78 research outputs found
Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has
commenced science observations of the Sun starting in late 2016. Since the Sun
is much larger than the field of view of individual ALMA dishes, the ALMA
interferometer is unable to measure the background level of solar emission when
observing the solar disk. The absolute temperature scale is a critical
measurement for much of ALMA solar science, including the understanding of
energy transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide an
absolute temperature scale, ALMA solar observing will take advantage of the
remarkable fast-scanning capabilities of the ALMA 12m dishes to make
single-dish maps of the full Sun. This article reports on the results of an
extensive commissioning effort to optimize the mapping procedure, and it
describes the nature of the resulting data. Amplitude calibration is discussed
in detail: a path that utilizes the two loads in the ALMA calibration system as
well as sky measurements is described and applied to commissioning data.
Inspection of a large number of single-dish datasets shows significant
variation in the resulting temperatures, and based on the temperature
distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3
mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of
order 100 K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods with very
different levels of solar activity. At a resolution of order 25 arcsec, the 1.3
mm wavelength images show temperatures on the disk that vary over about a 2000
K range.Comment: Solar Physics, accepted: 24 pages, 13 figure
The Case for a Low Extragalactic Gamma-ray Background
Measurements of the diffuse extragalactic gamma-ray background (EGRB) are
complicated by a strong Galactic foreground. Estimates of the EGRB flux and
spectrum, obtained by modeling the Galactic emission, have produced a variety
of (sometimes conflicting) results. The latest analysis of the EGRET data found
an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2
sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB
flux, finding the gamma-ray sky strongly dominated by Galactic foreground even
at high latitudes, with no conclusive evidence for an additional isotropic
component. The gamma-ray intensity measured towards the Galactic poles is
similar to or lower than previous estimates of I_x. The high latitude profile
of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for
|b|>70; overall it exhibits strong Galactic features and is well fit by a
simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99%
confidence level, with evidence for a much lower flux. We show that
correlations with Galactic tracers, previously used to identify the Galactic
foreground and estimate I_x, are not satisfactory; the results depend on the
tracers used and on the part of the sky examined, because the Galactic emission
is not linear in the Galactic tracers and exhibits spectral variations across
the sky. The low EGRB flux favored by our analysis places stringent limits on
extragalactic scenarios involving gamma-ray emission, such as radiation from
blazars, intergalactic shocks and production of ultra-high energy cosmic rays
and neutrinos. We suggest methods by which future gamma-ray missions such as
GLAST and AGILE could indirectly identify the EGRB.Comment: Accepted for publication in JCAP. Increased sizes of polar regions
examined, and added discussion of spectral data. Results unchange
Imprint of Intergalactic Shocks on the Radio Sky
Strong intergalactic shocks are a natural consequence of structure formation
in the universe. They are expected to deposit large fractions of their energy
in relativistic electrons (xi_e~0.05 according to SNR observations) and
magnetic fields (xi_B~0.01 according to cluster halo observations). We
calculate the synchrotron emission from such shocks using an analytical model,
calibrated with a hydrodynamical LCDM simulation. The resulting signal composes
a large fraction of the extragalactic radio background (ERB) below 500 MHz. The
associated angular fluctuations dominate the sky for frequencies nu<10 GHz and
angular scales arcmin-deg (after a modest removal of point sources), provided
that xi_e*xi_B>3*10^-4. The fluctuating signal is most pronounced for nu<500
MHz, dominating the sky even for xi_e*xi_B=5*10^-5. The signal will be easily
observable by next generation radio telescopes such as LOFAR and SKA, and is
marginally observable with present telescopes. It may be identified using
cross-correlations with tracers of large scale structure, possibly even in
existing <10 GHz CMB anisotropy maps and high resolution ~1 GHz radio surveys.
Detection of the signal will provide the first identification of intergalactic
shocks and of the WHIM, and gauge the unknown intergalactic magnetic field. We
show that existing observations of the diffuse <500 MHz radio background are
well fit by a simple, double-disk Galactic model, precluding a direct
identification of the diffuse ERB. Modelling the frequency-dependent anisotropy
pattern observed at very low (1-10 MHz) frequencies can disentangle the
distributions of Galactic cosmic-rays, ionized gas and magnetic fields. Space
missions such as ALFA will thus provide important insight into the structure
and composition of our Galaxy (abridged).Comment: Accepted for publication in ApJ. Presentation improved and references
adde
The Mirror of Anarchy: The Egoism of John Henry Mackay and Dora Marsden
The mirror of anarchy: the egoism of John Henry Mackay and Dora Marsde
- …