762 research outputs found
ARCADE 2 Measurement of the Extra-Galactic Sky Temperature at 3-90 GHz
The ARCADE 2 instrument has measured the absolute temperature of the sky at
frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic
instrument observing at balloon altitudes with no emissive windows between the
beam-forming optics and the sky. An external blackbody calibrator provides an
{\it in situ} reference. Systematic errors were greatly reduced by using
differential radiometers and cooling all critical components to physical
temperatures approximating the CMB temperature. A linear model is used to
compare the output of each radiometer to a set of thermometers on the
instrument. Small corrections are made for the residual emission from the
flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE
2 data alone show an extragalactic rise of mK at 3.3 GHz in addition
to a CMB temperature of K. Combining the ARCADE 2 data with
data from the literature shows a background power law spectrum of [K] from 22 MHz to 10 GHz ( GHz)
in addition to a CMB temperature of K.Comment: 11 pages 5 figures Submitted to Ap
Radiation from a Charge Uniformly Accelerated for All Time
A recent paper of Singal [Gen. Rel. Grav. 27 (1995), 953-967] argues that a
uniformly accelerated particle does not radiate, in contradiction to the
consensus of the research literature over the past 30 years. This note points
out some questionable aspects of Singal's argument and shows how similar
calculations can lead to the opposite conclusion.Comment: LaTeX, 9 pages, to appear in General Relativity and Gravitatio
ARCADE 2 Observations of Galactic Radio Emission
We use absolutely calibrated data from the ARCADE 2 flight in July 2006 to
model Galactic emission at frequencies 3, 8, and 10 GHz. The spatial structure
in the data is consistent with a superposition of free-free and synchrotron
emission. Emission with spatial morphology traced by the Haslam 408 MHz survey
has spectral index beta_synch = -2.5 +/- 0.1, with free-free emission
contributing 0.10 +/- 0.01 of the total Galactic plane emission in the lowest
ARCADE 2 band at 3.15 GHz. We estimate the total Galactic emission toward the
polar caps using either a simple plane-parallel model with csc|b| dependence or
a model of high-latitude radio emission traced by the COBE/FIRAS map of CII
emission. Both methods are consistent with a single power-law over the
frequency range 22 MHz to 10 GHz, with total Galactic emission towards the
north polar cap T_Gal = 0.498 +/- 0.028 K and spectral index beta = -2.55 +/-
0.03 at reference frequency 1 GHz. The well calibrated ARCADE 2 maps provide a
new test for spinning dust emission, based on the integrated intensity of
emission from the Galactic plane instead of cross-correlations with the thermal
dust spatial morphology. The Galactic plane intensity measured by ARCADE 2 is
fainter than predicted by models without spinning dust, and is consistent with
spinning dust contributing 0.4 +/- 0.1 of the Galactic plane emission at 22
GHz.Comment: 10 poges, 9 figures. Submitted to The Astrophysical Journa
ARCADE: Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission
The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission
(ARCADE) is a balloon-borne instrument designed to measure the temperature of
the cosmic microwave background at centimeter wavelengths. ARCADE searches for
deviations from a blackbody spectrum resulting from energy releases in the
early universe. Long-wavelength distortions in the CMB spectrum are expected in
all viable cosmological models. Detecting these distortions or showing that
they do not exist is an important step for understanding the early universe. We
describe the ARCADE instrument design, current status, and future plans.Comment: 12 pages, 6 figures. Proceedings of the Fundamental Physics With CMB
workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review
Direct Distance Measurements to Superluminal Radio Sources
We present a new technique for directly measuring the distances to
superluminal radio sources. By comparing the observed proper motions of
components in a parsec scale radio jet to their measured Doppler factors, we
can deduce the distance to the radio source independent of the standard rungs
in the cosmological distance ladder. This technique requires that the jet angle
to the line of sight and the ratio of pattern to flow velocities are
sufficiently constrained. We evaluate a number of possibilities for
