62 research outputs found
Is the observed high-frequency radio luminosity distribution of QSOs bimodal?
The distribution of QSO radio luminosities has long been debated in the
literature. Some argue that it is a bimodal distribution, implying that there
are two separate QSO populations (normally referred to as 'radio-loud' and
'radio-quiet'), while others claim it forms a more continuous distribution
characteristic of a single population. We use deep observations at 20 GHz to
investigate whether the distribution is bimodal at high radio frequencies.
Carrying out this study at high radio frequencies has an advantage over
previous studies as the radio emission comes predominantly from the core of the
AGN, hence probes the most recent activity. Studies carried out at lower
frequencies are dominated by the large scale lobes where the emission is built
up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our
sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF
Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of
0.2-0.5 mJy.
No evidence of bimodality is seen in either the 20 GHz luminosity
distribution or in the distribution of the R_20 parameter: the ratio of the
radio to optical luminosities traditionally used to classify objects as being
either radio-loud or radio-quiet. Previous results have claimed that at low
radio luminosities, star formation processes can dominate the radio emission
observed in QSOs. We attempt to investigate these claims by stacking the
undetected sources at 20 GHz and discuss the limitations in carrying out this
analysis. However, if the radio emission was solely due to star formation
processes, we calculate that this corresponds to star formation rates ranging
from ~10 solar masses/yr to ~2300 solar masses/yr.Comment: 13 pages, 11 figures. Accepted for publication in Ap
A 20 GHz bright sample for {\delta} > +72{\deg}: I. Catalogue
During 2010-2011, the Medicina 32-m dish hosted the 7-feed 18-26.5 GHz
receiver built for the Sardinia Radio Telescope, with the goal to perform its
commissioning. This opportunity was exploited to carry out a pilot survey at 20
GHz over the area for {\delta} > + 72.3{\deg}. This paper describes all the
phases of the observations, as they were performed using new hardware and
software facilities. The map-making and source extraction procedures are
illustrated. A customised data reduction tool was used during the follow-up
phase, which produced a list of 73 confirmed sources down to a flux density of
115 mJy. The resulting catalogue, here presented, is complete above 200 mJy.
Source counts are in agreement with those provided by the AT20G survey. This
pilot activity paves the way to a larger project, the K-band Northern Wide
Survey (KNoWS), whose final aim is to survey the whole Northern Hemisphere down
to a flux limit of 50 mJy (5{\sigma}).Comment: 10 pages, 10 figures. Accepted by MNRA
High-frequency radio properties of sources in the Fermi-LAT 1-year Point Source Catalogue
The high-frequency radio sky, like the gamma-ray sky surveyed by the Fermi
satellite, is dominated by flat spectrum radio quasars and BL Lac objects at
bright flux levels. To investigate the relationship between radio and gamma-ray
emission in extragalactic sources, we have cross-matched the Australia
Telescope 20 GHz survey catalog (AT20G) with the Fermi-LAT 1 year Point Source
Catalog (1FGL). The 6.0 sr of sky covered by both catalogs ({\delta} < 0\circ,
|b| > 1.\circ 5) contains 5890 AT20G radio sources and 604 1FGL gamma-ray
sources. The AT20G source positions are accurate to within ~1 arcsec and, after
excluding known Galactic sources, 43% of Fermi 1FGL sources have an AT20G
source within the 95% Fermi confidence ellipse. Monte Carlo tests imply that at
least 95% of these matches are genuine associations. Only five gamma-ray
sources (1% of the Fermi catalog) have more than one AT20G counterpart in the
Fermi error box. The AT20G matches also generally support the active galactic
nucleus (AGN) associations in the First LAT AGN Catalog. We find a trend of
increasing gamma-ray flux density with 20 GHz radio flux density. The Fermi
detection rate of AT20G sources is close to 100% for the brightest 20 GHz
sources, decreasing to 20% at 1 Jy, and to roughly 1% at 100 mJy. Eight of the
matched AT20G sources have no association listed in 1FGL and are presented here
as potential gamma-ray AGNs for the first time.We also identify an alternative
AGN counterpart to one 1FGL source. The percentage of Fermi sources with AT20G
detections decreases toward the Galactic plane, suggesting that the 1FGL
catalog contains at least 50 Galactic gamma-ray sources in the southern
