* Astronomy

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: 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: 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: 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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