Constraining the nature of High Frequency Peakers. I. The spectral variability

We investigate the spectral characteristics of 51 candidate High Frequency Peakers (HFPs), from the ``bright'' HFP sample, in order to determine the nature of each object, and to obtain a smaller sample of genuine young radio sources. Simultaneous multi-frequency VLA observations carried out at various epochs have been used to detect flux density and spectral shape variability in order to pinpoint contaminant objects, since young radio sources are not expected to be significantly variable on such a short time-scale. From the analysis of the spectral variability we find 13 contaminant objects, 11 quasars, 1 BL Lac, and 1 unidentified object, which we have rejected from the sample of candidate young radio sources. The 6 years elapsed between the first and latest observing run are not enough to detect any substantial evolution of the overall spectrum of genuine, non variable, young radio sources. If we also consider the pc-scale information, we find that the total radio spectrum we observe is the result of the superposition of the spectra of different regions (lobes, hot-spots, core, jets), instead of a single homogeneous radio component. This indicates that the radio source structure plays a relevant role in determining the spectral shape also in the rather common case in which the morphology appears unresolved even on high-resolution scales.Comment: 14 pages, 3 figures; accepted for pubblication in A&

Spectral variability in faint high frequency peakers

We present the analysis of simultaneous multi-frequency Very Large Array (VLA) observations of 57 out of 61 sources from the ``faint'' high frequency peaker (HFP) sample carried out in various epochs. Sloan Digital Sky Survey (SDSS) data have been used to identify the optical counterpart of each radio source. From the analysis of the multi-epoch spectra we find that 24 sources do not show evidence of spectral variability, while 12 objects do not possess a peaked spectrum anymore at least in one of the observing epochs. Among the remaining 21 sources showing some degree of variability, we find that in 8 objects the spectral properties change consistently with the expectation for a radio source undergoing adiabatic expansion. The comparison between the variability and the optical identification suggests that the majority of radio sources hosted in galaxies likely represent the young radio source population, whereas the majority of those associated with quasars are part of a different population similar to flat-spectrum objects, which possess peaked spectra during short intervals of their life, as found in other samples of high-frequency peaking objects. The analysis of the optical images from the SDSS points out the presence of companions around 6 HFP hosted in galaxies, suggesting that young radio sources resides in groups.Comment: 16 pages, 5 figures; accepted for publication in MNRA

Parsec-scale morphology and spectral index distribution in faint high frequency peakers

We investigate the parsec-scale structure of 17 high frequency peaking radio sources from the faint HFP sample. VLBA observations were carried out at two adjacent frequencies, 8.4 and 15.3 GHz, both in the optically-thin part of the spectrum, to obtain the spectral index information. We found that 64% of the sources are resolved into subcomponents, while 36% are unresolved even at the highest frequency. Among the resolved sources, 7 have a morphology and a spectral index distribution typical of young radio sources, while in other 4 sources, all optically associated with quasars, the radio properties resemble those of the blazar population. The equipartition magnetic field of the single components are a few tens milliGauss, similar to the values found in the hotspots of young sources with larger sizes. Such high magnetic fields cause severe radiative losses, precluding the formation of extended lobe structures emitting at centimeter wavelengths. The magnetic fields derived in the various components of individual source are usually very different, indicating a non self-similar source evolution, at least during the very first stages of the source growth.Comment: 14 pages, 5 figures, accepted for publication in MNRA