hemisphere that are yet to be identified.Comment: Replaced to match version published in Ap
Observations and properties of candidate high frequency GPS radio sources in the AT20G survey
We used the Australia Telescope Compact Array (ATCA) to obtain 40 GHz and 95
GHz observations of a number of sources that were selected from the Australia
Telescope Compact Array 20 GHz (AT20G) survey . The aim of the observations was
to improve the spectral coverage for sources with spectral peaks near 20 GHz or
inverted (rising) radio spectra between 8.6 GHz and 20 GHz. We present the
radio observations of a sample of 21 such sources along with optical spectra
taken from the ANU Siding Spring Observatory 2.3m telescope and the ESO-New
Technology Telescope (NTT). We find that as a group the sources show the same
level of variability as typical GPS sources, and that of the 21 candidate GPS
sources roughly 60% appear to be genuinely young radio galaxies. Three of the
21 sources studied show evidence of being restarted radio galaxies. If these
numbers are indicative of the larger population of AT20G radio sources then as
many as 400 genuine GPS sources could be contained within the AT20G with up to
25% of them being restarted radio galaxies.Comment: 21 pages, 24 figures, Table 1 truncated at 11 column
Search and modelling of remnant radio galaxies in the LOFAR Lockman Hole field
Accepted for publication in Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics, © 2017 ESO.The phase of radio galaxy evolution after the jets have switched off, often referred to as the remnant phase, is poorly understood and very few sources in this phase are known. In this work we present an extensive search for remnant radio galaxies in the Lockman Hole, a well-studied extragalactic field. We create mock catalogues of low-power radio galaxies based on Monte Carlo simulations to derive first-order predictions of the fraction of remnants in radio flux limited samples for comparison with our Lockman-Hole sample. We have combined LOFAR observations at 150 MHz with public surveys at higher frequencies to perform a complete selection and have used, for the first time, a combination of spectral criteria (e.g. the classical ultra-steep spectral index and high spectral curvature) as well as morphological criteria (e.g. low radio core prominence and relaxed shapes). Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. We have identified 23 candidate remnant radio galaxies which cover a variety of morphologies and spectral characteristics. We suggest that these different properties are related to different stages of the remnant evolution. We find that ultra-steep spectrum remnants represent only a fraction of our remnant sample suggesting a very rapid luminosity evolution of the radio plasma. Results from mock catalogues demonstrate the importance of dynamical evolution in the remnant phase of low-power radio galaxies to obtain fractions of remnant sources consistent with our observations. Moreover, these results confirm that ultra-steep spectrum remnants represent only a subset of the entire population (50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.Peer reviewe
The Australia Telescope 20 GHz Survey: The Source Catalogue
We present the full source catalogue from the Australia Telescope 20 GHz
(AT20G) Survey. The AT20G is a blind radio survey carried out at 20 GHz with
the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the
whole sky south of declination 0 deg. The AT20G source catalogue presented here
is an order of magnitude larger than any previous catalogue of high-frequency
radio sources, and includes 5890 sources above a 20 GHz flux-density limit of
40 mJy. All AT20G sources have total intensity and polarisation measured at 20
GHz, and most sources south of declination -15 deg also have near-simultaneous
flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected
in polarised total intensity at one or more of the three frequencies. We detect
a small but significant population of non-thermal sources that are either
undetected or have only weak detections in low-frequency catalogues. We
introduce the term Ultra-Inverted Spectrum (UIS) to describe these radio
sources, which have a spectral index alpha(5, 20) > +0.7 and which constitute
roughly 1.2 per cent of the AT20G sample. The 20 GHz flux densities measured
for the strongest AT20G sources are in excellent agreement with the WMAP 5-year
source catalogue of Wright et al. (2009), and we find that the WMAP source
catalogue is close to complete for sources stronger than 1.5 Jy at 23 GHz.Comment: 21 pages, accepted for publication in MNRA
Why Have Many of the Brightest Radio-loud Blazars Not Been Detected in Gamma-Rays by Fermi?
