2,008 research outputs found
The Blazar Sequence: Validity and Predictions
The "blazar sequence" posits that the most powerful BL Lacertae objects and
flat-spectrum radio quasars should have relatively small synchrotron peak
frequencies, nu_peak, and that the least powerful such objects should have the
highest nu_peak values. This would have strong implications for our
understanding of jet formation and physics and the possible detection of
powerful, moderately high-redshift TeV blazars. I review the validity of the
blazar sequence by using the results of very recent surveys and compare its
detailed predictions against observational data. I find that the blazar
sequence in its simplest form is ruled out. However, powerful flat-spectrum
radio quasars appear not to reach the nu_peak typical of BL Lacs. This could
indeed be related to some sort of sequence, although it cannot be excluded that
it is instead due to a selection effect.Comment: 9 pages, 4 figures, invited talk at the Workshop "The Multi-messenger
approach to high energy gamma-ray sources", Barcelona, Spain, July 4-7, 2006,
to appear in the proceeding
Mosaico de imagens de satélite do Pantanal no ano 2000.
O Pantanal devido a sua extensão e dificuldade de acesso representa um desafio para o desenvolvimento de atividades de produção e conservação da natureza. Nesse sentido as tecnologias de Sensoriamento Remoto podem contribuir muito para o conhecimento e manejo desse ecossistema. O presente trabalho apresenta o mosaico de imagens Landsat 7 ETM do ano 2000 do Pantanal, onde ao contrário dos demais produtos dessa natureza, apresenta uma maior precisão de posicionamento em função de um extenso trabalho de campo realizado para o georreferenciamento das imagens. Esse trabalho é o resultado da parceria entre a Embrapa Pantanal e a WWF Brasil e está disponível também na homepage da Embrapa Pantanal. Esperamos com essa iniciativa estar cumprindo com nosso objetivo de levar à comunidade informações sobre o Pantanal.bitstream/item/81191/1/DOC43.pd
Why is CDMA the solution for mobile satellite communication
It is demonstrated that spread spectrum Code Division Multiple Access (CDMA) systems provide an economically superior solution to satellite mobile communications by increasing the system maximum capacity with respect to single channel per carrier Frequency Division Multiple Access (FDMA) systems. Following the comparative analysis of CDMA and FDMA systems, the design of a model that was developed to test the feasibility of the approach and the performance of a spread spectrum system in a mobile environment. Results of extensive computer simulations as well as laboratory and field tests results are presented
On Quasar Masses and Quasar Host Galaxies
The mass of massive black holes in quasar cores can be deduced using the
typical velocities of Hb-emitting clouds in the Broad Line Region (BLR) and the
size of this region. However, this estimate depends on various assumptions and
is susceptible to large systematic errors. The Hb-deduced black hole mass in a
sample of 14 bright quasars is found here to correlate with the quasar host
galaxy luminosity, as determined with the Hubble Space Telescope (HST). This
correlation is similar to the black hole mass vs. bulge luminosity correlation
found by Magorrian et al. in a sample of 32 nearby normal galaxies. The
similarity of the two correlations is remarkable since the two samples involve
apparently different types of objects and since the black hole mass estimates
in quasars and in nearby galaxies are based on very different methods.
This similarity provides a ``calibration'' of the Hb-deduced black hole mass
estimate, suggesting it is accurate to +-0.5 on log scale. The similarity of
the two correlations also suggests that quasars reside in otherwise normal
galaxies, and that the luminosity of quasar hosts can be estimated to +-0.5 mag
based on the quasar continuum luminosity and the Hb line width. Future imaging
observations of additional broad-line active galaxies with the HST are required
in order to explore the extent, slope, and scatter of the black hole mass vs.
host bulge luminosity correlation in active galaxies.Comment: Accepted for publication in ApJ Letters, 7 pages, aas2pp4.st
X-Ray Spectral Variability of PKS 2005-489 During the Spectacular November 1998 Flare
We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-ray
Timing Explorer (RXTE) in October-December 1998. During these months, the
source underwent a spectacular flare; at its peak on November 10, its 2-10 keV
flux was , over 30 times
brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS
2005-489 hardened considerably, reaching near maximum. During the declining phase, the X-ray spectrum
steepened rapidly, reaching , then became somewhat harder
towards the end of December (). While such behavior has been
seen before, the simplicity, magnitude and duration of this flare allowed us to
study it in great detail. We argue that this flare was caused by either the
injection of particles into the jet or {\it in situ} particle acceleration, and
that the spectral steepening which followed the flare maximum was the result of
synchrotron cooling. Contrary to other recently observed blazar flares (e.g.,
Mkn 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the
location of the synchrotron peak during this flare.Comment: ApJ Letters in press, 6 pages, 2 figures Corrected reference
VLA Observations of a New Population of Blazars
We present the first deep VLA radio images of flat-spectrum radio quasars
(FSRQ) with multiwavelength emission properties similar to those of BL Lacs
with synchrotron X-rays. Our observations of twenty-five of these sources show
that their radio morphologies are similar to those of other radio quasars.
