Aris Karastergiou: Publications

LOFAR: The LOw-Frequency ARray

ArXiv 1305.355 (2013)

Authors:

MP van Haarlem, MW Wise, AW Gunst, G Heald, JP McKean, JWT Hessels, AG de Bruyn, R Nijboer, J Swinbank, R Fallows, M Brentjens, A Nelles, R Beck, H Falcke, R Fender, J Hörandel, LVE Koopmans, G Mann, G Miley, H Röttgering, BW Stappers, RAMJ Wijers, S Zaroubi, M van den Akker, A Alexov, J Anderson, K Anderson, A van Ardenne, M Arts, A Asgekar, IM Avruch, F Batejat, L Bähren, ME Bell, MR Bell, I van Bemmel, P Bennema, MJ Bentum, G Bernardi, P Best, L Bîrzan, A Bonafede, A-J Boonstra, R Braun, J Bregman, F Breitling, RH van de Brink, J Broderick, PC Broekema, WN Brouw, M Brüggen, HR Butcher, W van Cappellen, B Ciardi, T Coenen, J Conway, A Coolen, A Corstanje, S Damstra, O Davies, AT Deller, R-J Dettmar, G van Diepen, K Dijkstra, P Donker, A Doorduin, J Dromer, M Drost, A van Duin, J Eislöffel, J van Enst, C Ferrari, W Frieswijk, H Gankema, MA Garrett, F de Gasperin, M Gerbers, E de Geus, J-M Grießmeier, T Grit, P Gruppen, JP Hamaker, T Hassall, M Hoeft, H Holties, A Horneffer, A van der Horst, A van Houwelingen, A Huijgen, M Iacobelli, H Intema, N Jackson, V Jelic, A de Jong, E Juette, D Kant, A Karastergiou, A Koers, H Kollen, VI Kondratiev, E Kooistra, Y Koopman, A Koster, M Kuniyoshi, M Kramer, G Kuper, P Lambropoulos, C Law, J van Leeuwen, J Lemaitre, M Loose, P Maat, G Macario, S Markoff, J Masters, D McKay-Bukowski, H Meijering, H Meulman, M Mevius, E Middelberg, R Millenaar, JCA Miller-Jones, RN Mohan, JD Mol, J Morawietz, R Morganti, DD Mulcahy, E Mulder, H Munk, L Nieuwenhuis, R van Nieuwpoort, JE Noordam, M Norden, A Noutsos, AR Offringa, H Olofsson, A Omar, E Orrú, R Overeem, H Paas, M Pandey-Pommier, VN Pandey, R Pizzo, A Polatidis, D Rafferty, S Rawlings, W Reich, J-P de Reijer, J Reitsma, A Renting, P Riemers, E Rol, JW Romein, J Roosjen, M Ruiter, A Scaife, K van der Schaaf, B Scheers, P Schellart, A Schoenmakers, G Schoonderbeek, M Serylak, A Shulevski, J Sluman, O Smirnov, C Sobey, H Spreeuw, M Steinmetz, CGM Sterks, H-J Stiepel, K Stuurwold, M Tagger, Y Tang, C Tasse, I Thomas, S Thoudam, MC Toribio, B van der Tol, O Usov, M van Veelen, A-J van der Veen, S ter Veen, JPW Verbiest, R Vermeulen, N Vermaas, C Vocks, C Vogt, M de Vos, E van der Wal, R van Weeren, H Weggemans, P Weltevrede, S White, SJ Wijnholds, T Wilhelmsson, O Wucknitz, S Yatawatta, P Zarka, A Zensus, J van Zwieten

Abstract:

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.

Differential frequency-dependent delay from the pulsar magnetosphere

Astronomy and Astrophysics 552 (2013)

Authors:

TE Hassall, BW Stappers, P Weltevrede, JWT Hessels, A Alexov, T Coenen, A Karastergiou, M Kramer, EF Keane, VI Kondratiev, J Van Leeuwen, A Noutsos, M Pilia, M Serylak, C Sobey, K Zagkouris, R Fender, ME Bell, J Broderick, J Eislöffel, H Falcke, JM Grießmeier, M Kuniyoshi, JCA Miller-Jones, MW Wise, O Wucknitz, P Zarka, A Asgekar, F Batejat, MJ Bentum, G Bernardi, P Best, A Bonafede, F Breitling, M Brüggen, HR Butcher, B Ciardi, F De Gasperin, JP De Reijer, S Duscha, RA Fallows, C Ferrari, W Frieswijk, MA Garrett, AW Gunst, G Heald, M Hoeft, E Juette, P Maat, JP McKean, MJ Norden, M Pandey-Pommier, R Pizzo, AG Polatidis, W Reich, H Röttgering, J Sluman, Y Tang, C Tasse, R Vermeulen, RJ Van Weeren, SJ Wijnholds, S Yatawatta

Abstract:

