5,677 research outputs found
Super-hydrodynamic limit in interacting particle systems
This paper is a follow-up of the work initiated in [3], where it has been
investigated the hydrodynamic limit of symmetric independent random walkers
with birth at the origin and death at the rightmost occupied site. Here we
obtain two further results: first we characterize the stationary states on the
hydrodynamic time scale and show that they are given by a family of linear
macroscopic profiles whose parameters are determined by the current reservoirs
and the system mass. Then we prove the existence of a super-hyrdrodynamic time
scale, beyond the hydrodynamic one. On this larger time scale the system mass
fluctuates and correspondingly the macroscopic profile of the system randomly
moves within the family of linear profiles, with the randomness of a Brownian
motion.Comment: 22 page
A multi-frequency study of the SZE in giant radio galaxies
Radio-galaxy (RG) lobes contain relativistic electrons embedded in a tangled
magnetic field that produce, in addition to low-frequency synchrotron radio
emission, inverse-Compton scattering (ICS) of the cosmic microwave background
(CMB) photons. This produces a relativistic, non-thermal Sunyaev-Zel'dovich
effect (SZE). We study the spectral and spatial properties of the non-thermal
SZE in a sample of radio galaxies and make predictions for their detectability
in both the negative and the positive part of the SZE, with space experiments
like Planck, OLIMPO, and Herschel-SPIRE. These cover a wide range of
frequencies, from radio to sub-mm. We model the SZE in a general formalism that
is equivalent to the relativistic covariant one and describe the electron
population contained in the lobes of the radio galaxies with parameters derived
from their radio observations, namely, flux, spectral index, and spatial
extension. We further constrain the electron spectrum and the magnetic field of
the RG lobes using X-ray, gamma-ray, and microwave archival observations. We
determine the main spectral features of the SZE in RG lobes, namely, the
minimum, the crossover, and the maximum of the SZE. We show that these typical
spectral features fall in the frequency ranges probed by the available space
experiments. We provide the most reliable predictions for the amplitude and
spectral shape of the SZE in a sample of selected RGs with extended lobes. In
three of these objects, we also derive an estimate of the magnetic field in the
lobe at the muG level by combining radio (synchrotron) observations and X-ray
(ICS) observations. These data, together with the WMAP upper limits, set
constraints on the minimum momentum of the electrons residing in the RG lobes
and allow realistic predictions for the visibility of their SZE to be derived
with Planck, OLIMPO, and Herschel-SPIRE. [abridged]Comment: 26 pages, 21 figures; Astronomy and Astrophysics, in pres
Large Radio Telescopes for Anomalous Microwave Emission Observations
We discuss in this paper the problem of the Anomalous Microwave Emission
(AME) in the light of ongoing or future observations to be performed with the
largest fully steerable radio telescope in the world. High angular resolution
observations of the AME will enable astronomers to drastically improve the
knowledge of the AME mechanisms as well as the interplay between the different
constituents of the interstellar medium in our galaxy. Extragalactic
observations of the AME have started as well, and high resolution is even more
important in this kind of observations. When cross-correlating with IR-dust
emission, high angular resolution is also of fundamental importance in order to
obtain unbiased results. The choice of the observational frequency is also of
key importance in continuum observation. We calculate a merit function that
accounts for the signal-to-noise ratio (SNR) in AME observation given the
current state-of-the-art knowledge and technology. We also include in our merit
functions the frequency dependence in the case of multifrequency observations.
We briefly mention and compare the performance of four of the largest
radiotelescopes in the world and hope the observational programs in each of
them will be as intense as possible.Comment: Review accepted for publication in Advances in Astronom
A cryogenic waveplate rotator for polarimetry at mm and sub-mm wavelengths
Mm and sub-mm waves polarimetry is the new frontier of research in Cosmic
Microwave Background and Interstellar Dust studies. Polarimeters working in the
IR to MM range need to be operated at cryogenic temperatures, to limit the
systematic effects related to the emission of the polarization analyzer. In
this paper we study the effect of the temperature of the different components
of a waveplate polarimeter, and describe a system able to rotate, in a
completely automated way, a birefringent crystal at 4K. We simulate the main
systematic effects related to the temperature and non-ideality of the optical
components in a Stokes polarimeter. To limit these effects, a cryogenic
implementation of the polarimeter is mandatory. In our system, the rotation
produced by a step motor, running at room temperature, is transmitted down to
cryogenic temperatures by means of a long shaft and gears running on custom
cryogenic bearings. Our system is able to rotate, in a completely automated
way, a birefringent crystal at 4K, dissipating only a few mW in the cold
environment. A readout system based on optical fibers allows to control the
rotation of the crystal to better than 0.1{\deg}. This device fulfills the
stringent requirements for operation in cryogenic space experiments, like the
forthcoming PILOT, BOOMERanG and LSPE.Comment: Submitted to Astronomy and Astrophysics. v1: 10 pages, 8 figures. v2:
corrected labels for the bibliographic references (no changes in the
bibliography). v3: revised version. 9 pages, 7 figures. Added a new figure.
