92 research outputs found
Radio Spectra and NVSS Maps of Decametric Sources
We constructed radio spectra for ~1400 UTR-2 sources and find that 46% of
them have concave curvature. Inspection of NVSS maps of 700 UTR sources
suggests that half of all UTR sources are either blends of two or more sources
or have an ultra-steep spectrum (USS). The fraction of compact USS sources in
UTR may be near 10%. Using NVSS and the Digitized Sky Survey(s) we expect to
double the UTR optical identification rate from currently ~19%.Comment: 2 pages, no figures; to appear in Proc. "Observational Cosmology with
the New Radio Surveys", eds. M. Bremer, N. Jackson & I. Perez-Fournon, Kluwer
Acad. Pres
Measurement of the Integrated Faraday Rotations of BL Lac Objects
We present the results of multi-frequency polarization VLA observations of
radio sources from the complete sample of northern, radio-bright BL Lac objects
compiled by H. Kuhr and G. Schmidt. These were used to determine the integrated
rotation measures of 18 objects, 15 of which had never been measured
previously, which hindered analysis of the intrinsic polarization properties of
objects in the complete sample. These measurements make it possible to correct
the observed orientations of the linear polarizations of these sources for the
effect of Faraday rotation. The most probable origin for Faraday rotation in
these objects is the Galactic interstellar medium. The results presented
complete measurements of the integrated rotation measures for all 34 sources in
the complete sample of BL Lac objects.Comment: 9 pages, 7 figure
The BL Lac objects OQ 530 and S5 0716+714. Simultaneous observations in the X-rays, radio, optical and TeV bands
We present the results of the BeppoSAX observations of two BL Lacs, OQ 530
and S5 0716+714, as part of a ToO program for the simultaneous observation at
radio, optical, X-ray and TeV energies. Both sources are detected in the LECS
and MECS, with S5 0716+714 visible also in the PDS band, up to about 60 keV.
The X-ray spectra of both sources are better fitted by a double power-law
model, with a steep soft X-ray component flattening at harder energies, with
breaks at 0.3 and 1.5 keV, respectively. The concave shape of the spectra in
both objects is consistent with soft X-rays being produced by the synchrotron
and harder X-rays by the inverse Compton processes. Also the X-ray variability
properties confirm this scenario, in particular for S5 0716+714 our observation
shows variations by about a factor 3 over one hour below 3 keV and no
variability above. Their simultaneous broad band energy spectral distributions
can be successfully interpreted within the frame of a homogeneous synchrotron
and inverse Compton model, including a possible contribution from an external
source of seed photons with the different spectral states of S5 0716+714 being
reproduced by changing the injected power. The resulting parameters are fully
consistent with the two sources being intermediate objects within the
"sequence" scenario proposed for blazars.Comment: 10 pages, 8 figures, accepted by A&
A multifrequency analysis of radio variability of blazars
We have carried out a multifrequency analysis of the radio variability of
blazars, exploiting the data obtained during the extensive monitoring programs
carried out at the University of Michigan Radio Astronomy Observatory (UMRAO,
at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37
GHz). Two different techniques detect, in the Metsahovi light curves, evidences
of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023,
2200+420, and 2251+158). For the last three sources consistent periods are
found also at the three UMRAO frequencies and the Scargle (1982) method yields
an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz
periodicities of 0224+671 and 0945+408 (which were less extensively monitored
at Metsahovi and for which we get a significant false-alarm probability) are
not confirmed by the UMRAO database, where some indications of ill-defined
periods about a factor of two longer are retrieved. We have also investigated
the variability index, the structure function, and the distribution of
intensity variations of the most extensively monitored sources. We find a
statistically significant difference in the distribution of the variability
index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in
the sense that the former objects are more variable. For both populations the
variability index steadily increases with increasing frequency. The
distribution of intensity variations also broadens with increasing frequency,
and approaches a log-normal shape at the highest frequencies. We find that
variability enhances by 20-30% the high frequency counts of extragalactic
radio-sources at bright flux densities, such as those of the WMAP and Planck
surveys.Comment: A&A accepted. 12 pages, 16 figure
Optical and radio behaviour of the BL Lacertae object 0716+714
Eight optical and four radio observatories have been intensively monitoring
the BL Lac object 0716+714 in the last years: 4854 data points have been
collected in the UBVRI bands since 1994, while radio light curves extend back
to 1978. Many of these data are presented here for the first time. The
long-term trend shown by the optical light curves seems to vary with a
characteristic time scale of about 3.3 years, while a longer period of 5.5-6
years seems to characterize the radio long-term variations. In general, optical
colour indices are only weakly correlated with brightness. The radio flux
behaviour at different frequencies is similar, but the flux variation amplitude
decreases with increasing wavelength. The radio spectral index varies with
brightness (harder when brighter), but the radio fluxes seem to be the sum of
two different-spectrum contributions: a steady base level and a harder-spectrum
variable component. Once the base level is removed, the radio variations appear
as essentially achromatic, similarly to the optical behaviour. Flux variations
at the higher radio frequencies lead the lower-frequency ones with week-month
time scales. The behaviour of the optical and radio light curves is quite
different, the broad radio outbursts not corresponding in time to the faster
optical ones and the cross-correlation analysis indicating only weak
correlation with long time lags. However, minor radio flux enhancements
simultaneous with the major optical flares can be recognized, which may imply
that the mechanism producing the strong flux increases in the optical band also
marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for
publication in A&
The ISOPHOT-MAMBO survey of 3CR radio sources: Further evidence for the unified schemes
We present the complete set of ISOPHOT observations of 3CR radio galaxies and
quasars, which are contained in the ISO Data Archive, providing 75 mid- and
far-infrared spectral energy distributions (SEDs) between 5 and 200 micron. For
28 sources they are supplemented with MAMBO 1.2 mm observations and for 15
sources with new submillimetre data from the SCUBA archive.
