Westerbork Synthesis Radio Telescope Research Articles

The B3-VLA CSS sample

We present new polarisation observations at 13 cm, acquired using the Westerbork Synthesis Radio Telescope (WSRT), of 65 sources, from the B3-VLA sample of Compact Steep-Spectrum sources. These new data are combined with our VLA polarisation data, at 3.6, 6 and, 21 cm, presented in a previous paper. Due to the multi-channel frequency capabilities of the WSRT, these new 13 cm observations enable a more reliable determination of integrated Rotation Measures, and of depolarisation behaviour with wavelength. The new data are inconsistent with the depolarisation models that we used earlier, and we propose an alternative model which seems to work properly. We also revise our previous model for the external Faraday screen, and its dependence on the source redshift.

The H I content of the advanced merger NGC 4441

Context. NGC4441 is a candidate for a merger between a spiral and an elliptical galaxy (S+E merger), because it shows typical tidal structures such as an optical tail and two shells. With a far-infrared luminosity of similar to 5 x 10(9) L(circle dot) this galaxy belongs to the class of moderate luminosity mergers, in which the merging process induces (if at all) only a moderate starburst. Aims. The study of the atomic gas content allows us to investigate the merger history and the impact on the star formation. In particular, it is not clear from simulations whether an S+E merger leads to a gas concentration in the nucleus, resulting in a starburst, or whether the gas is spread out and therefore too diffuse for new star forming regions. Methods. We used the Westerbork Radio Synthesis Telescope to observe the properties of the Hi. By using this interferometer, we are able to study the large-scale Hi distribution and kinematics with high spatial and velocity resolution. Results. We found two Hi tails extending out to more than 40 kpc. In a central disc, the gas shows a fairly regular rotation pattern indicating that the gas started to settle after the merger. The total Hi mass adds up to 1.5 x 10(9) M(circle dot). By comparing the high resolution Hi maps with deep optical images, differences between the stellar and gaseous tidal features are apparent, which indicates an S+E merger origin.

TeV J2032+4130: a not-so-dark accelerator?

The HEGRA γ-ray source TeV J2032+4130 is considered the prototypical ‘dark accelerator’, since it was the first TeV source detected with no firm counterparts at lower frequencies. The Whipple collaboration observed this source in 2003-05 and the emission hotspot appears displaced about 9 arcmin to the northeast of the HEGRA position, though given the large positional uncertainties the HEGRA and Whipple positions are consistent. Here we report on Westerbork Synthesis Radio Telescope (WSRT), Very Large Array (VLA), Chandra and INTEGRAL data covering the locations of the Whipple and HEGRA hotspots. We confirm a dual-lobed radio source coincident with the Whipple hotspot, as well as a weak, partially non-thermal shell-like object, with a location and morphology very similar to the HEGRA source, in our WSRT and mosaicked VLA data sets, respectively. Due to its extended nature, it is likely that the latter structure is a more plausible counterpart of the reported very high-energy (VHE) γ-ray emissions in this region. If so, TeV J2032+4130 may not be a ‘dark accelerator’ after all. Further observations with the new generation of imaging Cherenkov telescopes are needed to pin down the precise location and morphology of the TeV emission region and thus clear up the confusion over its possible lower frequency counterparts.

PuMa-II: A Wide Band Pulsar Machine for the Westerbork Synthesis Radio Telescope

ABSTRACTThe Pulsar Machine II (PuMa-II) is the new flexible pulsar processing back-end system at the Westerbork Synthesis Radio Telescope (WSRT), specifically designed to take advantage of the upgraded WSRT. The instrument is based on a computer cluster running the Linux operating system, with minimal custom hardware. A maximum of 160 MHz analog bandwidth sampled as 8 × 20 MHz subbands with 8-bit resolution can be recorded on disks attached to separate computer nodes. Processing of the data is done in the additional 32 nodes allowing near real time coherent dedispersion for most pulsars observed at the WSRT. This has doubled the bandwidth for pulsar observations in general, and has enabled the use of coherent dedispersion over a bandwidth 8 times larger than was previously possible at the WSRT. PuMa-II is one of the widest bandwidth coherent dedispersion machines currently in use and has a maximum time resolution of 50 ns. The system is now routinely used for high-precision pulsar timing studies, polarization studies, single pulse work, and a variety of other observational work.

Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase

Context. We explore the physics behind one of the brightest radio afterglows ever, GRB 030329, at late times when the jet is nonrelativistic. Aims. We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements. Methods. We observed the GRB 030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268−1128 days after the burst. We modeled all the available radio data and derived the physical parameters. Results. The index of the electron energy distribution is p = 2.1, the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at tNR ∼ 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous findings. Conclusions. Our findings indicate that the blast wave is roughly spherical at tNR, and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB 030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further.

Ionized and neutral gas in the peculiar star/cluster complex in NGC 6946

The characteristics of ionized and H I gas in the peculiar star/cluster complex in NGC 6946, obtained with the 6-m telescope (BTA) Special Astrophysical Observatory Russian Academy of Sciences (RAS), the Gemini North telescope, and the Westerbork Synthesis Radio Telescope, are presented. The complex is unusual as hosting a super star cluster, the most massive known in an apparently non-interacting giant galaxy. It contains a number of smaller clusters and is bordered by a sharp C-shaped rim. We found that the complex is additionally unusual in having peculiar gas kinematics. The velocity field of the ionized gas reveals a deep oval minimum, similar to 300 pc in size, centred 7 arcsec east of the supercluster. The V-r of the ionized gas in the dip centre is 100 km s(-1) lower than in its surroundings, and emission lines within the dip appear to be shock-excited. This dip is near the centre of an H I hole and a semi-ring of H II regions. The H I (and less certainly, H II) velocity fields reveal expansion, with the velocity reaching similar to 30 km s(-1) at a distance about 300 pc from the centre of expansion, which is near the deep minimum position. The superstar cluster is at the western rim of the minimum. The sharp western rim of the whole complex is plausibly a manifestation of a regular dust arc along the complex edge. Different hypotheses about the complex and the V-r depression's origins are discussed, including a high-velocity H I cloud/dark minihalo impact, a blue compact dwarf galaxy merging, and a gas outflow due to release of energy from the supercluster stars.

Relationships between the H i 21-cm line strength, Mg ii equivalent width and metallicity in damped Lyman α absorption systems

We present the results of a survey for 21-cm absorption in four never previously searched damped Lyman α absorption systems (DLAs) with the Westerbork Synthesis Radio Telescope. The one detection is presented and discussed in Curran et al. and here we add our results to other recent studies in order to address the important issues regarding the detection of cold gas, through 21-cm absorption, in DLAs. Although, due to the DLAs identified with spiral galaxies, there is a mix of spin temperature/covering factor ratios at low redshift, two recent high redshift end points confirm that this ratio does not generally rise much above Tspin/f∼ 103 K over the whole redshift range searched (up to zabs= 3.39). That is, if the covering factors of many of these galaxies were a factor of ≥2 smaller than for the spirals (which span 120 ≤Tspin/f≤ 520 K), then no significant difference in the spin temperatures between these two classes would be required. Furthermore, although it is difficult to separate the relative contributions of the spin temperature and covering factor, the new results confirm that 21-cm detections tend to occur at low angular diameter distances, where the coverage of a given absorption cross-section is maximized. This indicates a dominant contribution by the covering factor. Indeed, the two new high redshift detections occur towards two extremely compact radio sources (≤0.04 arcsec), although the one other new detection, which may have an impact parameter in excess of 75 kpc, occurs towards one of the largest radio sources. Finally, we also find an apparent 21-cm line strength–Mg ii equivalent width correlation, which appears to be due to a coupling of the velocity structure between the components that each species traces. That is, the gas seen in 21-cm absorption could be the same as that seen in optical absorption. Combined with the known equivalent width–metallicity relation, this may be manifest as a spin temperature–metallicity anticorrelation, which is non-evolutionary in origin.

