LOFAR Radio Telescope Research Articles

New-generation radio interferometer

Studies of physical conditions in active galactic nuclei and cosmological studies require low-frequency observations of compact radio sources with high sensitivity and resolution. To advance considerably in this research, observations in the meter wavelength range with a sensitivity of about 1 mJy are needed. The sensitivity of low-frequency observations of extragalactic radio sources is limited by the confusion effect. To suppress it to a 1 mJy level, one needs antennas with apertures of 300–400 km. Currently, new-generation SKA and LOFAR radio telescopes are developed, which are aperture-synthesis systems based on small antennas which have dimensions of several hundreds of kilometers. Such measuring complexes will allow one to achieve a sensitivity of 1 mJy. In this work, we propose to make an interferometer with baseline exceeding 5000 km on the basis of in-phase kilometer antenna arrays. It is shown that such a system will have a sensitivity of about 1 mJy at a frequency of 100 MHz. Along with the high sensitivity, the proposed interferometer based on diffraction gratings will make it possible to form a multilobe directional pattern that will cover the entire observable hemisphere in terms of inclination. This opens up new opportunities for fast surveys of weak small-size radio sources.

Novel matched amplifiers with low noise positive feedback. Part II: Resistive–capacitive feedback

This article is a continuation of consideration for an amplifier with resistive positive feedback (RPF) (Bruck (2008), ‘Novel Matched LNA with Low Noise Positive Feedback. Part 1: General Features and Resistive Feedback’, International Journal of Electronics, 95, 441–456). We propose here new configuration schematics of a transformer-less selective LNA with resistive–capacitive positive feedback (RCPF). A circuit of an amplifier with a transistor connected into a circuit with a common base (CB) configuration is analysed in detail. RCPF and RPF circuits are compared. It is shown that the LNA RCPF provides any pass-band, a good level of input and output matching, a minimum noise temperature which is significantly lower than that of the LNA RPF, a rather high linearity, and stability of amplification. The simulation results and some experimental data for the amplifiers intended for use in the LOFAR radiotelescope (Konovalenko et al. (2003), ‘Thirty Element Array Antenna as a Prototype of a Huge Low-Frequency Radio Telescope,’ Experimental Astronomy, 16, 149–164; Konovalenko (2007), ‘Ukrainian Contribution to LOFAR’, A scientific workshop, organised by LOFAR/ASTRON' Emmen, Netherlands, 23–27. http://www.lofar.org/workshop) are given. It is assumed that such devices are of a special interest for high-frequency integral circuits (IC).

Finding pulsars with LOFAR

We investigate the number and type of pulsars that will be discovered with the low-frequency radio telescope LOFAR. We consider different search strategies for the Galaxy, for globular clusters and for other galaxies. We show that a 25-day all-sky Galactic survey can find approximately 900 new pulsars, probing the local pulsar population to a deep luminosity limit. For targets of smaller angular size such as globular clusters and galaxies many LOFAR stations can be combined coherently, to make use of the full sensitivity. Searches of nearby northern-sky globular clusters can find new low luminosity millisecond pulsars. Giant pulses from Crab-like extragalactic pulsars can be detected out to over a Mpc.

DEEP 1.4 GHz FOLLOW-UP OF THE STEEP SPECTRUM RADIO HALO IN A521

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.

Observations of 44 extragalactic radio sources with the VLBA at 92 cm

<i>Aims. <i/>We have analysed VLBA 92 cm archive data of 44 extragalactic sources in order to identify early targets and potential calibrator sources for the LOFAR radio telescope and the RadioAstron space VLBI mission. Some of these sources will also be suitable as “in-beam” calibrators, permitting deep, wide-field studies of other faint sources in the same field of view.<i>Methods. <i/>All publicly available VLBA 92 cm data observed between 1 January 2003 to December 31, 2006 have been analysed via an automatic pipeline, implemented within AIPS. The vast majority of the data are unpublished.<i>Results. <i/>The sample consists of 44 sources, 34 of which have been detected on at least one VLBA baseline. 30 sources have sufficient data to be successfully imaged. Most of the sources are compact, with a few showing extended structures. Of the 30 sources imaged, 13 are detected on the longest VLBA baselines (~9 M<i>λ<i/>), while all were detected on baselines greater than 2 M<i>λ<i/> (the maximum baseline of LOFAR including the current international baselines).

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

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.

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.

NEUTRINO DETECTION IN SALT DOMES UNDER LOFAR

Large volumes of natural materials are presently under study to construct a telescope, which will be used to search for ν's at the highest energies. Although these high-energy ν's have not been discovered yet, firm predictions for their existence have been made. The very origin of these high-energy ν's remains one of the burning questions in astroparticle physics. We consider the use of huge salt domes in the shallow underground of the north-eastern part of the Netherlands as a possible site for such a high-energy ν telescope. Initial measurement of the attenuation length for radio signals at 0.3 and 1.0 GHz will be reported. The physical location of the domes and the envisioned research program has a strong overlap with those of the new radio telescope LOFAR.