606 research outputs found
New Testament Baptism
https://digitalcommons.acu.edu/crs_books/1583/thumbnail.jp
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
Does the gamma-ray flux of the blazar 3C 454.3 vary on sub-hour timescales?
In the early days of April 2010, the blazar 3C 454.3 (z=0.859) underwent a
strong gamma-ray outburst, reaching fluxes (E > 100 MeV) in excess of 10^-5 ph
cm^-2 s^-1. The Fermi Gamma ray Space Telescope performed a 200 ks long pointed
observation starting from 5 April 2010 19:38 UTC. This allowed us to try
probing the variability of the gamma-ray emission on timescales of hours or
less. We found the variability on a few hours timescale. On sub-hour timescale
we found no evidence of significant variability, although the present
statistics is not yet conclusive and further observations are needed.Comment: 4 pages, 2 figures. Accepted for publication on MNRAS. Section 1
completely rewritten and enlarge
Christian Solutions to Modern Problems
https://digitalcommons.acu.edu/crs_books/1171/thumbnail.jp
First evidence for a gravitational lensing-induced echo in gamma rays with Fermi LAT
Aims. This article shows the first evidence for gravitational lensing
phenomena in high energy gamma-rays. This evidence comes from the observation
of a gravitational lens induced echo in the light curve of the distant blazar
PKS 1830-211. Methods. Traditional methods for the estimation of time delays in
gravitational lensing systems rely on the cross-correlation of the light curves
of the individual images. In this paper, we use 300 MeV-30 GeV photons detected
by the Fermi-LAT instrument. The Fermi-LAT instrument cannot separate the
images of known lenses. The observed light curve is thus the superposition of
individual image light curves. The Fermi-LAT instrument has the advantage of
providing long, evenly spaced, time series. In addition, the photon noise level
is very low. This allows to use directly Fourier transform methods. Results. A
time delay between the two compact images of PKS 1830-211 has been searched for
both by the autocorrelation method and the "double power spectrum" method. The
double power spectrum shows a 3 {\sigma} evidence for a time delay of 27.5
1.3 days, consistent with the result from Lovell et al. (1998). The
relative uncertainty on the time delay estimation is reduced from 20% to 5%.Comment: submitted to A&
On the Properties of Plastic Ablators in Laser-Driven Material Dynamics Experiments
Radiation hydrodynamics simulations were used to study the effect of plastic
ablators in laser-driven shock experiments. The sensitivity to composition and
equation of state was found to be 5-10% in ablation pressure. As was found for
metals, a laser pulse of constant irradiance gave a pressure history which
decreased by several percent per nanosecond. The pressure history could be made
more constant by adjusting the irradiance history. The impedance mismatch with
the sample gave an increase o(100%) in the pressure transmitted into the
sample, for a reduction of several tens of percent in the duration of the peak
load applied to the sample, and structured the release history by adding a
release step to a pressure close to the ablation pressure. Algebraic relations
were found between the laser pulse duration, the ablator thickness, and the
duration of the peak pressure applied to the sample, involving quantities
calculated from the equations of state of the ablator and sample using shock
dynamics.Comment: Typos fixe
Cold ultrarelativistic pulsar winds as potential sources of galactic gamma-ray lines above 100 GeV
The evidence of a line-like spectral feature at 130 GeV recently reported
from some parts of the galactic plane poses serious challenges for any
interpretation of this surprise discovery. It is generally believed that the
unusually narrow profile of the spectral line cannot be explained by
conventional processes in astrophysical objects, and, if real, is likely to be
associated with Dark Matter. In this paper we argue that cold ultrarelativistic
pulsar winds can be alternative sources of very narrow gamma-ray lines. We
demonstrate that Comptonization of a cold ultrarelativistic electron-positron
pulsar wind in the deep Klein-Nishina regime can readily provide very narrow
distinct gamma-ray line features. To verify this prediction, we produced photon
count maps based on the Fermi LAT data in the energy interval 100 to 140 GeV.
We confirm earlier reports of the presence of marginal gamma-ray line-like
signals from three regions of the galactic plane. Although the maps show some
structure inside these regions, unfortunately the limited photon statistics do
not allow any firm conclusion in this regard. The confirmation of 130 GeV line
emission by low-energy threshold atmospheric Cherenkov telescope systems, in
particular by the new 27 m diameter dish of the H.E.S.S. array, would be
crucial for resolving the spatial structure of the reported hotspots, and thus
for distinguishing between the Dark Matter and Pulsar origins of the `Fermi
Lines'.Comment: 5 pages. 4 figure
A Survey for Transient Astronomical Radio Emission at 611 MHz
We have constructed and operated the Survey for Transient Astronomical Radio
Emission (STARE) to detect transient astronomical radio emission at 611 MHz
originating from the sky over the northeastern United States. The system is
sensitive to transient events on timescales of 0.125 s to a few minutes, with a
typical zenith flux density detection threshold of approximately 27 kJy. During
18 months of around-the-clock observing with three geographically separated
instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these
events were rejected as locally generated interference, determined by requiring
the simultaneous observation of an event at all three sites for it to be
identified as having an astronomical origin. The remaining 3,898 events have
been found to be associated with 99 solar radio bursts. These results
demonstrate the remarkably effective RFI rejection achieved by a coincidence
technique using precision timing (such as GPS clocks) at geographically
separated sites. The non-detection of extra-solar bursting or flaring radio
sources has improved the flux density sensitivity and timescale sensitivity
limits set by several similar experiments in the 1970s. We discuss the
consequences of these limits for the immediate solar neighborhood and the
discovery of previously unknown classes of sources. We also discuss other
possible uses for the large collection of 611 MHz monitoring data assembled by
STARE.Comment: 24 pages, 6 figures; to appear in PAS
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