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TOPIC: LOFAR telescope


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RE: LOFAR telescope
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Title: LOFAR: The LOw-Frequency ARray
Authors: M. P. van Haarlem, M. W. Wise, A. W. Gunst, G. Heald, J. P. McKean, J. W. T. Hessels, A. G. de Bruyn, R. Nijboer, J. Swinbank, R. Fallows, M. Brentjens, A. Nelles, R. Beck, H. Falcke, R. Fender, J. Hörandel, L. V. E. Koopmans G. Mann, G. Miley, H. Röttgering, B. W. Stappers, R. A. M. J. Wijers, S. Zaroubi, M. van den Akker, A. Alexov, J. Anderson, K. Anderson, A. van Ardenne, M. Arts, A. Asgekar, I. M. Avruch, F. Batejat, L. Bähren, M. E. Bell, M. R. Bell, I. van Bemmel, P. Bennema, M. J. Bentum, G. Bernardi, P. Best, L. Bîrzan, A. Bonafede, A.-J. Boonstra, R. Braun, J. Bregman, F. Breitling, R. H. van de Brink, J. Broderick, P. C. Broekema, W. N. Brouw, M. Brüggen, H. R. Butcher, W. van Cappellen, B. Ciardi, T. Coenen, J. Conway, A. Coolen, A. Corstanje, S. Damstra, et al. (139 additional authors not shown)

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.

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Title: Detecting Radio Emission from Air Showers with LOFAR
Authors: Anna Nelles, Stijn Buitink, Arthur Corstanje, Emilio Enriquez, Heino Falcke, Wilfred Frieswijk, Jörg Hörandel, Maaijke Mevius, Satyendra Thoudam, Pim Schellart, Olaf Scholten, Sander ter Veen, Martin van den Akker, The LOFAR Collaboration

LOFAR (the Low Frequency Array) is the largest radio telescope in the world for observing low frequency radio emission from 10 to 240 MHz. In addition to its use as an interferometric array, LOFAR is now routinely used to detect cosmic ray induced air showers by their radio emission. The LOFAR core in the Netherlands has a higher density of antennas than any dedicated cosmic ray experiment in radio. On an area of 12 km² more than 2300 antennas are installed. They measure the radio emission from air showers with unprecedented precision and, therefore, give the perfect opportunity to disentangle the physical processes which cause the radio emission in air showers. In parallel to ongoing astronomical observations LOFAR is triggered by an array of particle detectors to record time-series containing cosmic-ray pulses. Cosmic rays have been measured with LOFAR since June 2011. We present the results of the first year of data.

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Title: Pilot pulsar surveys with LOFAR
Authors: Thijs Coenen, The LOFAR Pulsar Working Group

We are performing two complementary pilot pulsar surveys as part of LOFAR commissioning. The LOFAR Pilot Pulsar Survey (LPPS) is a shallow all-sky survey using an incoherent combination of LOFAR stations. The LOFAR Tied-Array Survey (LOTAS) is a deeper pilot survey using 19 simultaneous tied-array beams. These will inform a forthcoming deep survey of the entire northern hemisphere, which is expected to discover hundreds of pulsars. Here we present early results from LPPS and LOTAS, among which are two independent pulsar discoveries.

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Title: New results from LOFAR
Authors: Vladislav Kondratiev, Ben Stappers, the LOFAR Pulsar Working Group

The LOw Frequency Array, LOFAR, is a next generation radio telescope with its core in the Netherlands and elements distributed throughout Europe. It has exceptional collecting area and wide bandwidths at frequencies from 10 MHz up to 250 MHz. It is in exactly this frequency range where pulsars are brightest and also where they exhibit rapid changes in their emission profiles. Although LOFAR is still in the commissioning phase it is already collecting data of high quality. I will present highlights from our commissioning observations which will include: unique constraints on the site of pulsar emission, individual pulse studies, observations of millisecond pulsars, using pulsars to constrain the properties of the magneto-ionic medium and pilot pulsars surveys. I will also discuss future science projects and advances in the observing capabilities.

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Title: Observations of transients and pulsars with LOFAR international stations and the ARTEMIS backend
Authors: Maciej Serylak, Aris Karastergiou, Chris Williams, Wesley Armour, Michael Giles, LOFAR Pulsar Working Group

The LOw FRequency ARray - LOFAR is a new radio interferometer which design places emphasis on flexible digital hardware instead of mechanical solutions. The array elements, so-called stations, are located in the Netherlands and in neighbouring countries. The design of LOFAR allows independent use of its international stations which coupled with a dedicated backend make them very powerful telescopes in their own right. Such backend is called the Advanced Radio Transient Event Monitor and Identification System (ARTEMIS). It is a combined software/hardware solution for both targeted observations and real-time searches for millisecond radio transients which uses Graphical Processing Unit (GPU) technology to remove interstellar dispersion and detect millisecond radio bursts from astronomical sources in real-time.

