2,884 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
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
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
Proof Systems for Retracts in Simply Typed Lambda Calculus
Abstract. This paper concerns retracts in simply typed lambda calculus assuming βη-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.
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
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