arXiv subjects

R. Vermeulen

Publications and source records attributed to R. Vermeulen.

40 records · Page 3Linked to original sources

arXiv2011

Observing pulsars and fast transients with LOFAR

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

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arXiv2004

The unfriendly ISM in the radio galaxy 4C12.50 (PKS 1345+12)

The radio source 4C12.50 has often been suggested to be a prime candidate for the link between ultraluminous infrared galaxies and young radio galaxies. A VLBI study of the neutral hydrogen in the nuclear regions of this object shows that most of the gas detected close to the systemic velocity is associated with an off-nuclear cloud (~50 to 100 pc from the radio core) with a column density of ~10^22 T_spin/100 K) cm^(-2) and an HI mass of a few times 10^5 to 10^6 M_sun. We consider a number of possibilities to explain the results. In particular, we discus the possibility that this cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud -- at the edge of the northern radio jet/lobe -- suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet. The velocity profile of the gas is relatively broad (~150$ km/s) and we interpret this as kinematical evidence for interaction of the radio plasma with the cloud. We also consider the model where the cloud is part of a broader circumnuclear structure. Only a limited region of this structure would have sufficient background radio brightness and large enough column depth in neutral gas to obtain detectable HI absorption against the counterjet. The VLBI study of the neutral hydrogen in 4C12.50 suggests that HI detected near the systemic velocity (as it is often the case in radio galaxies) may not necessarily be connected with a circumnuclear disk or torus (as is very often assumed) but instead could be a tracer of the large-scale medium that surrounds the active nucleus and that may influence the growth of the young radio source.

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arXiv2001

Deep Westerbork 1.4 GHz Imaging of the Bootes Field

We present the results from our deep (16x12 hour) Westerbork Synthesis Radio Telescope (WSRT) observations of the approximately 7 square degree Bootes Deep Field, centered at 14h32m05.75s, 34d16'47.5" (J2000). Our survey consists of 42 discrete pointings, with enough overlap to ensure a uniform sensitivity across the entire field, with a limiting sensitivity of 28 microJy (1 sigma rms). The catalog contains 3172 distinct sources, of which 316 are resolved by the 13"x27" beam. The Bootes field is part of the optical/near-infrared imaging and spectroscopy survey effort conducted at various institutions. The combination of these data sets, and the deep nature of the radio observations will allow unique studies of a large range of topics including the redshift evolution of the luminosity function of radio sources, the K-z relation and the clustering environment of radio galaxies, the radio / far-infrared correlation for distant starbursts, and the nature of obscured radio loud AGN.

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arXiv2001

Astronomical Constraints on the Cosmic Evolution of the Fine Structure Constant and Possible Quantum Dimensions

We present measurements of absorption by the 21cm hyperfine transition of neutral hydrogen toward radio sources at substantial look-back times. These data are used in combination with observations of rotational transitions of common interstellar molecules, to set limits on the evolution of the fine structure constant: (dot alpha)/alpha < 3.5e-15/yr to a look-back time of 4.8 Gyr. The neutral hydrogen observations employed Very Long Baseline Interferometry in order to mitigate the substantial uncertainty arising from the fact that observations at very different wavelengths may probe different lines-of-site due to frequency dependent structure of the background source. We discuss the implication of these results on theories unifying natural forces based on compact quantum dimensions. In the context of string theory, the limit on the secular evolution of the scale factor of these compact dimensions, R, is (dot R)/R < 1e-15/yr. Including terrestrial and other astronomical measurements places limits (2sigma) on slow oscillations of R from the present to the epoch of cosmic nucleosynthesis, just seconds after the big bang, of (Delta R)/R < 1e-5.

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