arXiv subjects

A. G. de Bruyn

Publications and source records attributed to A. G. de Bruyn.

At least 73 records · Page 4Linked to original sources

arXiv2008

Fast Large-Scale Reionization Simulations

We present an efficient method to generate large simulations of the Epoch of Reionization (EoR) without the need for a full 3-dimensional radiative transfer code. Large dark-matter-only simulations are post-processed to produce maps of the redshifted 21cm emission from neutral hydrogen. Dark matter haloes are embedded with sources of radiation whose properties are either based on semi-analytical prescriptions or derived from hydrodynamical simulations. These sources could either be stars or power-law sources with varying spectral indices. Assuming spherical symmetry, ionized bubbles are created around these sources, whose radial ionized fraction and temperature profiles are derived from a catalogue of 1-D radiative transfer experiments. In case of overlap of these spheres, photons are conserved by redistributing them around the connected ionized regions corresponding to the spheres. The efficiency with which these maps are created allows us to span the large parameter space typically encountered in reionization simulations. We compare our results with other, more accurate, 3-D radiative transfer simulations and find excellent agreement for the redshifts and the spatial scales of interest to upcoming 21cm experiments. We generate a contiguous observational cube spanning redshift 6 to 12 and use these simulations to study the differences in the reionization histories between stars and quasars. Finally, the signal is convolved with the LOFAR beam response and its effects are analyzed and quantified. Statistics performed on this mock data set shed light on possible observational strategies for LOFAR.

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arXiv2008

Detection of diffuse radio emission at large distance from the center of the galaxy cluster A2255

Low-frequency radio observations of galaxy clusters are the key to detecting the diffuse extended emission associated with them. The presence and properties of such radio sources in galaxy clusters reveal the existence of magnetic fields on a large scale and allow theories to be tested concerning both the origin of relativistic particles in the ICM and their propagation. A deep radio bservation of the A2255 galaxy cluster was carried out at 85 cm with the WSRT. The good UV-coverage and sensitivity achieved by these observations allowed us to image the complex structure of the low-brightness, extended cluster sources (radio halo and relic). These sources show a larger extent than what has been imaged so far at this frequency, with two new structures located SW and NW of the cluster center and at projected distances of 2 Mpc from it. The physical properties of the newly detected structures, together with the active dynamical state of the cluster, support a connection with large-scale structure (LSS) formation shocks.

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arXiv2007

Three episodes of jet activity in the FRII radio galaxy B0925+420

We present Very Large Array images of a "Double-Double Radio Galaxy", a class of objects in which two pairs of lobes are aligned either side of the nucleus. In this object, B0925+420, we discover a third pair of lobes, close to the core and again in alignment with the other lobes. This first-known "Triple-Double" object strongly increases the likelihood that these lobes represent mutiple episodes of jet activity, as opposed to knots in an underlying jet. We model the lobes in terms of their dynamical evolution. We find that the inner pair of lobes is consistent with the outer pair having been displaced buoyantly by the ambient medium. The middle pair of lobes is more problematic - to the extent where an alternative model interpreting the middle and inner "lobes" as additional bow shocks within the outer lobes may be more appropriate - and we discuss the implications of this on our understanding of the density of the ambient medium.

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arXiv2007

WSRT Faraday tomography of the Galactic ISM at λ\sim 0.86 m

We investigate the distribution and properties of Faraday rotating and synchrotron emitting regions in the Galactic ISM in the direction of the Galactic anti-centre. We apply Faraday tomography to a radio polarization dataset that we obtained with the WSRT. We developed a new method to calculate a linear fit to periodic data, which we use to determine rotation measures from our polarization angle data. From simulations of a Faraday screen + noise we could determine how compatible the data are with Faraday screens. An unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an unresolved main component in the Faraday depth spectrum. For lines-of-sight with a single unresolved component we demonstrate that a Faraday screen in front of a synchrotron emitting region that contains a turbulent magnetic field component can explain the data.

