Diffuse polarized emission associated with the Perseus cluster

We report on full-polarization radio observations of the Perseus cluster (Abell 426) using the Westerbork Synthesis Radio Telescope (WSRT) at wavelengths from 81-95 cm. We have employed a novel technique, Rotation Measure synthesis (Brentjens and de Bruyn, 2005) to unravel the polarization properties of the emission across the full field of view and detect polarized emission over a wide range of RM from about 0 to 90 rad m^-2. The low RM emission is associated with our Galaxy, while the high RM emission is associated with the Perseus cluster. The latter reaches typical surface brightness levels of 0.5-1 mJy per beam and must be rather highly polarized. Most of the peripheral polarized emission appears too bright, by about 1-2 orders of magnitude, to be explainable as Thomson scattered emission of the central radio source off the thermal electrons in the cluster. The bulk of the emission associated with the Perseus cluster is probably related to buoyant bubbles of relativistic plasma, probably relics from still active or now dormant AGN within the cluster. A lenticular shaped structure measuring 0.5-1 Mpc is strikingly similar to the structures predicted by Ensslin et al. (1998). At the western edge of the cluster, we detect very long, linear structures that may be related to shocks caused by infall of gas into the Perseus cluster.Comment: 18 pages, 17 figures, accepted by A&A, corrected small typo, added referenc

Wide field polarimetry around the Perseus cluster at 350 MHz

This paper investigates the fascinating diffuse polarization structures at 350 MHz that have previously been tentatively attributed to the Perseus cluster and, more specifically, tries to find out whether the structures are located at (or near) the Perseus cluster, or in the Milky Way. A wide field, eight point Westerbork Synthesis Radio Telescope mosaic of the area around the Perseus cluster was observed in full polarization. The frequency range was 324 to 378 MHz and the resolution of the polarization maps was 2'x3'. The maps were processed using Faraday rotation measure synthesis to counter bandwidth depolarization. The RM-cube covers Faraday depths of -384 to +381 rad m^{-2} in steps of 3 rad m^{-2}. There is emission all over the field at Faraday depths between -50 and +100 rad m^{-2}. All previously observed structures were detected. However, no compelling evidence was found supporting association of those structures with either the Perseus cluster or large scale structure formation gas flows in the Perseus-Pisces super cluster. On the contrary, one of the structures is clearly associated with a Galactic depolarization canal at 1.41 GHz. Another large structure in polarized intensity, as well as Faraday depth at a Faraday depth of +30 rad m^{-2}, coincides with a dark object in WHAM H-alpha maps at a kinematic distance of 0.5 \pm 0.5 kpc. All diffuse polarized emission at 350 MHz towards the Perseus cluster is most likely located within 1 kpc from the Sun. The layers that emit the polarized radiation are less than 40 pc/B|| thick.Comment: 16 pages, accepted for publication in A&

Interaction of antibodies with renal cell surface antigens

Over the past ten years it has been increasingly recognized that immune complex nephritides, characterized by the accumulation of granular immune deposits in renal structures, may not only be the consequence of deposition of circulating immune complexes but also of antigen-antibody complexes aggregated locally or formed in situ [1]. One of the sites at which antibodies may interact locally with antigens in the kidney is at the surface of renal cells [1, 2]. There is an abundance of antigens on cells: part of them are intrinsic plasma membrane proteins, while others may be present because they are secretion products or they have been ‘planted“ on the cell surface for immunological or physico-chemical reasons. In this article renal lesions that may arise from the binding of antibodies with these various kinds of cell surface antigens will be reviewed. Most of our insights come from animal models of renal disease. In the closing section the potential impact of data derived from experimental pathology on our concepts of human nephropathies will be evaluated