Bubble-Driven Inertial Micropump

The fundamental action of the bubble-driven inertial micropump is investigated. The pump has no moving parts and consists of a thermal resistor placed asymmetrically within a straight channel connecting two reservoirs. Using numerical simulations, the net flow is studied as a function of channel geometry, resistor location, vapor bubble strength, fluid viscosity, and surface tension. Two major regimes of behavior are identified: axial and non-axial. In the axial regime, the drive bubble either remains inside the channel or continues to grow axially when it reaches the reservoir. In the non-axial regime the bubble grows out of the channel and in all three dimensions while inside the reservoir. The net flow in the axial regime is parabolic with respect to the hydraulic diameter of the channel cross-section but in the non-axial regime it is not. From numerical modeling, it is determined that the net flow is maximal when the axial regime crosses over to the non-axial regime. To elucidate the basic physical principles of the pump, a phenomenological one-dimensional model is developed and solved. A linear array of micropumps has been built using silicon-SU8 fabrication technology, and semi-continuous pumping across a 2 mm-wide channel has been demonstrated experimentally. Measured variation of the net flow with fluid viscosity is in excellent agreement with simulation results.Comment: 18 pages, 18 figures, single colum

e-VLBI observations of GHz-Peaked Spectrum (GPS) radio sources in nearby galaxies from the AT20G survey

GHz-peaked spectrum (GPS) radio sources are thought to be young objects which later evolve into FR-I and FR-II radio galaxies. We have used the Australia Telescope 20GHz (AT20G) survey catalogue to select a uniform sample of GPS sources with spectral peaks above 5GHz, which should represent the youngest members of this class. In this paper, we present e-VLBI observations of ten such objects which are associated with nearby (z<0.15) galaxies and so represent a new population of local, low--power GPS sources. Our e-VLBI observations were carried out at 4.8GHz with the Australia Telescope Long Baseline Array (LBA) using a real--time software correlator. All ten sources were detected, and were unresolved on scales of ~100mas, implying that they are typically less than 100pc in linear size.Comment: 7 pages, 7 figures, 3 table

Screening of variants for lactase persistence/non-persistence in populations from South Africa and Ghana

<p>Abstract</p> <p>Background</p> <p>Lactase non-persistence is a condition where lactase activity is decreased in the intestinal wall after weaning. In European derived populations a single nucleotide polymorphism (SNP) C/T_-13910residing 13.9 kb upstream from the lactase gene has been shown to define lactase activity, and several other single nucleotide polymorphisms (G/C_-14010T/G_-13915, C/G_-13907and T/C_-13913) in the same region have been identified in African and Middle East populations.</p> <p>Results</p> <p>The T_-13910allele most common in European populations was present in 21.8% mixed ancestry (N = 62) individuals and it was absent in the Xhosa (N = 109) and Ghana (N = 196) subjects. Five other substitutions were also found in the region covering the previously reported variants in African and Middle East populations. These included the G/C_-14010variant common in Kenyan and Tanzanian populations, which was present in 12.8% of Xhosa population and in 8.1% of mixed ancestry subjects. Two novel substitutions (C/T_-14091and A/C_-14176) and one previously reported substitution G/A_-13937(rs4988234) were less common and present only in the Xhosa population. One novel substitution G/A_-14107was present in the Xhosa and Ghanaian populations. None of the other previously reported variants were identified.</p> <p>Conclusion</p> <p>Identification of the G/C_-14010variant in the Xhosa population, further confirms their genetic relatedness to other nomadic populations members that belong to the Bantu linguistic group in Tanzania and Kenya. Further studies are needed to confirm the possible relationship of the novel substitutions to the lactase persistence trait.</p

Multifrequency study of GHz-peaked spectrum sources and candidates with the RATAN-600 radio telescope