We check the orientation-dependent unified scheme, in which the powerful FR2
narrow line galaxies are quasars viewed at high inclination, so that their
nuclei are hidden behind a dust torus intercepting the optical-ultraviolet AGN
radiation and reemitting it in the infrared. We find that (1) both the quasars
and the galaxies show a high mid- to far-infrared luminosity ratio typical for
powerful AGNs and (2) -- when matched in 178 MHz luminosity -- both show the
same ratio of isotropic far-infrared to isotropic 178 MHz lobe power.
Therefore, from our large sample investigated here we find strong evidence for
the orientation-dependent unification of the powerful FR2 galaxies with the
quasars.Comment: 16 pages, 7 figures, 3 tables, accepted by Astronomy & Astrophysic
Multi-colour optical monitoring of eight red blazars
We present the observational results of multi-colour optical monitoring of
eight red blazars from 2003 September to 2004 February. The aim of our
monitoring is to investigate the spectral variability as well as the flux
variations at short and long time scales. The observations were carried out
using the 1.0 m robotic telescope of Mt. Lemmon Optical Astronomy Observatory,
in Arizona, USA, the 0.6 m telescope of Sobaeksan Optical Astronomy Observatory
and the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory, in the
Republic of Korea. During the observations, all sources show strong flux
variations with amplitudes of larger than 0.5 mag. Variations with amplitudes
of over 1 mag are found in four sources. Intraday variations with amplitudes
larger than 0.15 mag, and a rapid brightness increase with a rate of ~0.2 mag
per day in four days, are detected in S5 0716+71. We investigate the
relationship between the colour index and source brightness for each source. We
find that two out of three FSRQs tend to be redder when they are brighter, and,
conversely, all BL Lac objects tend to be bluer. In particular, we find a
significant anti-correlation between the V-I colour index and R magnitude for
3C 454.3. This implies that the spectrum became steeper when the source was
brighter, which is opposite to the common trend for blazars. In contrast,
significant positive correlations are found in 3C 66A, S5 0716+71, and BL Lac.
However, there are only very weak correlations for PKS 0735+17 and OJ 287. We
propose that the different relative contributions of the thermal versus
non-thermal radiation to the optical emission may be responsible for the
different trends of the colour index with brightness in FSRQs and BL Lac
objects.Comment: 15 pages, 12 figures. Accepted for publication in A&
Nonthermal Emission from Star-Forming Galaxies
The detections of high-energy gamma-ray emission from the nearby starburst
galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of
star-driven nonthermal processes and phenomena in non-AGN star-forming
galaxies. We review basic aspects of the related processes and their modeling
in starburst galaxies. Since these processes involve both energetic electrons
and protons accelerated by SN shocks, their respective radiative yields can be
used to explore the SN-particle-radiation connection. Specifically, the
relation between SN activity, energetic particles, and their radiative yields,
is assessed through respective measures of the particle energy density in
several star-forming galaxies. The deduced energy densities range from O(0.1)
eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high
star-formation rates.Comment: 17 pages, 5 figures, to be published in Astrophysics and Space
Science Proceeding
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
The sensitivity of a search for sources of TeV neutrinos can be improved by
grouping potential sources together into generic classes in a procedure that is
known as source stacking. In this paper, we define catalogs of Active Galactic
Nuclei (AGN) and use them to perform a source stacking analysis. The grouping
of AGN into classes is done in two steps: first, AGN classes are defined, then,
sources to be stacked are selected assuming that a potential neutrino flux is
linearly correlated with the photon luminosity in a certain energy band (radio,
IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino
production in AGN, this correlation is motivated by hadronic AGN models, as
briefly reviewed in this paper.
The source stacking search for neutrinos from generic AGN classes is
illustrated using the data collected by the AMANDA-II high energy neutrino
detector during the year 2000. No significant excess for any of the suggested
groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic
Testing the Correlation of Ultra-High Energy Cosmic Rays with High Redshift Sources
We study the correlation between compact radio quasars or 3EG gamma-ray
blazars and the arrival directions of cosmic rays above 10^19 eV using an
updated list of air shower detections. Our Monte Carlo simulations reveal no
significant correlations above random and some previous positive results appear
to be an effect of the small sample size. Consequently, unless somehow severely
deflected, there is no evidence for ultra-high energy cosmic ray primaries
being new particles or particles with new interactions beyond the electroweak
scale, produced in high-redshift active galactic nuclei.Comment: 5 latex pages, uses revtex, no figures. revised and expanded versio
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