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arXiv2007

LOFAR - Opening up a new window on the Universe

LOFAR, the Low Frequency Array, is a next-generation radio telescope that is being built in Northern Europe and expected to be fully operational at the end of this decade. It will operate at frequencies from 15 to 240 MHz (corresponding to wavelengths of 20 to 1.2 m). Its superb sensitivity, high angular resolution, large field of view and flexible spectroscopic capabilities will represent a dramatic improvement over previous facilities at these wavelengths. As such, LOFAR will carry out a broad range of fundamental astrophysical studies. The design of LOFAR has been driven by four fundamental astrophysical applications: (i) The Epoch of Reionisation, (ii) Extragalactic Surveys and their exploitation to study the formation and evolution of clusters, galaxies and black holes, (iii) Transient Sources and their association with high energy objects such as gamma ray bursts, and (iv) Cosmic Ray showers and their exploitation to study the origin of ultra-high energy cosmic rays. In this conference the foreseen LOFAR work on the epoch of reionisation has been covered by de Bruyn and on cosmic ray showers by Falcke. During this contribution we will first present the LOFAR project with an emphasis on the challenges faced when carrying out sensitive imaging at low radio frequencies. Subsequently, we will discuss LOFAR's capabilities to survey the low-frequency radio sky. Main aims for the planned surveys are studies of z>6 radio galaxies, diffuse emission associated with distant clusters and starbursting galaxies at z>2.

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arXiv2006

A very brief description of LOFAR - the Low Frequency Array

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope.

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arXiv2006

The WENSS and Dwingeloo surveys and the Galactic magnetic field

In this paper we use the diffuse Galactic synchrotron emission to study the properties of the Galactic magnetic field. We combined data from the low-frequency WENSS survey with single-dish observations carried out by Brouw and Spoelstra to (partially) fill in the central gap in the (u,v) plane that is missing from the interferometer data. The small bandwidth of the WENSS data meant that we could not determine rotation measures (RM) directly. Instead we interpreted the polarization angle gradients that we derived as gradients in RM, and to do this we found a new way to efficiently and reliably fit linear gradients to periodic data. RM are available for the single-dish observations, and we found that we have to rescale these single-dish RM to match the RM gradients that we derive from the combined WENSS/single-dish dataset with the single-dish only RM gradients. We tentatively show that the difference in beamsize between the datasets can be responsible for this. We interpret the scaled-up RM we find in terms of a simple toy model, and by combining our results with those by Haverkorn et al. (2004) we reconstruct the full 3D magnetic field vector along a number of lines-of-sight. For these lines-of-sight we derive the properties of the magnetic field component perpendicular to the Galactic plane.

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arXiv2006

Detecting UHE Cosmics & Neutrinos off the Moon; an Optimal Radio Window

We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. Through an example it is shown that this new approach offers, for the first time, the realistic possibility of measuring UHE neutrino fluxes below the Waxman-Bahcall limit. It is shown that in only one month of observing with the upcoming LOFAR radio telescope, cosmic-ray events can be measured beyond the GZK-limit, at a sensitivity level of two orders of magnitude below the extrapolated values.

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arXiv2006

Optimal Radio Window for the Detection of Ultra-High-Energy Cosmic Rays and Neutrinos off the Moon

When high-energy cosmic rays impinge on a dense dielectric medium, radio waves are produced through the Askaryan effect. We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. Through an example it is shown that this new approach offers, for the first time, the realistic possibility of measuring UHE neutrino fluxes below the Waxman-Bahcall limit. It is shown that in only one month of observing with the upcoming LOFAR radio telescope, cosmic-ray events can be measured beyond the GZK-limit, at a sensitivity level of two orders of magnitude below the extrapolated values.

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arXiv2005

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.

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arXiv2005

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.

