3,490 research outputs found
The discovery of high power - high synchrotron peak blazars
We study the quasi-simultaneous near-IR, optical, UV, and X-ray photometry of
eleven gamma-ray selected blazars for which redshift estimates larger than 1.2
have been recently provided. Four of these objects turn out to be high-power
blazars with the peak of their synchrotron emission between ~ 3 x 10^15 and ~
10^16 Hz, and therefore of a kind predicted to exist but never seen before.
This discovery has important implications for our understanding of physical
processes in blazars, including the so-called "blazar sequence", and might also
help constraining the extragalactic background light through gamma-ray
absorption since two sources are strongly detected even in the 10 - 100 GeV
Fermi-LAT band. Based on our previous work and their high powers, these sources
are very likely high-redshift flat-spectrum radio quasars with their emission
lines swamped by the non-thermal continuum.Comment: 5 pages, 6 colour figures, accepted for publication in the Monthly
Notices of the Royal Astronomical Societ
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
Protostars: Forges of cosmic rays?
Galactic cosmic rays (CR) are particles presumably accelerated in supernova
remnant shocks that propagate in the interstellar medium up to the densest
parts of molecular clouds, losing energy and their ionisation efficiency
because of the presence of magnetic fields and collisions with molecular
hydrogen. Recent observations hint at high levels of ionisation and at the
presence of synchrotron emission in protostellar systems, which leads to an
apparent contradiction. We want to explain the origin of these CRs accelerated
within young protostars as suggested by observations. Our modelling consists of
a set of conditions that has to be satisfied in order to have an efficient CR
acceleration through diffusive shock acceleration. We analyse three main
acceleration sites, then we follow the propagation of these particles through
the protostellar system up to the hot spot region. We find that jet shocks can
be strong accelerators of CR protons, which can be boosted up to relativistic
energies. Other promising acceleration sites are protostellar surfaces, where
shocks caused by impacting material during the collapse phase are strong enough
to accelerate CR protons. In contrast, accretion flow shocks are too weak to
efficiently accelerate CRs. Though CR electrons are weakly accelerated, they
can gain a strong boost to relativistic energies through re-acceleration in
successive shocks. We suggest a mechanism able to accelerate both CR protons
and electrons through the diffusive shock acceleration mechanism, which can be
used to explain the high ionisation rate and the synchrotron emission observed
towards protostellar sources. The existence of an internal source of energetic
particles can have a strong and unforeseen impact on the ionisation of the
protostellar disc, on the star and planet formation processes, and on the
formation of pre-biotic molecules.Comment: 22 pages, 15 figures, accepted by Astronomy and Astrophysic
BeppoSAX Observations of 1-Jy BL Lacertae Objects - II
We present new BeppoSAX LECS and MECS observations, covering the energy range
0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the
1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which
we interpret as inverse Compton emission. One object shows evidence for a
low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the
synchrotron component merging into the inverse Compton one around ~ 2 keV. A
variable synchrotron tail would explain why the ROSAT spectra of our sources
are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The
broad-band spectral energy distributions fully confirm this picture and model
fits using a synchrotron inverse Compton model allow us to derive the physical
parameters (intrinsic power, magnetic field, etc.) of our sources. By combining
the results of this paper with those previously obtained on other sources we
present a detailed study of the BeppoSAX properties of a well-defined
sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s)
1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous
radio and X-ray powers for the 1-Jy sources in which the X-ray band is
dominated by inverse Compton emission, which points to a strong link between
X-ray and radio emission components in these objects.Comment: 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.htm
An X-ray selected sample of radio-loud quasars
We construct the first X-ray selected sample of broad line radio-loud AGN
from the EMSS survey. In order to test unifying schemes for radio-loud objects,
their spectral and statistical properties (both flat and steep spectrum
objects) are examined and compared with those of other samples of blazars. The
X-ray selection allows us to explore properties of radio-loud quasars 10-100
weaker in the radio band than classical samples. The most convincing
interpretation of our results is that there are no radio-loud quasars whose
synchrotron emission peak reaches the EUV-soft X-ray band at these (radio) flux
levels. Moreover, due to the comparatively weak non-thermal emission, a
quasi-thermal component appears to contribute at optical-UV energies. The
detection of sources at low radio fluxes also reveals a large population of
steep spectrum quasars and the lack of the predicted turnover in the quasar
(radio) counts. The evolution of X-ray selected radio-loud quasars does not
significantly differ from that of radio-selected ones, and flat spectrum and
steep spectrum sources appear to behave quite similarly.Comment: 11 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
The Deep X-ray Radio Blazar Survey (DXRBS). II. New Identifications
We have searched the archived, pointed ROSAT Position Sensitive Proportional
Counter data for blazars by correlating the WGACAT X-ray database with several
publicly available radio catalogs, restricting our candidate list to
serendipitous X-ray sources with a flat radio spectrum (alpha_r <= 0.70). This
makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new
identifications and spectra for 106 sources, including 86 radio-loud quasars,
11 BL Lacertae objects, and 9 narrow-line radio galaxies. Together with our
previously published objects and already known sources, our sample now contains
298 identified objects: 234 radio-loud quasars (181 flat-spectrum quasars: FSRQ
[alpha_r <= 0.50] and 53 steep-spectrum quasars: SSRQ), 36 BL Lacs, and 28
narrow-line radio galaxies. Redshift information is available for 96% of these.