Context. Gigahertz peaked spectrum (GPS) radio sources are a class of extragalactic radio sources characterized by a spectral peak in the gigahertz domain. They are a mixed class of quasars and galaxies. A large proportion of the sources studied in the literature have only few data points in the radio domain, and the determination of variability and shape of the simultaneous spectra is inadequate. Sources currently included in the GPS source lists are very heterogeneous. Aims. We present the observational results from 12 observing campaigns (carried out between 2006 and 2010) at the RATAN-600 radio telescope to obtain the simultaneous radio spectra, which is valuable and necessary to derive genuine GPS sources from flatspectrum radio sources caught in a flaring state when their spectra are temporarily inverted. The sample contains both quasar- and galaxy-type GPS (122 sources) identified in the literature. Methods. The observations were carried out at six frequencies (1.1, 2.3, 4.8, 7.7, 11.2 and 21.7 GHz). The flux densities were measured at several epochs. A six-frequency broadband radio spectrum was obtained by observing simultaneously with an accuracy of up to a minute at 1.4, 2.7, 3.9, 6.25, 13, and 30 cm. Results. The original GPS source samples were highly contaminated. Finally, we selected 29% GPS source candidates within the sample. We found some difference in spectral properties between GPS galaxies and quasars within the sample. The GPS galaxies demonstrate a steeper spectral index in the optically thin part of the spectra. There are only relatively few (17) sources whose radio spectra strictly agree with the spectra of homogeneous self-absorbed synchrotron sources. The narrowest radio spectra are found in both ultra-high-z (z ≥ 1.8) and low-z (0.02 ≤ z ≤ 0.7, FWHM ∼ 0.9) convex spectrum radio sources. The majority of quasars within this sample should be considered as flat-spectrum radio sources with a temporarily inverted spectrum, and not as genuine GPS sources. The number of truly convex-spectrum sources remains low, and the lists of GPS sources should accordingly be revised. © 2012 ESO

High--frequency predictions for number counts and spectral properties of extragalactic radio sources. New evidences of a break at mm wavelengths in spectra of bright blazar sources

We present models to predict high frequency counts of extragalactic radio sources using physically grounded recipes to describe the complex spectral behaviour of blazars, that dominate the mm-wave counts at bright flux densities. We show that simple power-law spectra are ruled out by high-frequency (nu>100 GHz) data. These data also strongly constrain models featuring the spectral breaks predicted by classical physical models for the synchrotron emission produced in jets of blazars (Blandford & Konigl 1979; Konigl 1981). A model dealing with blazars as a single population is, at best, only marginally consistent with data coming from current surveys at high radio frequencies. Our most successful model assumes different distributions of break frequencies, nu_M, for BL Lacs and Flat-Spectrum Radio Quasars (FSRQs). The former objects have substantially higher values of nu_M, implying that the synchrotron emission comes from more compact regions; therefore, a substantial increase of the BL Lac fraction at high radio frequencies and at bright flux densities is predicted. Remarkably, our best model is able to give a very good fit to all the observed data on number counts and on distributions of spectral indices of extragalactic radio sources at frequencies above 5 and up to 220 GHz. Predictions for the forthcoming sub-mm blazar counts from Planck, at the highest HFI frequencies, and from Herschel surveys are also presented.Comment: 21 pages, 16 figures; Eq.B.2 and labels in few Figures correcte

Statistical analyses of long-term variability of AGN at high radio frequencies

We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: The structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole.Comment: 19 pages, 12 figures, Accepted for publication in A&

Radio spectra and polarisation properties of radio-loud Broad Absorption Line Quasars

We present multi-frequency observations of a sample of 15 radio-emitting Broad Absorption Line Quasars (BAL QSOs), covering a spectral range between 74 MHz and 43 GHz. They display mostly convex radio spectra which typically peak at about 1-5 GHz (in the observer's rest-frame), flatten at MHz frequencies, probably due to synchrotron self-absorption, and become steeper at high frequencies, i.e., >~ 20 GHz. VLA 22-GHz maps (HPBW ~ 80 mas) show unresolved or very compact sources, with linear projected sizes of <= 1 kpc. About 2/3 of the sample look unpolarised or weakly polarised at 8.4 GHz, frequency in which reasonable upper limits could be obtained for polarised intensity. Statistical comparisons have been made between the spectral index distributions of samples of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding steeper spectra among non-BAL QSOs. However constraining this comparison to compact sources results in no significant differences between both distributions. This comparison is consistent with BAL QSOs not being oriented along a particular line of sight. In addition, our analysis of the spectral shape, variability and polarisation properties shows that radio BAL QSOs share several properties common to young radio sources like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources.Comment: 18 pages, 11 Postscript figures, 12 Tables. Accepted for publication in MNRA