A strong upper limit on the pulsed radio luminosity of the compact object 1RXS J141256.0+792204

The ROSAT X-ray source 1RXS J141256.0+792204 has recently been identified as a likely compact object whose properties suggest it could be a very nearby radio millisecond pulsar at d = 80 - 260pc. We investigated this hypothesis by searching for radio pulsations using the Westerbork Synthesis Radio Telescope. We observed 1RXS J141256.0+792204 at 385 and 1380MHz, recording at high time and frequency resolution in order to maintain sensitivity to millisecond pulsations. These data were searched both for dispersed single pulses and using Fourier techniques sensitive to constant and orbitally modulated periodicities. No radio pulsations were detected in these observations, resulting in pulsed radio luminosity limits of L_400 ~ 0.3 (d/250pc)^2 mJy kpc^2 and L_1400 ~ 0.03 (d/250pc)^2 mJy kpc^2 at 400 and 1400MHz respectively. The lack of detectable radio pulsations from 1RXS J141256.0+792204 brings into question its identification as a nearby radio pulsar, though, because the pulsar could be beamed away from us, this hypothesis cannot be strictly ruled out.

WSRT Ultradeep Neutral Hydrogen Imaging of Galaxy Clusters at z ≈ 0.2: A Pilot Survey of Abell 963 and Abell 2192

A pilot study with the powerful new backend of the Westerbork Synthesis Radio Telescope (WSRT) of two galaxy clusters at z = 0.2 has revealed neutral hydrogen emission from 42 galaxies. The WSRT probes a total combined volume of 3.4 × 104 Mpc3 at resolutions of 54 × 86 kpc2 and 19.7 km s-1, surveying both clusters and the large-scale structure in which they are embedded. In Abell 963, a dynamically relaxed, lensing Butcher-Oemler cluster with a high blue fraction, most of the gas-rich galaxies are located between 1 and 3 Mpc in projection, northeast from the cluster core. Their velocities are slightly redshifted with respect to the cluster, and this is likely a background group. None of the blue galaxies in the core of Abell 963 are detected in H I, although they have colors and luminosities similar to those of the H I detected galaxies in the cluster outskirts and field. Abell 2192 is less massive and more diffuse. Here the gas-rich galaxies are more uniformly distributed. The detected H I masses range from 5 × 109 to 4 × 1010 M☉. Some galaxies are spatially resolved, providing rudimentary rotation curves useful for detailed kinematic studies of galaxies in various environments. This is a pilot for ultradeep integrations down to H I masses of 8 × 108 M☉, providing a complete survey of the gas content of galaxies at z = 0.2, probing environments ranging from cluster cores to voids.

Statistical properties of giant pulses from the Crab pulsar

We have studied the statistics of giant pulses from the Crab pulsar for the first time with particular reference to their widths. We have analyzed data collected during 3.5 hours of observations conducted with the Westerbork Synthesis Radio Telescope operated in a tied-array mode at a frequency of 1200 MHz. The PuMa pulsar backend provided voltage recording of X and Y linear polarization states in two conjugate 10 MHz bands. We restricted the time resolution to 4 microseconds to match the scattering on the interstellar inhomogeneities. In total about 18000 giant pulses (GP) were detected in full intensity with a threshold level of 6 sigma. Cumulative probability distributions (CPD) of giant pulse energies were analyzed for groups of GPs with different effective widths in the range 4 to 65 microseconds. The CPDs were found to manifest notable differences for the different GP width groups. The slope of a power-law fit to the high-energy portion of the CPDs evolves from -1.7 to -3.2 when going from the shortest to the longest GPs. There are breaks in the CPD power-law fits indicating flattening at low energies with indices varying from -1.0 to -1.9 for the short and long GPs respectively. The GPs with a stronger peak flux density were found to be of shorter duration. We compare our results with previously published data and discuss the importance of these peculiarities in the statistical properties of GPs for the heoretical understanding of the emission mechanism responsible for GP generation.