However, their range of extended powers is more similar to that of BL Lacertae
objects (BL Lacs) and extends down to the low values typical of FR I radio
galaxies. Five out of our nine lobe-dominated sources have extended radio
powers in the range typical of both FR I and FR II radio galaxies, but their
extended radio structure is clearly FR II-like. Therefore, we have not yet
found a large population of radio quasars hosted by FR Is. Two thirds of our
sources have a core-dominated radio morpholgy and thus X-rays likely dominated
by the jet. We find that their ratios of radio core to total X-ray luminosity
are low and in the regime indicative of synchrotron X-rays. This result shows
that also blazars with strong emission lines can produce jets of high-energy
synchrotron emission and undermines at least in part the ``blazar sequence''
scenario which advocates that particle Compton cooling by an external radiation
field governs the frequency of the synchrotron emission peak.Comment: 26 pages, 33 figures. Accepted for publication in Ap
The Sedentary Multi-Frequency Survey. I. Statistical Identification and Cosmological Properties of HBL BL Lacs
We have assembled a multi-frequency database by cross-correlating the NVSS
catalog of radio sources with the RASSBSC list of soft X-ray sources, obtaining
optical magnitude estimates from the Palomar and UK Schmidt surveys as provided
by the APM and COSMOS on-line services. By exploiting the nearly unique
broad-band properties of High-Energy Peaked (HBL) BL Lacs we have statistically
identified a sample of 218 objects that is expected to include about 85% of BL
Lacs and that is therefore several times larger than all other published
samples of HBLs. Using a subset (155 objects) that is radio flux limited and
statistically well-defined we have derived the \vovm distribution and the
LogN-LogS of extreme HBLs (fx/fr >= 3E-10 erg/cm2/s/Jansky) down to 3.5 mJy. We
find that the LogN-LogS flattens around 20 mJy and that = 0.42 +/- 0.02.
This extends to the radio band earlier results, based on much smaller X-ray
selected samples, about the anomalous cosmological observational properties of
HBL BL Lacs. A comparison with the expected radio LogN-LogS of all BL Lacs
(based on a beaming model) shows that extreme HBLs make up roughly 2% of the BL
Lac population, independently of radio flux. This result, together with the
flatness of the radio logN-logS at low fluxes, is in contrast with the
predictions of a recent model which assumes an anti-correlation between peak
frequency and bolometric luminosity. The extreme fx/fr flux ratios and high
X-ray fluxes of these BL Lacs makes them good candidate TeV sources, some of
the brighter (and closer) ones possibly detectable with the current generation
of Cerenkov telescopes.Comment: 11 pages, LaTeX, 6 ps figures. Accepted for publication in MNRA
From M-ary Query to Bit Query: a new strategy for efficient large-scale RFID identification
The tag collision avoidance has been viewed as one of the most important research problems in RFID communications and bit tracking technology has been widely embedded in query tree (QT) based algorithms to tackle such challenge. Existing solutions show further opportunity to greatly improve the reading performance because collision queries and empty queries are not fully explored. In this paper, a bit query (BQ) strategy based Mary query tree protocol (BQMT) is presented, which can not only eliminate idle queries but also separate collided tags into many small subsets and make full use of the collided bits. To further optimize the reading performance, a modified dual prefixes matching (MDPM) mechanism is presented to allow multiple tags to respond in the same slot and thus significantly reduce the number of queries. Theoretical analysis and simulations are supplemented to validate the effectiveness of the proposed BQMT and MDPM, which outperform the existing QT-based algorithms. Also, the BQMT and MDPM can be combined to BQMDPM to improve the reading performance in system efficiency, total identification time, communication complexity and average energy cost
Banco de dados geográfico da Estrada-Parque-Pantanal, MS: enfoque na atividade de pesca esportiva.
Esta publicação tem por objetivo: Gerar um protótipo de banco de dados georreferenciado sobre a Estrada-Parque-Pantanal para servir de modelo na organização e disponibilização de informações sobre essa unidade de conservação.bitstream/item/81189/1/DOC44.pd
The Deep X-Ray Radio Blazar Survey (DXRBS). III. Radio Number Counts, Evolutionary Properties, and Luminosity Function of Blazars
Our knowledge of the blazar surface densities and luminosity functions, which
are fundamental parameters, relies still on samples at relatively high flux
limits. As a result, our understanding of this rare class of active galactic
nuclei is mostly based on relatively bright and intrinsically luminous sources.
We present the radio number counts, evolutionary properties, and luminosity
functions of the faintest blazar sample with basically complete (~ 95%)
identifications. Based on the Deep X-ray Radio Blazar Survey (DXRBS), it
includes 129 flat-spectrum radio quasars (FSRQ) and 24 BL Lacs down to a 5 GHz
flux and power ~ 50 mJy and ~ 10^{24} W/Hz, respectively, an order of magnitude
improvement as compared to previously published (radio-selected) blazar
samples. DXRBS FSRQ are seen to evolve strongly, up to redshift ~ 1.5, above
which high-power sources show a decline in their comoving space density. DXRBS
BL Lacs, on the other hand, do not evolve. High-energy (HBL) and low-energy
(LBL) peaked BL Lacs share the same lack of cosmological evolution, which is at
variance with some previous results. The observed luminosity functions are in
good agreement with the predictions of unified schemes, with FSRQ getting close
to their expected minimum power. Despite the fact that the large majority of
our blazars are FSRQ, BL Lacs are intrinsically ~ 50 times more numerous.
Finally, the relative numbers of HBL and LBL in the radio and X-ray bands are
different from those predicted by the so-called "blazar sequence" and support a
scenario in which HBL represent a small minority (~ 10%) of all BL Lacs.Comment: 19 pages, 14 figures (1 color), accepted for publication in the
Astrophysical Journa
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