A morphological algorithm for improving radio-frequency interference detection

A technique is described that is used to improve the detection of radio-frequency interference in astronomical radio observatories. It is applied on a two-dimensional interference mask after regular detection in the time-frequency domain with existing techniques. The scale-invariant rank (SIR) operator is defined, which is a one-dimensional mathematical morphology technique that can be used to find adjacent intervals in the time or frequency domain that are likely to be affected by RFI. The technique might also be applicable in other areas in which morphological scale-invariant behaviour is desired, such as source detection. A new algorithm is described, that is shown to perform quite well, has linear time complexity and is fast enough to be applied in modern high resolution observatories. It is used in the default pipeline of the LOFAR observatory.Comment: Accepted for publication in A&

FLAGCAL:A flagging and calibration package for radio interferometric data

We describe a flagging and calibration pipeline intended for making quick look images from GMRT data. The package identifies and flags corrupted visibilities, computes calibration solutions and interpolates these onto the target source. These flagged calibrated visibilities can be directly imaged using any standard imaging package. The pipeline is written in "C" with the most compute intensive algorithms being parallelized using OpenMP.Comment: 15 Pages, 6 figures, 2 Tables, Accepted for publication in the Experimental Astronomy Journa

Beach bird surveys in Belgium (poster)

Oil contamination is still a major cause of mortality in many coastal and seabird species around Europe. In a comparison with other North Sea areas, oil rates of most Belgian beached bird species are significantly higher than in northern areas such as the Shetlands and Norway, and more or less in line with oil rates at other European continental coasts. Wrecks of starved unoiled guillemots (and other species) became an almost annual event at the North Sea coasts from the first half of the ‘80s onwards, also in Belgium.Oil rates of beached bird corpses are an appropriate condition indicator of oil pollution at sea. Oil rate of most bird species/taxa in Belgium indicate a decline in oil pollution for the period 1962-99, though only Laridae, guillemot and razorbill show significant reductions. For the other taxa no significant decrease in proportion of oiled birds could be demonstrated, often due to the relatively small study area and hence insufficient number of birds collected. Assuming that a sample of at least ten complete corpses is required to calculate reliable oil rates, only the guillemot (as species) and auks (as taxon) can provide the necessary data in Belgium these days.Long-term oil pollution monitoring in Belgium should be continued with a major focus on a set of abundant bird taxa, sensitive to oil pollution and occurring in various marine habitats and the collection of additional data during the rest of the winter. Most appropriate for this set of limited bird taxa to focus on are grebes (inshore), Laridae, guillemot and razorbill (midshore) and kittiwake and fulmar (offshore).Birds dying at sea may eventually wash ashore. As such, beached bird surveys can be an important source of information concerning mortality of seabirds in the marine environment. However, there has been a lot of debate on the question how numbers of casualties on beaches relate to the actual mortality at sea and which factors affect this relationship. The temporal patterns of beached birds usually follow those of seabirds at sea with a time lag of at least one month. Considering the short Belgian shoreline and the prevailing frequency distribution of winds, probably only 10% of all birds washing ashore died in Belgian marine waters. With a dominant SSW circulation and a net residual current in northeastern direction, many birds must end up on Dutch, German or Scandinavian beaches. Accordingly, there is a higher probability that Belgian beaches receive birds that died in northern France or south England than from other North Sea border states. Based on the number of birds found on the beach and brought in at the MEC, and taking into account that 50-80 % of the corpses have disappeared already within the first 9 days (the mean interval between succeeding weekly surveys), we estimate that the total number of bird corpses beaching on the Belgian coast each winter might be as high as 5,000-10,000 birds

Seabirds in Belgian marine waters: implications for policy and management (poster)

In 1992, the Institute of Nature Conservation (INC) started ship-based surveys on the Belgian Continental Shelf (BCS) to study the spatial and temporal distribution of seabirds. Among other things, the study showed that the Belgian marine waters hold internationally important numbers (> 1% of the biogeographical populations) of several coastal and marine bird species. Also the area proved to be of major importance for several species of migratory birds. Out of the 121 bird species encountered at sea during the period 1992-98, 23 were selected as being true marine species that occur in relatively high densities within the Belgian waters. Of these 23 species six were so-called ‘focal species’, being seabirds which are included in the highest priority lists of international conservation instruments (EC-Birds Directive, Bern Convention or Bonn Convention) and which attain at least 1% of the flyway population in Belgian marine waters in a particular season. Based on the distribution patterns, conservation value, and sensitivity for disturbance or oil pollution of the six focal seabirds several areas of high ornithological importance as well as areas sensitive for disturbance or pollution could be distinguished at the BCS. The avian hotspots at the Westkustbanken and Vlaamse Banken turned out to be the most sensitive areas for disturbance and oil pollution throughout the year, while the sensitivity of other areas varies with the seasons