The subpulse modulation properties of pulsars at 92 cm and the frequency dependence of subpulse modulation

A large sample of pulsars has been observed to study their subpulse modulation at an observing wavelength (when achievable) of both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. In this paper we present the 92-cm data and a comparison is made with the already published 21-cm results. We analysed 191 pulsars at 92 cm using fluctuation spectra. The sample of pulsars is as unbiased as possible towards any particular pulsar characteristics. For 15 pulsars drifting subpulses are discovered for the first time and 26 of the new drifters found in the 21-cm data are confirmed. We discovered nulling for 8 sources and 8 pulsars are found to intermittently emit single pulses that have pulse energies similar to giant pulses. It is estimated that at least half of the total population of pulsars have drifting subpulses when observations with a high enough signal-to-noise ratio would be available. It could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. Drifting subpulses are in general found at both frequencies, although the chance of detecting drifting subpulses is possibly slightly higher at 92 cm. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission can be divided in a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. The measured values of P3 at the two frequencies are highly correlated, but there is no evidence for a correlation with other pulsar parameters.

The main-interpulse interaction of PSR B1702–19

This paper reports on single-pulse radio observations of PSR B1702-19 and their implications for pulsar emission theories. These observations were made with the Westerbork Synthesis Radio Telescope at 1380 and 328 MHz. The PA-swing is used to constrain possible geometries of the pulsar and the single-pulse data is analysed for subpulse modulation correlations between the main pulse and interpulse. We confirm earlier conclusions that the dipole axis of this pulsar is almost perpendicular to its rotation axis, and report that both its main pulse and interpulse are modulated with a periodicity around 10.4 times the pulsar's rotation. Allowing for the half-period delay between main pulse and interpulse the modulation is found to be precisely in phase. Despite small secular variations in the periodicity, the phase-locking continues over all timescales ranging up to several years. The precision of the phase locking is difficult for current emission theories to explain if the main pulse and interpulse originate from opposing magnetic poles. We therefore also explore the possibility of a bidirectional model, in which all the modulated emission comes from one pole, but is seen from two sides and slightly displaced by aberration and time-delay. In this model the unmodulated emission is directed to us from the opposite pole, requiring the emission of the main pulse to originate from two different poles. This is difficult to reconcile with the observed smooth PA-swing. Whichever model turns out to be correct, the answer will have important implications for emission theories.

The Effect of Star Formation on the Far-Infrared-Radio Correlation within Galaxies

Using data obtained for 12 galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT) SINGS radio continuum survey, we study how star formation activity affects the far-infrared-radio correlation within galaxies by testing a phenomenological model that describes the radio image as a smeared version of the far-infrared image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk-averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.

Statistics of the Drifting Subpulse Phenomenon

We present the statistical results of a systematic, unbiased search for subpulse modulation of 187 pulsars performed with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands at an observing wavelength of 21 cm (Weltevrede et al. 2006). We have increased the list of pulsars that show the drifting subpulse phenomenon by 42, indicating that more than 55% of the pulsars show this phenomenon. The large number of new drifters we have found allows us, for the first time, to do meaningful statistics on the drifting phenomenon. We find that the drifting phenomenon is correlated with the pulsar age such that drifting is more likely to occur in older pulsars. Pulsars that drift more coherently seem to be older and have a lower modulation index. Contrary claims from older studies, both P3 (the repetition period of the drifting subpulse pattern) and the drift direction are found to be uncorrelated with other pulsar parameters.