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arXiv2005

CLASS B0631+519: Last of the CLASS lenses

We report the discovery of a new gravitational lens system from the CLASS survey, CLASS B0631+519. VLA, MERLIN and VLBA observations show a doubly-imaged radio core, a doubly-imaged lobe and a second lobe that is probably quadruply-imaged. The maximum image separation is 1.16 arcseconds. The VLBA resolves the most magnified image of the flat-spectrum radio core into a number of sub-components spread across approximately 20 milli-arcseconds. Optical and near-infrared imaging with the ACS and NICMOS cameras on the HST show that there are two galaxies along the line of sight to the lensed source, as was previously discovered by optical spectroscopy. The nearer galaxy at z=0.0896 is a small blue irregular, while the more distant galaxy at z=0.6196 is an elliptical type and appears to contribute most of the lensing effect. The host galaxy of the lensed source is visible in NICMOS imaging as a set of arcs that form an almost complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the ring and indicates that the irregular galaxy has a (localised) effect on the flux density distribution in the Einstein ring at the 5-10% level.

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arXiv2004

CLASS B2108+213: A new wide separation gravitational lens system

We present observations of CLASS B2108+213, the widest separation gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact components separated by 4.56 arcsec with a faint third component in between which we believe is emission from a lensing galaxy. 5-GHz VLBA observations reveal milliarcsecond-scale structure in the two lensed images that is consistent with gravitational lensing. Optical emission from the two lensed images and two lensing galaxies within the Einstein radius is detected in Hubble Space Telescope imaging. Furthermore, an optical gravitational arc, associated with the strongest lensed component, has been detected. Surrounding the system are a number of faint galaxies which may help explain the wide image separation. A plausible mass distribution model for CLASS B2108+213 is also presented.

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arXiv2004

The Microarcsecond Sky and Cosmic Turbulence

Radio waves are imprinted with propagation effects from ionized media through which they pass. Owing to electron density fluctuations, compact sources (pulsars, masers, and compact extragalactic sources) can display a wide variety of scattering effects. These scattering effects, particularly interstellar scintillation, can be exploited to provide *superresolution*, with achievable angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained by very long baseline interferometry on terrestrial baselines. Scattering effects also provide a powerful sub-AU probe of the microphysics of the interstellar medium, potentially to spatial scales smaller than 100 km, as well as a tracer of the Galactic distribution of energy input into the interstellar medium through a variety of integrated measures. Coupled with future gamma-ray observations, SKA observations also may provide a means of detecting fainter compact gamma-ray sources. Though it is not yet clear that propagation effects due to the intergalactic medium are significant, the SKA will either detect or place stringent constraints on intergalactic scattering.

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arXiv2004

Structure in the polarized Galactic synchrotron emission, in particular `depolarization canals'

The polarized component of the diffuse radio synchrotron emission of our Galaxy shows structure, which is apparently unrelated to the structure in total intensity, on many scales. The structure in the polarized emission can be due to several processes or mechanisms. Some of those are related to the observational setup, such as beam depolarization -- the vector combination and (partial) cancellation of polarization vectors within a synthesized beam --, or the insensitivity of a synthesis telescope to structure on large scales, also known as the 'missing short spacings problem'. Other causes for structure in the polarization maps are intrinsic to the radiative transfer of the emission in the warm ISM, which induces Faraday rotation and depolarization. We use data obtained with the Westerbork Synthesis Radio Telescope at 5 frequencies near 350 MHz to estimate the importance of the various mechanisms in producing structure in the linearly polarized emission. In the two regions studied here, which are both at positive latitudes in the second Galactic quadrant, the effect of 'missing short spacings' is not important. The properties of the narrow depolarization 'canals' that are observed in abundance lead us to conclude that they are mostly due to beam depolarization, and that they separate regions with different rotation measures. As beam depolarization only creates structure on the scale of the synthesized beam, most of the structure on larger scales must be due to depth depolarization. We do not discuss that aspect of the observations here, but in a companion paper we derive information about the properties of the ISM from the structure of the polarized emission.