Samenwerking in de detailhandel: Theorie en praktijk

In dit artikel staat de vraag "Waarom zijn ondernemers lid van een inkoopcombinatie?" centraal. Uitgaande van de 'strategic behaviour'-benadering en de transactiekostentheorie wordt beschreven of de redenen van samenwerking van toepassing zijn op een zevental geinterviewde leden. De resultaten moeten vanwege het geringe aantal waarnemingen met een zekere voorzichtigheid genterpreteerd worden. Uit de interviews kwam naar voren dat vrijwel alle voordelen en slechts een van de nadelen zoals onderscheiden in de 'strategic behaviour'- benadering daadwerkelijk ervaren werden door de ondernemers. Bij de transactiekostentheorie is een onderscheid gemaakt naar directe en indirecte transactiekosten. Het bleek dat samenwerking via de inkoopcombinatie alleen maar voordelen en geen nadelen op het gebied van de produktiekosten en directe transactiekosten oplevert. Er was geen uitspraak mogelijk of en in hoeverre er indirecte transactiekostenvoordelen of - nadelen werden ervaren.Op basis van de waarnemingen zou geconcludeerd kunnen worden dat samenwerken via een inkoopcombinatie een efficint alternatief is voor ondernemers, aangezien de voordelen ervan groter zijn dan de nadelen. Tevens geldt dat de voordelen minus de nadelen van de inkoopcombinatie duidelijk opwegen tegen andere alternatieven. Tenslotte kwam uit de interviews naar voren dat de aantrekkelijkheid van de inkoopcombinatie in de loop van de tijd reeds is toegenomen en dat de verwachting bestaat dat deze trend zich zal voortzetten in de toekomst.Retailing;Cooperation;Small Business;small and medium sized business

Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties

We describe singularities in the distribution of polarized intensity as a function of Faraday depth (i.e. the Faraday spectrum) caused by line-of-sight (LOS) magnetic field reversals. We call these features Faraday caustics because of their similarity to optical caustics. They appear as sharply peaked and asymmetric profiles in the Faraday spectrum, that have a tail that extends to one side. The direction in which the tail extends depends on the way in which the LOS magnetic field reversal occurs (either changing from oncoming to retreating or vice versa). We describe how Faraday caustics will form three-dimensional surfaces that relate to boundaries between regions where the LOS magnetic field has opposite polarity. We present examples from simulations of the predicted polarized synchrotron emission from the Milky Way. We derive either the probability or luminosity distribution of Faraday caustics produced in a Gaussian magnetic field distribution as a function of their strength, F, and find that for strong Faraday caustics P(F)\proptoF^{-3} . If fully resolved, this distribution is also shown to depend on the Taylor microscale, which relates to the largest scale over which dissipation is important in a turbulent flow.Comment: 14 pages, 9 figures, Accepted for publication in Astronomy & Astrophysic

Wavelet-based Faraday Rotation Measure Synthesis

Faraday Rotation Measure (RM) Synthesis, as a method for analyzing multi-channel observations of polarized radio emission to investigate galactic magnetic fields structures, requires the definition of complex polarized intensity in the range of the negative lambda square. We introduce a simple method for continuation of the observed complex polarized intensity into this domain using symmetry arguments. The method is suggested in context of magnetic field recognition in galactic disks where the magnetic field is supposed to have a maximum in the equatorial plane. The method is quite simple when applied to a single Faraday-rotating structure on the line of sight. Recognition of several structures on the same line of sight requires a more sophisticated technique. We also introduce a wavelet-based algorithm which allows us to consider a set of isolated structures. The method essentially improves the possibilities for reconstruction of complicated Faraday structures using the capabilities of modern radio telescopes.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter

Faraday Rotation Measure Synthesis

We extend the rotation measure work of Burn (1966) to the cases of limited sampling of lambda squared space and non-constant emission spectra. We introduce the rotation measure transfer function (RMTF), which is an excellent predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation measure synthesis can be implemented very efficiently on modern computers. Because the analysis is easily applied to wide fields, one can conduct very fast RM surveys of weak spatially extended sources. Difficult situations, for example multiple sources along the line of sight, are easily detected and transparently handled. Under certain conditions, it is even possible to recover the emission as a function of Faraday depth within a single cloud of ionized gas. Rotation measure synthesis has already been successful in discovering widespread, weak, polarized emission associated with the Perseus cluster (De Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as good as traditional linear fits to polarization angle versus lambda squared plots. However, when the situation is more complex or very weak polarized emission at high rotation measures is expected, it is the only viable option.Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected typo