30 glitches in slow pulsars

We have analyzed 5.5 years of timing observations of 7 slowly rotating radio pulsars, made with the Westerbork Synthesis Radio Telescope. We present improved timing solutions and 30, mostly small, new glitches. Particularly interesting are our results on PSR J1814-1744, which is one of the pulsars with similar rotation parameters and magnetic field strength to the Anomalous X-ray Pulsars (AXPs). Although the high-B radio pulsars do not show X-ray emission, and no radio emission is detected for AXPs, the roughly similar glitch parameters provide us with another tool to compare these classes of neutron stars. Furthermore, we were able to detect glitches one to two orders of magnitude smaller than before, for example in our well-sampled observations of PSR B0355+54. We double the total number of known glitches in PSR B1737-30, and improve statistics on glitch sizes for this pulsar individually and pulsars in general. We detect no significant variations in dispersion measure for PSRs B1951+32 and B2224+65, two pulsars located in high-density surroundings. We discuss the effect of small glitches on timing noise, and show it is possible to resolve timing-noise looking structures in the residuals of PSR B1951+32 by using a set of small glitches.

A survey for redshifted molecular and atomic absorption lines - I. The Parkes half-Jansky flat-spectrum red quasar sample

We are currently undertaking a large survey for redshifted atomic and molecular absorption lines at radio frequencies. In this paper, we present the results from the first phase of this survey: the search for Hi 21-cm and OH 18-cm absorption lines in the hosts of reddened quasars and radio galaxies. Although we observed each source for up to several hours with two of the world's most sensitive radio telescopes, the Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT), only one clear and one tentative detection were obtained: H I absorption at z = 0.097 in PKS 1555-140 and OH absorption at z = 0.126 in PKS 2300-189, respectively, with the Australia Telescope Compact Array (ATCA). For the latter, no H i absorption was detected at the same redshift as the borderline OH detection. In order to determine why no clear molecular absorption was detected in any of the 13 sources searched, we investigate the properties of the five redshifted systems currently known to exhibit OH absorption. In four of these, molecules were first detected via millimetre-wave transitions, and the flat radio spectra indicate compact background continuum sources, which may suggest a high degree of coverage of the background source by the molecular clouds in the absorber. Furthermore, for these systems we find a relationship between the molecular line strength and red optical-near-infrared (V - K) colours, thus supporting the notion that the reddening of these sources is due to dust, which provides an environment conducive to the formation of molecules. Upon comparison with the V - K colours of our sample, this relationship suggests that, presuming the reddening occurs at the host galaxy redshift at least in some of the targets, many of our observations still fall short of the sensitivity required to detect OH absorption, although a confirmation of the 'detection' of OH in 2300-189 could contravene this.

Neutral hydrogen in nearby elliptical and lenticular galaxies: the continuing formation of early-type galaxies

We present the results of deep Westerbork Synthesis Radio Telescope observations of neutral hydrogen in 12 nearby elliptical and lenticular galaxi es. The selected objects come from a representative sample of nearby galaxies earlier studied at optical wavelengths with the integral-field spectrograph SAURON. They are field galaxies, or (in two cases) located in poor group environments. We detect H I - both in regular discs as well as in clouds and tails offset from the host galaxy - in 70% of the galaxies. This detection rate is much higher than in previous, shallower single-dish surveys, and is similar to that for the ionised gas. The results suggest that at faint detection levels the presence of H I is a relatively common characteristic of field early-type galaxies, confirming what was suggested t wenty years ago by Jura based on IRAS observations. The observed total H I masses range between a few times 10 6 M⊙ to just over 10 9 M⊙. The presence of regular disc-like structures is a situatio n as common as H I in offset clouds and tails around early-type galaxies. All g alaxies where H I isdetected also contain ionised gas, whereas no H I is found around galaxies without ionised gas. Galaxies with regular H I discs tend to have strong emission from ionised gas. In these cases, the similar kinematics of the neutral hydrogen and ionised gas suggest that they form one structure. The kinematical axis of the stellar component is nearly always misaligned with respect to that of the gas. We do not find a clear trend between the presence of H I and the global age of the stellar population or the global dynamical characteristics of t he galaxies. More specifically, H I detections are uniformly spread through the (V/σ, ǫ) diagram. If fast and slow rotators - galaxies with high and low specific angular momentum - represent th e relics of different formation paths, this does not appear in the presence and characterist ics of the H I. Our observations support the idea that gas accretion is common and does not happen exclusively in peculiar early-type galaxies. The links observed between the large-scale gas and the characteristics on the nuclear scale (e.g., the presence of kinematically de coupled cores, radio continuum emission etc.), suggest that for the majority of the cases th e gas is acquired through merging, but the lack of correlation with the stellar population age s uggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either cases, the data suggest that early-type galaxies continue to build their mass up to the pr esent.