Thus our selection technique is ~ 90% efficient at finding radio-loud quasars
and BL Lacs. Reaching 5 GHz radio fluxes ~ 50 mJy and 0.1-2.0 keV X-ray fluxes
a few x 10^-14 erg/cm^2/s, DXRBS is the faintest and largest flat-spectrum
radio sample with nearly complete (~ 85%) identification. We review the
properties of the DXRBS blazar sample, including redshift distribution and
coverage of the X-ray-radio power plane for quasars and BL Lacs. Additionally,
we touch upon the expanded multiwavelength view of blazars provided by DXRBS.
By sampling for the first time the faint end of the radio and X-ray luminosity
functions, this sample will allow us to investigate the blazar phenomenon and
the validity of unified schemes down to relatively low powers.Comment: 33 pages, 5 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/survey.htm
Mosaico de imagens de satélite do Pantanal no ano 2000.
O Pantanal devido a sua extensĂŁo 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 contrĂĄrio dos demais produtos dessa natureza, apresenta uma maior precisĂŁo de posicionamento em função de um extenso trabalho de campo realizado para o georreferenciamento das imagens. Esse trabalho Ă© o resultado da parceria entre a Embrapa Pantanal e a WWF Brasil e estĂĄ disponĂvel tambĂ©m na homepage da Embrapa Pantanal. Esperamos com essa iniciativa estar cumprindo com nosso objetivo de levar Ă comunidade informaçÔes sobre o Pantanal.bitstream/item/81191/1/DOC43.pd
Protonated CO2 in massive star-forming clumps
Interstellar CO2 is an important reservoir of carbon and oxygen, and one of
the major constituents of the icy mantles of dust grains, but it is not
observable directly in the cold gas because has no permanent dipole moment. Its
protonated form, HOCO+, is believed to be a good proxy for gaseous CO2.
However, it has been detected in only a few star-forming regions so far, so
that its interstellar chemistry is not well understood. We present new
detections of HOCO+ lines in 11 high-mass star-forming clumps. Our observations
increase by more than three times the number of detections in star-forming
regions so far. We have derived beam-averaged abundances relative to H2 in
between 0.3 and 3.8 x 10^{-11}. We have compared these values with the
abundances of H13CO+, a possible gas-phase precursor of HOCO+, and CH3OH, a
product of surface chemistry. We have found a positive correlation with H13CO+,
while with CH3OH there is no correlation. We suggest that the gas-phase
formation route starting from HCO+ plays an important role in the formation of
HOCO+, perhaps more relevant than protonation of CO2 (upon evaporation of this
latter from icy dust mantles).Comment: 5 pages, 4 figures, 1 table, accepted for publication in MNRA
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
Proof Systems for Retracts in Simply Typed Lambda Calculus
Abstract. This paper concerns retracts in simply typed lambda calculus assuming ÎČη-equality. We provide a simple tableau proof system which characterises when a type is a retract of another type and which leads to an exponential decision procedure.
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