Updated with a more realistic simulation for the interstellar dust and with
the latest cryogenic test
Optimal strategy for polarization modulation in the LSPE-SWIPE experiment
Context. Cosmic microwave background (CMB) B-mode experiments are required to control systematic effects with an unprecedented level of accuracy. Polarization modulation by a half wave plate (HWP) is a powerful technique able to mitigate a large number of the instrumental systematics. Aims. Our goal is to optimize the polarization modulation strategy of the upcoming LSPE-SWIPE balloon-borne experiment, devoted to the accurate measurement of CMB polarization at large angular scales. Methods. We departed from the nominal LSPE-SWIPE modulation strategy (HWP stepped every 60 s with a telescope scanning at around 12 deg/s) and performed a thorough investigation of a wide range of possible HWP schemes (either in stepped or continuously spinning mode and at different azimuth telescope scan-speeds) in the frequency, map and angular power spectrum domain. In addition, we probed the effect of high-pass and band-pass filters of the data stream and explored the HWP response in the minimal case of one detector for one operation day (critical for the single-detector calibration process). We finally tested the modulation performance against typical HWP-induced systematics. Results. Our analysis shows that some stepped HWP schemes, either slowly rotating or combined with slow telescope modulations, represent poor choices. Moreover, our results point out that the nominal configuration may not be the most convenient choice. While a large class of spinning designs provides comparable results in terms of pixel angle coverage, map-making residuals and BB power spectrum standard deviations with respect to the nominal strategy, we find that some specific configurations (e.g., a rapidly spinning HWP with a slow gondola modulation) allow a more efficient polarization recovery in more general real-case situations. Conclusions. Although our simulations are specific to the LSPE-SWIPE mission, the general outcomes of our analysis can be easily generalized to other CMB polarization experiments
Cold-electron bolometers for future mm and sub-mm sky surveys
Future sky surveys in the mm/sub-mm range, like the forthcoming balloon-borne
missions LSPE, OLIMPO, SPIDER etc., will need detectors insensitive to cosmic
rays (CRs) and with a NEP of the order of W/sqrt(Hz).
The Cold-Electron Bolometers (CEBs) technology is promising, having the
required properties, since the absorber volume is extremely small and the
electron system of the absorber is thermally insulated from the phonon system.
We have developed an experimental setup to test the optical performance and the
CRs insensitivity of CEBs, with the target of integrating them in the OLIMPO
and LSPE focal planes.Comment: 6 figure
Women on boards and corporate environmental performance in Italian companies: The importance of nomination background
On compressive radial tidal forces
Radial tidal forces can be compressive instead of disruptive, a possibility
that is frequently overlooked in high level physics courses. For example,
radial tidal compression can emerge in extended stellar systems containing a
smaller stellar cluster. For particular conditions the tidal field produced by
this extended mass distribution can exert on the cluster it contains
compressive effects instead of the common disruptive forces. This interesting
aspect of gravity can be derived from standard relations given in many
textbooks and introductory courses in astronomy and can serve as an opportunity
to look closer at some aspects of gravitational physics, stellar dynamics, and
differential geometry. The existence of compressive tides at the center of huge
stellar systems might suggest new evolutionary scenarios for the formation of
stars and primordial galactic formation processes.Comment: 22 pages, 2 figure
Low-resolution spectroscopy of the Sunyaev-Zel'dovich effect and estimates of cluster parameters
The Sunyaev-Zel'dovich (SZ) effect is a powerful tool for studying clusters
of galaxies and cosmology. Large mm-wave telescopes are now routinely detecting
and mapping the SZ effect in a number of clusters, measure their comptonisation
parameter and use them as probes of the large-scale structure and evolution of
the universe. We show that estimates of the physical parameters of clusters
(optical depth, plasma temperature, peculiar velocity, non-thermal components
etc.) obtained from ground-based multi-band SZ photometry can be significantly
biased, owing to the reduced frequency coverage, to the degeneracy between the
parameters and to the presence of a number of independent components larger
than the number of frequencies measured. We demonstrate that low-resolution
spectroscopic measurements of the SZ effect that also cover frequencies
GHz are effective in removing the degeneracy. We used accurate simulations of
observations with lines-of-sight through clusters of galaxies with different
experimental configurations (4-band photometers, 6-band photometer, multi-range
differential spectrometer, full coverage spectrometers) and different
intracluster plasma stratifications. We find that measurements carried out with
ground-based few-band photometers are biased towards high electron temperatures
and low optical depths, and require coverage of high frequency and/or
independent complementary observations to produce unbiased information; a
differential spectrometer that covers 4 bands with a resolution of $\sim 6 \
GHz$ eliminates most if not all bias; full-range differential spectrometers are
the ultimate resource that allows a full recovery of all parameters.Comment: in pres
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