252 research outputs found
Mt. Graham: Optical turbulence vertical distribution at standard and high vertical resolution
A characterization of the optical turbulence vertical distribution and all
the main integrated astroclimatic parameters derived from the CN2 and the wind
speed profiles above Mt. Graham is presented. The statistic includes
measurements related to 43 nights done with a Generalized Scidar (GS) used in
standard configuration with a vertical resolution of ~1 km on the whole 20-22
km and with the new technique (HVR-GS) in the first kilometer. The latter
achieves a resolution of ~ 20-30 m in this region of the atmosphere.
Measurements done in different periods of the year permit us to provide a
seasonal variation analysis of the CN2. A discretized distribution of the
typical CN2 profiles useful for the Ground Layer Adaptive Optics (GLAO)
simulations is provided and a specific analysis for the LBT Laser Guide Star
system ARGOS case is done including the calculation of the 'gray zones' for J,
H and K bands. Mt. Graham confirms to be an excellent site with median values
of the seeing without dome contribution equal to 0.72", the isoplanatic angle
equal to 2.5" and the wavefront coherence time equal to 4.8 msec. We provide a
cumulative distribution of the percentage of turbulence developed below H*
where H* is included in the (0,1 km) range. We find that 50% of the whole
turbulence develops in the first 80 m from the ground. The turbulence
decreasing rate is very similar to what has been observed above Mauna Kea.Comment: 12 pages, 6 figures, Proc. SPIE Conference "Ground-based and Airborne
Telescopes III", 27 June 2010, San Diego, California, US
The low-frequency radio catalog of flat spectrum sources
A well known property of the gamma-ray sources detected by COS-B in the
1970s, by the Compton Gamma-ray Observatory in the 1990s and recently by the
Fermi observations is the presence of radio counterparts, in particular for
those associated to extragalactic objects. This observational evidence is the
basis of the radio-gamma-ray connection established for the class of active
galactic nuclei known as blazars. In particular, the main spectral property of
the radio counterparts associated with gamma-ray blazars is that they show a
flat spectrum in the GHz frequency range. Our recent analysis dedicated to
search blazar-like candidates as potential counterparts for the unidentified
gamma-ray sources (UGSs) allowed us to extend the radio-gamma-ray connection in
the MHz regime. We also showed that below 1 GHz blazars maintain flat radio
spectra. Thus on the basis of these new results, we assembled a low-frequency
radio catalog of flat spectrum sources built by combining the radio
observations of the Westerbork Northern Sky Survey (WENSS) and of the
Westerbork in the southern hemisphere (WISH) catalog with those of the NRAO
Very Large Array Sky survey (NVSS). This could be used in the future to search
for new, unknown blazar-like counterparts of the gamma-ray sources. First we
found NVSS counterparts of WSRT radio sources and then we selected flat
spectrum radio sources according to a new spectral criterion specifically
defined for radio observations performed below 1 GHz. We also described the
main properties of the catalog listing 28358 radio sources and their logN-logS
distributions. Finally a comparison with with the Green Bank 6-cm radio source
catalog has been performed to investigate the spectral shape of the
low-frequency flat spectrum radio sources at higher frequencies.Comment: 10 pages, 10 figures, 1 table, ApJS published in 2014 (pre-proof
version uploaded
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