
predict the spatial variation of the Chandra and XMM-Newton backgrounds to
and precision respectively. To verify the cross-calibration of
Chandra and XMM-Newton-based cluster mass measurements, we derive the mass
profiles of the 21 clusters that have been observed with both satellites,
extracting surface brightness and temperature profiles from identical regions
of the respective datasets. We obtain consistent results for the gas and total
hydrostatic cluster masses: the average ratio of Chandra- to XMM-Newton-based
measurements of and at are and
, respectively with an intrinsic scatter of for gas
masses and for hydrostatic masses. Comparison of our hydrostatic mass
measurements at with the latest LoCuSS weak-lensing results indicate
that the data are consistent with non-thermal pressure support at this radius
of . We also investigate the scaling relation between our hydrostatic
cluster masses and published integrated Compton parameter
measurements from the Sunyaev-Zel'dovich Array. We measure a scatter in mass at
fixed of at , which is consistent with
theoretical predictions of scatter.Comment: 21 pages, 11 figure
The Shape of the Black Hole Photon Ring: A Precise Test of Strong-Field General Relativity
We propose a new test of strong-field general relativity (GR) based on the
universal interferometric signature of the black hole photon ring. The photon
ring is a narrow ring-shaped feature, predicted by GR but not yet observed,
that appears on images of sources near a black hole. It is caused by extreme
bending of light within a few Schwarzschild radii of the event horizon and
provides a direct probe of the unstable bound photon orbits of the Kerr
geometry. We show that the precise shape of the observable photon ring is
remarkably insensitive to the astronomical source profile and can therefore be
used as a stringent test of GR. We forecast that a tailored space-based
interferometry experiment targeting M87* could test the Kerr nature of the
source to the sub-sub-percent level.Comment: 20 pages, 9 figures. v2: minor change
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