1,279 research outputs found
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
Radio Luminosities and Classificatory Criteria of BL Lacertae Objects
Using the sample of radio selected BL Lacertae objects (RBLs) and X-ray
selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we
calculated the luminosities of radio, optical and X-ray of each source and made
the statistical analysis among the luminosities at different wave-bands,
broad-band spectral indices from radio to X-ray () and peak
frequencies (). Our results are as follows: (i) there is a positive
correlation between radio luminosity and and a
negative correlation between and . High-energy peak BL Lacs
(HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the
dividing lines are probably those of (erg/sec) and
(or )0.75 for - plot and
those of (erg/sec) and for the
- plot; (ii) there is a weak positive correlation between
optical luminosity and and a negatively weak
correlation between and ; (iii) there is no correlation
between X-ray luminosity and or between and
. From our analysis, we find that synchrotron radiation is the main
X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs.Comment: 9 pages, 3 figures. Submitted to A&
Unresolved Unidentified Source Contribution to the Gamma-ray Background
The large majority of EGRET point sources remain without an identified
low-energy counterpart, and a large fraction of these sources are most likely
extragalactic. Whatever the nature of the extragalactic EGRET unidentified
sources, faint unresolved objects of the same class must have a contribution to
the diffuse extragalactic gamma-ray background (EGRB). Understanding this
component of the EGRB, along with other guaranteed contributions from known
sources, is essential if we are to use this emission to constrain exotic
high-energy physics. Here, we follow an empirical approach to estimate whether
a potential contribution of unidentified sources to the EGRB is likely to be
important, and we find that it is. Additionally, we show how upcoming GLAST
observations of EGRET unidentified sources, as well as of their fainter
counterparts, can be combined with GLAST observations of the Galactic and
extragalactic diffuse backgrounds to shed light on the nature of the EGRET
unidentified sources even without any positional association of such sources
with low-energy counterparts.Comment: 11 pages, 5 figures, submitted to Ap
The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution
We examine the constraints on the luminosity-dependent density evolution
model for the evolution of blazars given the observed spectrum of the diffuse
gamma-ray background (DGRB), blazar source-count distribution, and the blazar
spectral energy distribution sequence model, which relates the observed the
blazar spectrum to its luminosity. We show that the DGRB observed by the Large
Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced
entirely by gamma-ray emission from blazars and nonblazar active galactic
nuclei, and that our blazar evolution model is consistent with and constrained
by the spectrum of the DGRB and flux source-count distribution function of
blazars observed by Fermi-LAT. Our results are consistent with previous work
that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model
includes only three free parameters, and forecasts that >~ 95% of the flux from
blazars will be resolved into point sources by Fermi-LAT with 5 years of
observation, with a corresponding reduction of the flux in the DGRB by a factor
of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's
sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in
Phys. Rev.
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
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
Resolving Non-Determinism in Choreographies
Resolving non-deterministic choices of choreographies is a crucial task. We introduce a novel notion of realisability for choreographies âcalled whole-spectrum implementationâ that rules out deterministic implementations of roles that, no matter which context they are placed in, will never follow one of the branches of a non-deterministic choice. We show that, under some conditions, it is decidable whether an implementation is whole-spectrum. As a case study, we analyse the POP protocol under the lens of whole-spectrum implementation
Testing the Blazar Paradigm: ASCA Observations of FSRQs with Steep Soft X-ray Spectra
We present the first observations at medium-hard X-rays with ASCA in 1998
August--November of four Flat Spectrum Radio Quasars (FSRQs), characterized by
unusually steep soft X-ray spectra (photon index, \Gamma_{0.2-2.4 keV} \sim
2-2.5), as previously measured with ROSAT. Such steep X-ray slopes are similar
to those observed in synchrotron-dominated BL Lacs and are unexpected in the
context of the recent blazar paradigm, where sources with strong emission lines
(such as FSRQs) are dominated in soft X-rays by a flat inverse Compton tail. We
find that the ASCA spectra of the four FSRQs are consistent with a power law
model with \Gamma_{2-10 keV} \sim 1.8, flatter than their ROSAT spectra. This
indicates the onset of an inverse Compton component at energies \gtrsim 2 keV,
in agreement with the blazar unification scheme. However, these objects are
still anomalous within the blazar class for their steep soft X-ray continua
which, together with non-simultaneous data at longer wavelengths, hint at the
possibility that the synchrotron emission extends to soft X-rays. This would
imply an anomalously high synchotron peak frequency for a quasar with luminous
broad lines, challenging current blazar unification schemes. Alternatively, a
plausible explanation for the steep optical-to-soft X-ray continua of the four
FSRQs is thermal emission from the accretion disk, similar to the blazars
3C~273 and 3C~345. In the Appendix, we present fits to the SIS data in an
effort to contribute to the ongoing calibration of the the time-dependence of
the SIS response at low energies.Comment: accepted for publication in The Astrophysical Journa
Black Hole Masses and Host Galaxy Evolution of Radio-loud Active Galactic Nuclei
We report stellar velocity dispersion measurements for a sample of 28 AGN
host galaxies including our previous work. Using the mass-dispersion
() and the fundamental plane relations, we estimate the
black hole mass for a sample of 66 BL Lac objects and investigate the role of
black hole mass in the energetics of BL Lac objects. The black hole mass range
for different BL Lac spectral types is similar, . Neither X-ray nor radio luminosity correlates with black hole
mass. Low-frequency-peaked BL Lac objects have higher Eddington ratios on
average, because of either more beaming or higher intrinsic power. For the
black hole mass range , the radio
luminosity of BL Lac objects and flat-spectrum radio quasars spans over 4
orders of magnitude, with BL Lac objects being low-power AGNs. We also
investigate the evolution of host galaxies for 39 AGNs out to
with measuredstellar velocity dispersions. Comparing the mass-to-light ratio
evolution in the observed frame with population synthesis models, we find that
single burst star formation models with are
consistent with the observations. From our model, we estimated
the intrinsic mass-to-light ratio evolution in the Cousins band, , consistent with that of normal early
type galaxies.Comment: ApJ accepted, 22 pages, 11 figure
PSM in Italy: Troubled RAI in a Troubled Country
The chapter explores public service media in Italy. In the comparative literature the Italian RAI is often taken as a paradigmatic case of a highly (party) politicized public service broadcaster. Political interference has arguably been a constant feature of RAIâs sixty-year-long history, although the forms in which this phenomenon has manifested itself have changed considerably over time. After briefly contextualising historically and comparatively the case of public service media in Italy, the chapter sets out to discuss recent developments, including the effects of recent reforms to RAIâs governance and funding regimes. It then places these developments and the current debate over the role and future of RAI against the backdrop of a changing political landscape, the countryâs ongoing economic problems and major social and cultural transformations
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