920 research outputs found
Electromagnetic interferences from plasmas generated in meteoroids impacts
It is shown that the plasma, generated during an impact of a meteoroid with
an artificial satellite, can produce electromagnetic radiation below the
microwave frequency range. This interference is shown to exceed local noise
sources and might disturb regular satellite operations.Comment: 6 pages, no figures. This version macthes the published versio
Study of the variability of Blazars gamma-ray emission
The gamma-ray emission of blazar jets shows a pronounced variability and this
feature provides limits to the size and to the speed of the emitting region. We
study the gamma-ray variability of bright blazars using data from the first 18
months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space
Telescope. From the daily light-curves of the blazars characterized by a
remarkable activity, we firstly determine the minimum variability time-scale,
giving an upper limit for the size of the emitting region of the sources,
assumed to be spheroidal blobs in relativistic motion. These regions must be
smaller than ~10^-3 parsec. Another interesting time-scale is the duration of
the outbursts. We conclude that they cannot correspond to radiation produced by
a single blob moving relativistically along the jet, but they are either the
signature of emission from a standing shock extracting energy from a modulated
jet, or the superposition of a number of flares occurring on a shorter
time-scale. We also derive lower limits on the bulk Lorentz factor needed to
make the emitting region transparent for gamma-rays interacting through
photon-photon collisions.Comment: 11 pages, 7 figures, accepted for publication on Advances in Space
Research. Poster presented at COSPAR 2010 (Bremen), event E11 (Time
variability at high energies: a probe of AGN physics
Blazar nuclei in radio-loud narrow-line Seyfert 1?
It has been suggested that some radio-loud narrow-line Seyfert 1 contain
relativistic jets, on the basis of their flat-spectrum radio nuclei and studies
on variability. We present preliminary results of an ongoing investigation of
the X-ray and multiwavelength properties of 5 radio-loud NLS1 based on archival
data from Swift and XMM-Newton. Some sources present interesting
characteristics, very uncharacteristic for a radio-quiet narrow-line Seyfert 1,
such as very hard X-ray spectra, and correlated optical and ultraviolet
variability. However, none of the studied sources show conclusive evidence for
relativistic jets. gamma-ray observations with Fermi are strongly recommended
to definitely decide on the presence or not of relativistic jets.Comment: 9 pages, 4 figures. Talk presented at the 37th COSPAR Assembly
(Montreal, Canada, July 13-20, 2008), Session E17. Accepted for publication
on Advances in Space Researc
On radar measurements of the terrestrial mass accretion rate of meteoroids
Radar data recorded during 1993 and 1994 by using the forwardscatter meteor radar of the National Research Council (CNR), enable us to obtain a measurement of the terrestrial mass accretion rate in the cosmic dust mass range 10212 to 1027 kg. This value results to be 5.4 Q 107 kg per year and is in good agreement with previous estimates obtained from other authors (LOVE S. G. and
BROWNLEE D. E., Science, 262 (1993) 550). Calculations are possible only by taking into account some statistical data series and extrapolation toward higher mass ranges appears to be misleading
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