We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGN) to examine the gamma-ray detection statistics of the brightest radio-loud blazars in the northern sky. We find that 23% of these AGN were not detected above 0.1 GeV by the Fermi LAT during the 4-year 3FGL catalog period partly because of an instrumental selection effect, and partly due to their lower Doppler boosting factors. Blazars with synchrotron peaks in their spectral energy distributions located below 10^13.4 Hz also tend to have high-energy peaks that lie below the 0.1 GeV threshold of the LAT, and are thus less likely to be detected by Fermi. The non-detected AGN in the 1.5 Jy sample also have significantly lower 15 GHz radio modulation indices and apparent jet speeds, indicating that they have lower than average Doppler factors. Since the effective amount of relativistic Doppler boosting is enhanced in gamma-rays (particularly in the case of external inverse-Compton scattering), this makes them less likely to appear in the 3FGL catalog. Based on their observed properties, we have identified several bright radio-selected blazars that are strong candidates for future detection by Fermi
The Australia Telescope 20 GHz (AT20G) Survey: The Bright Source Sample
The Australia Telescope 20 GHz (AT20G) Survey is a blind survey of the whole
Southern sky at 20 GHz (with follow-up observations at 4.8 and 8.6 GHz) carried
out with the Australia Telescope Compact Array (ATCA) from 2004 to 2007. The
Bright Source Sample (BSS) is a complete flux-limited subsample of the AT20G
Survey catalogue comprising 320 extragalactic (|b|>1.5 deg) radio sources south
of dec = -15 deg with S(20 GHz) > 0.50 Jy. Of these, 218 have near simultaneous
observations at 8 and 5 GHz. In this paper we present an analysis of radio
spectral properties in total intensity and polarisation, size, optical
identifications and redshift distribution of the BSS sources. The analysis of
the spectral behaviour shows spectral curvature in most sources with spectral
steepening that increases at higher frequencies (the median spectral index
\alpha, assuming S\propto \nu^\alpha, decreases from \alpha_{4.8}^{8.6}=0.11
between 4.8 and 8.6 GHz to \alpha_{8.6}^{20}=-0.16 between 8.6 and 20 GHz),
even if the sample is dominated by flat spectra sources (85 per cent of the
sample has \alpha_{8.6}^{20}>-0.5). The almost simultaneous spectra in total
intensity and polarisation allowed us a comparison of the polarised and total
intensity spectra: polarised fraction slightly increases with frequency, but
the shapes of the spectra have little correlation. Optical identifications
provided an estimation of redshift for 186 sources with a median value of 1.20
and 0.13 respectively for QSO and galaxies.Comment: 34 pages, 19 figures, tables of data included, replaced with version
published in MNRA
High-energy sources at low radio frequency : the Murchison Widefield Array view of Fermi blazars
This is the accepted version of the following article: Giroletti, M. et al., A&A, 588 (2016) A141, which has been published in final form at DOI: http://dx.doi.org/10.1051/0004-6361/201527817. This article may be used for non-commercial purposes in accordance with the EDP Sciences self-archiving policies.Low-frequency radio arrays are opening a new window for the study of the sky, both to study new phenomena and to better characterize known source classes. Being flat-spectrum sources, blazars are so far poorly studied at low radio frequencies. We characterize the spectral properties of the blazar population at low radio frequency compare the radio and high-energy properties of the gamma-ray blazar population, and search for radio counterparts of unidentified gamma-ray sources. We cross-correlated the 6,100 deg^2 Murchison Widefield Array Commissioning Survey catalogue with the Roma blazar catalogue, the third catalogue of active galactic nuclei detected by Fermi-LAT, and the unidentified members of the entire third catalogue of gamma-ray sources detected by \fermilat. When available, we also added high-frequency radio data from the Australia Telescope 20 GHz catalogue. We find low-frequency counterparts for 186 out of 517 (36%) blazars, 79 out of 174 (45%) gamma-ray blazars, and 8 out of 73 (11%) gamma-ray blazar candidates. The mean low-frequency (120--180 MHz) blazar spectral index is : blazar spectra are flatter than the rest of the population of low-frequency sources, but are steeper than at GHz frequencies. Low-frequency radio flux density and gamma-ray energy flux display a mildly significant and broadly scattered correlation. Ten unidentified gamma-ray sources have a (probably fortuitous) positional match with low radio frequency sources. Low-frequency radio astronomy provides important information about sources with a flat radio spectrum and high energy. However, the relatively low sensitivity of the present surveys still misses a significant fraction of these objects. Upcoming deeper surveys, such as the GaLactic and Extragalactic All-Sky MWA (GLEAM) survey, will provide further insight into this population.Peer reviewedFinal Published versio
The radio/gamma-ray connection in Active Galactic Nuclei in the era of the Fermi Large Area Telescope
We present a detailed statistical analysis of the correlation between radio
and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi
during its first year of operation, with the largest datasets ever used for
this purpose. We use both archival interferometric 8.4 GHz data (from the VLA
and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz
measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of
199 objects). Our unprecedentedly large sample permits us to assess with high
accuracy the statistical significance of the correlation, using a
surrogate-data method designed to simultaneously account for common-distance
bias and the effect of a limited dynamical range in the observed quantities. We
find that the statistical significance of a positive correlation between the cm
radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the
whole AGN sample, with a probability <1e-7 for the correlation appearing by
chance. Using the OVRO data, we find that concurrent data improve the
significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size
allows us to study the dependence of correlation strength and significance on
specific source types and gamma-ray energy band. We find that the correlation
is very significant (chance probability <1e-7) for both FSRQs and BL Lacs
separately; a dependence of the correlation strength on the considered
gamma-ray energy band is also present, but additional data will be necessary to
constrain its significance.Comment: Accepted for publications by ApJ. Contact authors: M. Giroletti, V.
Pavlidou, A. Reime
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