4,678 research outputs found
The mass area of jets
We introduce a new characteristic of jets called mass area. It is defined so
as to measure the susceptibility of the jet's mass to contamination from soft
background. The mass area is a close relative of the recently introduced
catchment area of jets. We define it also in two variants: passive and active.
As a preparatory step, we generalise the results for passive and active areas
of two-particle jets to the case where the two constituent particles have
arbitrary transverse momenta. As a main part of our study, we use the mass area
to analyse a range of modern jet algorithms acting on simple one and
two-particle systems. We find a whole variety of behaviours of passive and
active mass areas depending on the algorithm, relative hardness of particles or
their separation. We also study mass areas of jets from Monte Carlo simulations
as well as give an example of how the concept of mass area can be used to
correct jets for contamination from pileup. Our results show that the
information provided by the mass area can be very useful in a range of
jet-based analyses.Comment: 36 pages, 12 figures; v2: improved quality of two plots, added entry
in acknowledgments, nicer form of formulae in appendix A; v3: added section
with MC study and pileup correction, version accepted by JHE
Boosted top: experimental tools overview
An overview of tools and methods for the reconstruction of high-boost top
quark decays at the LHC is given in this report. The focus is on hadronic
decays, in particular an overview of the current status of top quark taggers in
physics analyses is presented. The most widely used jet substructure
techniques, normally used in combination with top quark taggers, are reviewed.
Special techniques to treat pileup in large cone jets are described, along with
a comparison of the performance of several boosted top quark reconstruction
techniques.Comment: Proceedings from the 7th International Workshop on Top Quark Physics,
Cannes, Franc
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
The Curious Adventure of the Ultrahigh Energy Cosmic Rays
These lectures discuss the mysteries involving the production and
extragalactic propagation of ultrahigh energy cosmic rays and suggested
possible solutions.Comment: Lectures given at the D. Chalonge Euroschool, Erice, Italy, November
2000, 25 pages, 7 ps figs., expanded revision with color fig.
Relations between concurrent hard X-ray sources in solar flares
Context: Solar flares release a large fraction of their energy into
non-thermal electrons, but it is not clear where and how. Bremsstrahlung X-rays
are observed from the corona and chromosphere.
Aims: We aim to characterize the acceleration process by the coronal source
and its leakage toward the footpoints in the chromosphere. The relations
between the sources reflect the geometry and constrict the configuration of the
flare.
Methods: We studied solar flares of GOES class larger than M1 with three or
more hard X-ray sources observed simultaneously in the course of the flare. The
events were observed with the X-ray satellite RHESSI from February 2002 until
July 2005. We used imaging spectroscopy methods to determine the spectral
evolution of each source in each event. The images of all of the five events
show two sources visible only at high energies (footpoints) and one source only
visible at low energies (coronal or looptop source, in two cases situated over
the limb).
Results: We find soft-hard-soft behavior in both, coronal source and
footpoints. The coronal source is nearly always softer than the footpoints. The
footpoint spectra differ significantly only in one event out of five.
Conclusions: The observations are consistent with acceleration in the coronal
source and an intricate connection between the corona and chromosphere.Comment: accepted for publication in A&A, 11 pages, 9 figure
Chandra Snapshot Observations of Low-Luminosity AGNs with a Compact Radio Source
The results of Chandra snapshot observations of 11 LINERs (Low-Ionization
Nuclear Emission-line Regions), three low-luminosity Seyfert galaxies, and one
HII-LINER transition object are presented. Our sample consists of all the
objects with a flat or inverted spectrum compact radio core in the VLA survey
of 48 low-luminosity AGNs (LLAGNs) by Nagar et al. (2000). An X-ray nucleus is
detected in all galaxies except one and their X-ray luminosities are in the
range 5x10^38 to 8x10^41 erg/s. The X-ray spectra are generally steeper than
expected from thermal bremsstrahlung emission from an advection-dominated
accretion flow (ADAF). The X-ray to Halpha luminosity ratios for 11 out of 14
objects are in good agreement with the value characteristic of LLAGNs and more
luminous AGNs, and indicate that their optical emission lines are predominantly
powered by a LLAGN. For three objects, this ratio is less than expected.
Comparing with properties in other wavelengths, we find that these three
galaxies are most likely to be heavily obscured AGN. We use the ratio RX = \nu
L\nu (5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure
of radio loudness. In contrast to the usual definition of radio loudness (RO =
L\nu(5 GHz)/L\nu(B)), RX can be used for heavily obscured (NH >~ 10^23 cm^-2,
AV>50 mag) nuclei. Further, with the high spatial resolution of Chandra, the
nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear
optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert
galaxies, quasars and radio galaxies and confirm the suggestion that a large
fraction of LLAGNs are radio loud.Comment: 15 pages, accepted for publication in Ap
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