USING THE WESTERBORK RADIO OBSERVATORY TO DETECT UHE COSMIC PARTICLES INTERACTING ON THE MOON

Ultra-High-Energy (UHE) particles of cosmological origin (cosmic-rays and neutrinos), carry information on the most spectacular events known. These extremely energetic (energies larger than 1 ZeV= 1021 eV) cosmic-rays or neutrinos initiate in the lunar regolith a cascade of charged particles which acts as a radio pulse emitter. The instantaneous power produced can be detected here at the Earth, with a radio telescope operating at the optimal frequency window around 150 MHz. Using 12 telescopes of the Westerbork Synthesis Radio Telescope, WSRT, with a field of view covering the whole lunar surface, our calculations show that one should identify 10 UHE events within an observation time of 500 hours, assuming an extrapolated power law dependence of the highest ever measured cosmic-ray events, around an energy of 1020 eV. A null result will determine unambiguously the GKZ effect for the cosmic-ray flux and improve the present world upper limit on the neutrino flux above 1 ZeV, by three orders of magnitude, allowing for the first time to test the Waxman-Bahcall neutrino flux limit.

RM‐synthesis of the Perseus cluster

AbstractWe present low frequency radio polarimetric observations of the Perseus cluster. The data were taken with the Westerbork Synthesis Radio Telescope (WSRT) between 315–360 MHz. We have discovered faint, extended, highly polarized emission that we associate with the Perseus cluster (de Bruyn & Brentjens 2005). We propose that at least one of these structures is associated with large‐scale structure formation gas inflow from the Perseus‐Pisces supercluster. The extragalactic emission is seen through a polarized Galactic synchrotron foreground screen. The discovery was made possible by a novel rotation measure analysis technique, called Faraday Rotation Measure Synthesis, or RM‐synthesis for short (Brentjens & de Bruyn 2005). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

H i 21 cm Observations of the PG 1216+069 Sub-damped Lyα Absorber Field at z = 0.00632

The Westerbork Synthesis Radio Telescope finds a weak 21 cm line emission feature at the coordinates (R.A., decl., velocity) of the sub-damped Lyα (DLA) absorber observed at zabs = 0.00632 in the spectrum of PG 1216+069. The emission feature, WSRT J121921+0639, lies within 30'' of the quasar sight line, is detected at a 99.8% (3 σ) confidence level, has M between 5 and 15 × 106 M☉, and has a velocity spread between 20 and 60 km s-1. Other H I emitters in the field include VCC 297 at a projected distance of 86 h kpc and a previously unreported H I cloud, WSRT J121919+0624, at 112 h kpc with M ~ 3 × 108 M☉. The optically identified foreground galaxy that is closest to the quasar sight line appears to be VCC 339 (~L*/25) at 29 h kpc with a velocity offset of 292 km s-1. A low surface brightness galaxy with the H I mass of the sub-DLA absorber WSRT J121921+0639 would likely have mB ~ 17, and its diffuse optical emission would need to compete with the light of both the background QSO and a brighter foreground star ~10'' from the QSO sight line.