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arXiv2004

Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging

We describe a first attempt to derive properties of the regular and turbulent Galactic magnetic field from multi-frequency polarimetric observations of the diffuse Galactic synchrotron background. A single-cell-size model of the thin Galactic disk is constructed which includes random and regular magnetic fields and thermal and relativistic electrons. The disk is irradiated from behind with a uniform partially polarized background. Radiation from the background and from the thin disk is Faraday rotated and depolarized while propagating through the medium. The model parameters are estimated from a comparison with 350 MHz observations in two regions at intermediate latitudes done with the Westerbork Synthesis Radio Telescope. We obtain good consistency between the estimates for the random and regular magnetic field strengths and typical scales of structure in the two regions. The regular magnetic field strength found is a few microGauss, and the ratio of random to regular magnetic field strength is 0.7 +/- 0.5, for a typical scale of the random component of 15 +/- 10 pc. Furthermore, the regular magnetic field is directed almost perpendicular to the line of sight. This modeling is a potentially powerful method to estimate the structure of the Galactic magnetic field, especially when more polarimetric observations of the diffuse synchrotron background at intermediate latitudes become available.

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arXiv2003

HI kinematics in a massive spiral galaxy at z=0.89

We present a kinematic model of the neutral-hydrogen in the spiral galaxy of the lens system PKS1830-211, based on a Multi Element Radio-Linked Interferometer (MERLIN) 1.4-GHz radio map and the integrated and redshifted 21-cm hydrogen absorption-line profile as measured with the Westerbork Synthesis Radio Telescope (WSRT). Degeneracies in the models do not allow a unique determination of the kinematic center and forthcoming deeper Hubble Space Telescope observations with the Advanced Camera for Surveys (ACS) are required to break this degeneracy. Even so, we measure the inclination of the hydrogen disk: i=(17-32) deg., indicating a close-to face-on spiral galaxy. The optical depth increases with radius over the extent of the Einstein ring, suggesting HI depletion towards the lens center. The latter could be due to star formation or conversion of HI in to molecular hydrogen because of a higher metalicity/dust content in the galaxy center. The neutral hydrogen optical depth gives N_HI= 2x10^21 cm^-2 at r=5.0 h_{70}^-1 kpc in the disk (T_s=100K), comparable to local spiral galaxies. Our study shows that planned new radio telescopes (i.e. ALMA, LOFAR and SKA) -- which will discover large numbers of similar lens systems -- are powerful new tools to probe the internal structure, kinematics and evolution of spiral galaxies in detail to z>=1, as well as their neutral-hydrogen (and molecular) content, thereby complementing studies of HI emission from spirals at z<=1 and damped Ly-alpha systems to z>>1.

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arXiv2003

Detection of OH and wide HI absorption toward B0218+357

We present deep GMRT OH and WSRT HI absorption spectra of the $z = 0.6846$ gravitational lens toward B0218+357. Both the 1665 MHz and 1667 MHz OH lines are clearly detected for the first time while a new wide absorption component was detected (at low significance) in the HI spectrum; the OH spectra yielded an OH column density of $N_{OH} = 2.3 \times 10^{15}$ cm^{-2}. The ratio of 1667 and 1665 MHz equivalent widths is $\sim 1.8$ while the redshift of peak OH absorption is $z = 0.68468 \pm 0.000008$ for both lines; this redshift agrees with that obtained from the HI line. The velocity spread (between nulls) of the HI absorption is $\sim 140$ km/s, while that of both OH lines is $\sim 100$ km/s; the HI and OH spectra are broadly similar in that they each have two principal narrow components and a wide absorption trough. We argue that the wide absorption is likely to arise from source components in the Einstein ring and derive a rotation velocity of $\sim 150$ km/s at a distance of 1.5 kpc from the centre of the $z \sim 0.6846$ galaxy.

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