constraining these parameters and demonstrate the technique on a well defined
component in the parsec scale jet of the quasar 3C279 (z = 0.536). We find an
angular size distance to 3C279 of greater than 1.8 (+0.5,-0.3) n^{1/8} Gpc,
where n is the ratio of the energy density in the magnetic field to the energy
density in the radiating particles in that jet component. For an Einstein-de
Sitter Universe, this measurement would constrain the Hubble constant to be H <
65 n^{-1/8} km/s/Mpc at the two sigma level. Similar measurements on higher
redshift sources may help discriminate between cosmological models.Comment: 18 pages, 8 figures, to be published in The Astrophysical Journa
The ARCADE 2 Instrument
The second generation Absolute Radiometer for Cosmology, Astrophysics, and
Diffuse Emission (ARCADE 2) instrument is a balloon-borne experiment to measure
the radiometric temperature of the cosmic microwave background and Galactic and
extra-Galactic emission at six frequencies from 3 to 90 GHz. ARCADE 2 utilizes
a double-nulled design where emission from the sky is compared to that from an
external cryogenic full-aperture blackbody calibrator by cryogenic switching
radiometers containing internal blackbody reference loads. In order to further
minimize sources of systematic error, ARCADE 2 features a cold fully open
aperture with all radiometrically active components maintained at near 2.7 K
without windows or other warm objects, achieved through a novel thermal design.
We discuss the design and performance of the ARCADE 2 instrument in its 2005
and 2006 flights.Comment: 12 pages, 14 figues, 3 tables, 2 figures added, Accepted to Ap
Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies
The clear sign of a hidden quasar inside a radio galaxy is the appearance of
quasar spectral features in its polarized (scattered) light. However that
observational test requires suitably placed scattering material to act as a
mirror, allowing us to see the nuclear light. A rather robust and more general
test for a hidden quasar is to look for the predicted high mid-IR luminosity
from the nuclear obscuring matter. The nuclear waste heat is detected and well
isolated in the nearest narrow line radio galaxy, Cen A. This confirms other
indications that Cen A does contain a modest quasar-like nucleus. However we
show here that M87 does not: at high spatial resolution, the mid-IR nucleus is
seen to be very weak, and consistent with simple synchrotron emission from the
base of the radio jet. This fairly robustly establishes that there are "real"
narrow line radio galaxies, without the putative accretion power, and with
essentially all the luminosity in kinetic form. Next we show the intriguing
mid-IR morphology of Cygnus A, reported previously by us and later discussed in
detail by Radomski et al. (2002). All of this mid-IR emission is consistent
with reprocessing by a hidden quasar, known to exist from spectropolarimetry by
Ogle et al. (1997) and other evidence.Comment: 21 pages, 5 figure
Interpretation of the ARCADE 2 Absolute Sky Brightness Measurement
We use absolutely calibrated data between 3 and 90 GHz from the 2006 balloon flight of the ARCADE 2 instrument, along with previous measurements at other frequencies, to constrain models of extragalactic emission. Such emission is a combination of the cosmic microwave background (CMB) monopole, Galactic foreground emission, the integrated contribution of radio emission from external galaxies, any spectral distortions present in the CMB, and any other extragalactic source. After removal of estimates of foreground emission from our own Galaxy, and an estimated contribution of external galaxies, we present fits to a combination of the flat-spectrum CMB and potential spectral distortions in the CMB.We find 2σ upper limits to CMB spectral distortions ofμ \u3c 6×10−4 and |Yff| \u3c 1×10−4. We also find a significant detection of a residual signal beyond that, which can be explained by the CMB plus the integrated radio emission from galaxies estimated from existing surveys. This residual signal may be due to an underestimated galactic foreground contribution, an unaccounted for contribution of a background of radio sources, or some combination of both. The residual signal is consistent with emission in the form of a power law with amplitude 18.4 ± 2.1 K at 0.31 GHz and a spectral index of −2.57 ± 0.05
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