LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

We present an analysis of multi-epoch global VLBI observations of the Compact Symmetric Objects: 2352+495 and 0710+439 at 5 GHz. Analysis of data spread over almost two decades shows strong evidence for an increase in separation of the outer components of both sources at a rate of ∼ 0.2h −1 c (for q • =0.5 and H • = 100h kms −1 Mpc −1 ). Dividing the overall sizes of the sources by their separation rates implies that these Compact Symmetric Objects have a kinematic age ≪ 10 4 years. These results (and those for other CSOs) strongly argue that CSOs are indeed very young sources and that they are probably evolve into the much larger classical doubles.

We present VLBI images of the compact high-luminosity radio galaxy 2352+495 that show symmetric structure on either side of a prominent central core. This contrasts strongly with the asymmetric nuclear structure exhibited by the great majority of powerful extragalactic sources. The outer structure of 2352 + 495 takes the form of. two "n;iini-lob~s" contain~ng hot spots! in this r~spect this compact radio galaxy resembles extende? radio galaxies, but its overa!l ~tze,-150 pc, ts-1000 times smaller. A reanalysis of existing data on th~ radio galaxy 0710+439 shows s1mtlar compact structure, and together these VLBI images confirm the ex1~tence of a ~lass of two-sided compact symmetric objects (CSOs). We show that, in contrast to nuclear rad10. sources m ot~er. powerful objects, the observed structure of CSOs is not dominated by relativistic beammg effects. It ts hkely that many objects previously classified as "compact doubles" will prove to be CSOs when mapped with VLBI with high dynamic range.

The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ∼2 years of observing, 405 cosmic-ray events in the energy range of 10 16 −10 18 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ∼1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.

Abstract.We have observed HNC 1-0, CN 1-0 & 2-1 line emission in a sample of 13 IR luminous (LIRGs, LIR > 10 11 L ) starburst and Seyfert galaxies. HNC 1-0 is detected in 9, CN 1-0 is detected in 10 and CN 2-1 in 7 of the galaxies and all are new detections. We also report the first detection of HC3N (10-9) emission in Arp 220. The excitation of HNC and CN emission requires densities n > 10 4 cm −3 . We compare their intensities to that of the usual high density tracer HCN. The I(HCN) I(HNC) 1-0 and I(HCN) I(CN)1-0 line intensity ratios vary significantly, from 0.5 to > ∼ 6, among the galaxies. This implies that the actual properties of the dense gas is varying among galaxies who otherwise have similar I(CO) I(HCN)line intensity ratios. We suggest that the HNC emission is not a reliable tracer of cold (10 K) gas at the center of LIR galaxies, as it often is in the disk of the Milky Way. Instead, the HNC abundance may remain substantial, despite high gas temperatures, because the emission is emerging from regions where the HCN and HNC formation and destruction processes are dominated by ion-neutral reactions which are not strongly dependent on kinetic temperature. We find five galaxies (Mrk 231, NGC 7469, NGC 7130, IC 694 and NGC 2623) where the I(HCN) I(HNC)intensity ratio is close to unity. Four are classified as active galaxies and one as a starburst. In other active galaxies, however, the I(HCN) I(HNC)is >4. The CN emission is on average a factor of two fainter than the HCN for the luminous IR galaxies, but the variation is large and there seems to be a trend of reduced relative CN luminosity with increasing IR luminosity. This trend is discussed in terms of other PDR tracers such as the [C II] 158 µm line emission. One object, NGC 3690, has a CN luminosity twice that of HCN and its ISM is thus strongly affected by UV radiation. We discuss the I(HCN) I(HNC)and I(HCN) I(CN)line ratios as indicators of starburst evolution. However, faint HNC emission is expected both in a shock dominated ISM as well as for a cloud ensemble dominated by dense warm gas in the very early stages of a starburst. Additional information will help resolve the dichotomy.