229 research outputs found
Sky Surface Brightness at Mount Graham: UBVRI Science Observations with the Large Binocular Telescope
We present the measurements of sky surface brightness on Mount Graham
International Observatory obtained during the first binocular-mode science runs
at the Large Binocular Telescope (LBT). A total of 860 images obtained on 23
moonless nights in the period Feb 2008-Jun 2008 were analyzed with our data
quality assessment procedure. These data, taken at the solar minimum, show that
Mt.Graham, in photometric conditions, still has one of the darkest skies,
competing with the other first-class observatories. The zenith-corrected values
are 21.98, 22.81, 21.81, 20.82 and 19.78 mag/arcsec^2 in U, B, V R and I,
respectively. In photometric conditions, the sky background is ~0.1
mag/arcsec^2 higher than the median when observing toward Tucson and Phoenix
but it may be up to ~0.5 mag/arcsec^2 higher in non-photometric conditions. The
sky at Mt.Graham is ~0.32 mag/arcsec^2 brighter at airmass ~1.4 than at zenith
but no significant trend was found with the time of the night. We demonstrated
the dependence of the sky background at Mt.Graham on the solar activity for the
first time. In fact in 2008, at B and V bands, the sky was ~0.3 mag /arcsec^2
darker than in 1999-2002. With these results we conclude that Mt.Graham is
still a first-class observing site, comparable to the darkest sites in Hawaii,
Chile and Canary Islands.Comment: 16 pages, 4 Figures, 1 Table. Accepted for publication on PAS
Inverse Compton X-rays from the radio galaxy 3C 219
We report the results from a Chandra observation of the powerful nearby
(z=0.1744) radio galaxy 3C 219. We find evidence for non-thermal X-ray emission
from the radio lobes which fits fairly well with a combination of inverse
Compton scattering of Cosmic Microwave Background radiation and of nuclear
photons with the relativistic electrons in the lobes. The comparison between
radio synchrotron and IC emission yields a magnetic field strength
significantly lower (about a factor 3) than that calculated under minimum
energy conditions; the source energetics is then dominated by the relativistic
particles.Comment: 5 pages, 2 color figures, Accepted for publication in MNRAS pink
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El Roque de Los Muchachos Site Characteristics. III. Analysis of Atmospheric Dust and Aerosol Extinction
Canary Islands are normally interested by dominant North-East winds that, in
some meteorological conditions, can transport sand at high altitude from the
Sahara desert. The dust may affect the efficiency of the telescopes and
decreases the transparency of the sky. In order to maximize the scientific
return of the telescopes located at the ORM, we present an analysis of the
atmospheric dust content and its effects on astronomical observations. B, V and
I dust aerosol astronomical extinction are derived. Using a 5 years series
database of data taken from the four channel TNG dust monitor, we compute a
mean hourly and daily values of the dust content. We have detected particles
having size 0.3, 0.5, 1.0 and 5.0 um. Using a power law we have derived the
content of 10.0 um particles. We found a typical local dust concentration
ranging from 3x10^6 particles per cubic meter at 0.3 um, to 10^3 at 5.0 um and
10 at 10.0 um, increasing up to 3 order of magnitudes during the dust storms,
with a relative higher increase of 1.0, 5.0 and 10.0 um particles. The number
of local dust storm events is the same in winter- and summertime, but, the
average background and storm-related increases in the dust concentration in
summer are significantly higher than in winter. In a uniform approximation,
during the dust storms, an average height of the dust layer of 2.5 km above the
telescope is inferred. During the sand storms La Palma Island is affected by an
almost uniform layer extending up to 5 km above the sea level, down, at least
the height of the telescope. The visible extinction is dominated by particles
at 1.0, 5.0 and 10.0 um. In agreement with the results from Carlsberg Automatic
Meridian Circle (CAMC) we find a typical extinction during dust storms of about
0.2 mag/airmass.Comment: Accepted for publication in A&A. 9 pages, 11 figures. This work is
the continuation of a series of papers concerning a detailed study of the
Astroclimatology at ORM. The two previous papers (both Lombardi et al.) have
reference PASP.2006.118.1198-1204 and PASP.2007.119.292-30
El Roque de Los Muchachos Site Characteristics. II. Analysis of Wind, Relative Humidity and Air Pressure
In this paper we present an analysis of wind speed, wind direction, relative
humidity and air pressure taken at TNG, CAMC and NOT at Observatorio del Roque
de Los Muchachos, in the Canary Islands. Data are compared in order to check
local variations and both long term and short term trends of the microclimate.
Furthermore, influence of wind speed on the astronomical seeing is estimated to
the aim to better understand the influence of wide scale parameters on local
meteorological data. The three telescopes show different prevailing wind
direction, wind speed, relative humidity and air pressure confirming
differences in local microclimate. We found that seeing deteriorates when wind
speed is lower than 3.3 m/s. Comparison in terms of wind speed and high
relative humidity (> 90%) shows that TNG seems to have optimal observational
conditions with respect to CAMC and NOT. Air pressure analysis shows that ORM
is dominated by high pressure. Short time variations of pressure anticipate
temperature variations tipically by 2-3 hours, this property vanishes in time
scales higher than some hours and disappear in longer time scales.Comment: 10 pages, 9 figures, accepted for publication in PAS
El Roque de Los Muchachos site characteristics, I.Temperature analysis
In this paper we present an analysis of temperature taken at two telescopes
located at the Observatorio del Roque de Los Muchachos in the Canary Islands.
More than 20 years of measurements at CAMC are included. The analysis of the
data from TNG and CAMC are compared in order to check local variations and long
term trends. Furthermore, the temperatures at different heights are correlated
to the quality of astronomical seeing. We considered the correlation of NAO
Index and annual downtime with mean annual temperatures. The final aim of this
work is to better understand the influence of wide scale parameters on local
meteorological data. The analysis is done using a statistical approach. From
each long series of data we compute the hourly averages and than the monthly
averages in order to reduce the short time fluctuations due to the day/night
cycle. A particular care is used to minimize any effect due to biases in case
of lacking of data. Finally, we compute the annual average from the monthly
ones. The two telescopes show similar trends. There is an increase of
temperatures of about 1.0 deg/10yrs from the annual means and a more rapid
increase of the annual minimums then the maximums. We found that positive NAO
Index reduces the increase of temperatures, and accelerates the decrease.
Moreover, there is no evidence that positive NAO Index corresponds to a lower
number of non-observable nights. Finally, seeing deteriorates when the gradient
of temperatures between 2 and 10 m above the ground is greater than -0.6 deg.Comment: 8 pages, 9 figures, accepted by PAS
Chandra detection of the radio and optical double hot spot of 3C 351
In this letter we report a Chandra X-ray detection of the double northern hot
spot of the radio quasar 3C 351. The hot spot has also been observed in the
optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio
Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are
interpreted as the results of the synchrotron and synchrotron-self-Compton
(SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times
smaller than the equipartition values. In the framework of shock acceleration
theory, we show that the requirement for such a relatively small field strength
is in agreement with the fitted synchrotron spectral models and with the sizes
of the hot spots. Finally, we show that the combination of a lower magnetic
field strength with the high frequencies of the synchrotron cut-off in the
fitted synchrotron spectra provides strong evidence for electron acceleration
in the hot spots.Comment: 16 pag. + 2 .PS figures (fig.2 color), ApJ Letter in pres
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