Measurement of the anisotropy power spectrum of the radio synchrotron background

We present the first targeted measurement of the power spectrum of anisotropies of the radio synchrotron background, at 140 MHz where it is the overwhelmingly dominant photon background. This measurement is important for understanding the background level of radio sky brightness, which is dominated by steep-spectrum synchrotron radiation at frequencies below 0.5 GHz and has been measured to be significantly higher than that which can be produced by known classes of extragalactic sources and most models of Galactic halo emission. We determine the anisotropy power spectrum on scales ranging from 2 degrees to 0.2 arcminutes with LOFAR observations of two 18 square degree fields -- one centered on the Northern hemisphere coldest patch of radio sky where the Galactic contribution is smallest and one offset from that location by 15 degrees. We find that the anisotropy power is higher than that attributable to the distribution of point sources above 100 micro-Jy in flux. This level of radio anisotropy power indicates that if it results from point sources, those sources are likely at low fluxes and incredibly numerous, and likely clustered in a specific manner.Comment: 8 pages, 5 figures, published in MNRAS, updated to published versio

Kinetic analysis of the role of the tyrosine 13, phenylalanine 56 and glutamine 54 network in the U1A/U1 hairpin II interaction

The A protein of the U1 small nuclear ribonucleoprotein particle, interacting with its stem–loop RNA target (U1hpII), is frequently used as a paradigm for RNA binding by recognition motif domains (RRMs). U1A/U1hpII complex formation has been proposed to consist of at least two steps: electrostatically mediated alignment of both molecules followed by locking into place, based on the establishment of close-range interactions. The sequence of events between alignment and locking remains obscure. Here we examine the roles of three critical residues, Tyr13, Phe56 and Gln54, in complex formation and stability using Biacore. Our mutational and kinetic data suggest that Tyr13 plays a more important role than Phe56 in complex formation. Mutational analysis of Gln54, combined with molecular dynamics studies, points to Arg52 as another key residue in association. Based on our data and previous structural and modeling studies, we propose that electrostatic alignment of the molecules is followed by hydrogen bond formation between the RNA and Arg52, and the sequential establishment of interactions with loop bases (including Tyr13). A quadruple stack, sandwiching two bases between Phe56 and Asp92, would occur last and coincide with the rearrangement of a C-terminal helix that partially occludes the RRM surface in the free protein

Diffuse Sources, Clustering and the Excess Anisotropy of the Radio Synchrotron Background

We present the largest low frequency (120~MHz) arcminute resolution image of the radio synchrotron background (RSB) to date, and its corresponding angular power spectrum of anisotropies (APS) with angular scales ranging from $3^\circ$ to $0.3^\prime$. We show that the RSB around the North Celestial Pole has a significant excess anisotropy power at all scales over a model of unclustered point sources based on source counts of known source classes. This anisotropy excess, which does not seem attributable to the diffuse Galactic emission, could be linked to the surface brightness excess of the RSB. To better understand the information contained within the measured APS, we model the RSB varying the brightness distribution, size, and angular clustering of potential sources. We show that the observed APS could be produced by a population of faint clustered point sources only if the clustering is extreme and the size of the Gaussian clusters is $\lesssim 1'$. We also show that the observed APS could be produced by a population of faint diffuse sources with sizes $\lesssim 1'$, and this is supported by features present in our image. Both of these cases would also cause an associated surface brightness excess. These classes of sources are in a parameter space not well probed by even the deepest radio surveys to date.Comment: 13 pages, 14 figures. Accepted for publication in MNRA

Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole

In the coming years a new insight into galaxy formation and the thermal history of the Universe is expected to come from the detection of the highly redshifted cosmological 21 cm line. The cosmological 21 cm line signal is buried under Galactic and extragalactic foregrounds which are likely to be a few orders of magnitude brighter. Strategies and techniques for effective subtraction of these foreground sources require a detailed knowledge of their structure in both intensity and polarization on the relevant angular scales of 1-30 arcmin. We present results from observations conducted with the Westerbork telescope in the 140-160 MHz range with 2 arcmin resolution in two fields located at intermediate Galactic latitude, centred around the bright quasar 3C196 and the North Celestial Pole. They were observed with the purpose of characterizing the foreground properties in sky areas where actual observations of the cosmological 21 cm line could be carried out. The polarization data were analysed through the rotation measure synthesis technique. We have computed total intensity and polarization angular power spectra. Total intensity maps were carefully calibrated, reaching a high dynamic range, 150000:1 in the case of the 3C196 field. [abridged]Comment: 20 pages, 22 figures, accepted for publication in A&A. A version with full resolution figures is available at http://www.astro.rug.nl/~bernardi/NCP_3C196/bernardi.pd