772 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
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