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Title: M87 at metre wavelengths: the LOFAR picture
Authors: F. de Gasperin, E. Orru', M. Murgia, A. Merloni, H. Falcke, R. Beck, R. Beswick, L. Birzan, A. Bonafede, M. Bruggen, G. Brunetti, K. Chyzy, J. Conway, J.H. Croston, T. Ensslin, C. Ferrari, G. Heald, S. Heidenreich, N. Jackson, G. Macario, J. McKean, G. Miley, R. Morganti, A. Offringa, R. Pizzo, D. Rafferty, H. Roettgering, A. Shulevski, M. Steinmetz, C. Tasse, S. van der Tol, W. van Driel, R. J. van Weeren, J. E. van Zwieten, A. Alexov, J. Anderson, A. Asgekar, M. Avruch, M. Bell, M. R. Bell, M. Bentum, G. Bernardi, P. Best, F. Breitling, J. W. Broderick, A. Butcher, B. Ciardi, R. J. Dettmar, J. Eisloeffel, W. Frieswijk, H. Gankema, M. Garrett, M. Gerbers, J. M. Griessmeier, A. W. Gunst, T. E. Hassall, J. Hessels, M. Hoeft, A. Horneffer, A. Karastergiou, J. Koehler, Y. Koopman, G. Kuper, et al. (32 additional authors not shown)

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 solar masses, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.

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Title: The LOFAR radio environment
Authors: A. R. Offringa, A. G. de Bruyn, S. Zaroubi, G. van Diepen, O. Martinez-Ruby, P. Labropoulos, M. A. Brentjens, B. Ciardi, S. Daiboo, G. Harker, V. Jelic, S. Kazemi, L. V. E. Koopmans, G. Mellema, V. N. Pandey, R. F. Pizzo, J. Schaye, H. Vedantham, V. Veligatla, S. J. Wijnholds, S. Yatawatta, P. Zarka, A. Alexov, J. Anderson, A. Asgekar, M. Avruch, R. Beck, M. Bell, M. R. Bell, M. Bentum, G. Bernardi, P. Best, L. Birzan, A. Bonafede, F. Breitling, J. W. Broderick, M. Bruggen, H. Butcher, J. Conway, M. de Vos, R. J. Dettmar, J. Eisloeffel, H. Falcke, R. Fender, W. Frieswijk, M. Gerbers, J. M. Griessmeier, A. W. Gunst, T. E. Hassall, G. Heald, J. Hessels, M. Hoeft, A. Horneffer, A. Karastergiou, V. Kondratiev, Y. Koopman, M. Kuniyoshi, G. Kuper, P. Maat, G. Mann, J. McKean, H. Meulman, M. Mevius, et al. (33 additional authors not shown)

Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions: Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.

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Title: LOFAR insights into the epoch of reionisation from the cross power spectrum of 21cm emission and galaxies
Authors: R. P. C. Wiersma, B. Ciardi, R. M. Thomas, G. J. A. Harker, S. Zaroubi, G. Bernardi, M. Brentjens, A. G. de Bruyn, S. Daiboo, V. Jelic, S. Kazemi, L. V. E. Koopmans, P. Labropoulos, O. Martinez, G. Mellema, A. Offringa, V. N. Pandey, J. Schaye, V. Veligatla, H. Vedantham, S. Yatawatta

Using a combination of N-body simulations, semi-analytic models and radiative transfer calculations, we have estimated the theoretical cross power spectrum between galaxies and the 21cm emission from neutral hydrogen during the epoch of reionisation. In accordance with previous studies, we find that the 21cm emission is initially correlated with halos on large scales (> 30 Mpc), anti-correlated on intermediate (~ 5 Mpc), and uncorrelated on small (< 3 Mpc) scales. This picture quickly changes as reionisation proceeds and the two fields become anti-correlated on large scales. The normalisation of the cross power spectrum can be used to set constraints on the average neutral fraction in the intergalactic medium and its shape can be a tool to study the topology of reionisation. When we apply a drop-out technique to select galaxies and add to the 21cm signal the noise expected from the LOFAR telescope, we find that while the normalisation of the cross power spectrum remains a useful tool for probing reionisation, its shape becomes too noisy to be informative. On the other hand, for a Lyalpha Emitter (LAE) survey both the normalisation and the shape of the cross power spectrum are suitable probes of reionisation. A closer look at a specific planned LAE observing program using Subaru Hyper-Suprime Cam reveals concerns about the strength of the 21cm signal at the planned redshifts. If the ionised fraction at z ~ 7 is lower that the one estimated here, then using the cross power spectrum may be a useful exercise given that at higher redshifts and neutral fractions it is able to distinguish between two toy models with different topologies.

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World's largest radio telescope LOFAR open to worldwide community

International LOFAR Telescope Regular Proposal Call Open to the Worldwide Community
Submission deadline: Monday September 17, at 12 UT.

Cycle 0 will run from 01 December 2012 to 31 May 2013.

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Onsala Space Observatory
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World's largest radio telescope bigger with Swedish LOFAR station

On Monday, Sweden's Minister for Education and Research, Jan Björklund, will open Onsala Space Observatory's newest telescope. Part of Lofar, the world's largest radio telescope, it is the biggest telescope built in Sweden in the last 35 years. Lofar will map radio signals which have travelled across the universe for billions of years. Scientists expect Lofar to answer questions about the nature of our universe.
Lofar (Low Frequency Array) is a new kind of radio telescope. It can see radio waves with low frequencies, similar to those that give us FM radio. Rather than collecting signals from individual radio sources, Lofar continuously monitors large swathes of sky. Lofar is more sensitive to the longest observable radio waves than any other telescope. It can see many billions of light years out into space, back to the time before the first stars formed, a few hundred million years after the Big Bang.

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