Some radio pulsars show clear "drifting subpulses", in which subpulses are seen to drift in pulse longitude in a systematic pattern. Here we examine how the drifting subpulses of PSR B0809+74 evolve with time and observing frequency. We show that the subpulse period (P3) is constant on timescales of days, months and years, and between 14-5100 MHz. Despite this, the shapes of the driftbands change radically with frequency. Previous studies have concluded that, while the subpulses appear to move through the pulse window approximately linearly at low frequencies (<500 MHz), a discrete step of ~180 in subpulse phase is observed at higher frequencies (>820 MHz) near to the peak of the average pulse profile. We use LOFAR, GMRT, GBT, WSRT and Effelsberg 100-m data to explore the frequency-dependence of this phase step. We show that the size of the subpulse phase step increases gradually, and is observable even at low frequencies. We attribute the subpulse phase step to the presence of two separate driftbands, whose relative arrival times vary with frequency - one driftband arriving 30 pulses earlier at 20 MHz than it does at 1380 MHz, whilst the other arrives simultaneously at all frequencies. The drifting pattern which is observed here cannot be explained by either the rotating carousel model or the surface oscillation model, and could provide new insight into the physical processes happening within the pulsar magnetosphere. © ESO, 2013.

LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A

ArXiv 1302.3128 (2013)

Authors:

Ashish Asgekar, JBR Oonk, S Yatawatta, RJ van Weeren, JP McKean, G White, N Jackson, J Anderson, IM Avruch, F Batejat, R Beck, ME Bell, MR Bell, I van Bemmel, MJ Bentum, G Bernardi, P Best, L Birzan, A Bonafede, R Braun, F Breitling, RH van de Brink, J Broderick, WN Brouw, M Bruggen, HR Butcher, W van Cappellen, B Ciardi, JE Conway, F de Gasperin, E de Geus, A de Jong, M de Vos, S Duscha, J Eisloffel, H Falcke, RA Fallows, C Ferrari, W Frieswijk, MA Garrett, J-M Griesmeier, T Grit, AW Gunst, TE Hassall, G Heald, JWT Hessels, M Hoeft, M Iacobelli, H Intema, E Juette, A Karastergiou, J Kohler, VI Kondratiev, M Kuniyoshi, G Kuper, C Law, J van Leeuwen, P Maat, G Macario, G Mann, S Markoff, D McKay-Bukowski, M Mevius, JCA Miller-Jones, JD Mol, R Morganti, DD Mulcahy, H Munk, MJ Norden, E Orru, H Paas, M Pandey-Pommier, VN Pandey, R Pizzo, AG Polatidis, W Reich, H Rottgering, B Scheers, A Schoenmakers, J Sluman, O Smirnov, C Sobey, M Steinmetz, M Tagger, Y Tang, C Tasse, R Vermeulen, C Vocks, RAMJ Wijers, MW Wise, O Wucknitz, P Zarka

Abstract:

Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15-80 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 10^-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band.

Synchronous x-ray and radio mode switches: a rapid global transformation of the pulsar magnetosphere.

Science 339:6118 (2013) 436-439

Authors:

W Hermsen, JWT Hessels, L Kuiper, J van Leeuwen, D Mitra, J de Plaa, JM Rankin, BW Stappers, GAE Wright, R Basu, A Alexov, T Coenen, J-M Grießmeier, TE Hassall, A Karastergiou, E Keane, VI Kondratiev, M Kramer, M Kuniyoshi, A Noutsos, M Serylak, M Pilia, C Sobey, P Weltevrede, K Zagkouris, A Asgekar, IM Avruch, F Batejat, ME Bell, MR Bell, MJ Bentum, G Bernardi, P Best, L Bîrzan, A Bonafede, F Breitling, J Broderick, M Brüggen, HR Butcher, B Ciardi, S Duscha, J Eislöffel, H Falcke, R Fender, C Ferrari, W Frieswijk, MA Garrett, F de Gasperin, E de Geus, AW Gunst, G Heald, M Hoeft, A Horneffer, M Iacobelli, G Kuper, P Maat, G Macario, S Markoff, JP McKean, M Mevius, JCA Miller-Jones, R Morganti, H Munk, E Orrú, H Paas, M Pandey-Pommier, VN Pandey, R Pizzo, AG Polatidis, S Rawlings, W Reich, H Röttgering, AMM Scaife, A Schoenmakers, A Shulevski, J Sluman, M Steinmetz, M Tagger, Y Tang, C Tasse, S ter Veen, R Vermeulen, RH van de Brink, RJ van Weeren, RAMJ Wijers, MW Wise, O Wucknitz, S Yatawatta, P Zarka

Abstract:

Pulsars emit from low-frequency radio waves up to high-energy gamma-rays, generated anywhere from the stellar surface out to the edge of the magnetosphere. Detecting correlated mode changes across the electromagnetic spectrum is therefore key to understanding the physical relationship among the emission sites. Through simultaneous observations, we detected synchronous switching in the radio and x-ray emission properties of PSR B0943+10. When the pulsar is in a sustained radio-"bright" mode, the x-rays show only an unpulsed, nonthermal component. Conversely, when the pulsar is in a radio-"quiet" mode, the x-ray luminosity more than doubles and a 100% pulsed thermal component is observed along with the nonthermal component. This indicates rapid, global changes to the conditions in the magnetosphere, which challenge all proposed pulsar emission theories.

Initial deep LOFAR observations of Epoch of Reionization windows: I. The North Celestial Pole

ArXiv 1301.163 (2013)

Authors:

S Yatawatta, AG de Bruyn, MA Brentjens, P Labropoulos, VN Pandey, S Kazemi, S Zaroubi, LVE Koopmans, AR Offringa, V Jelic, O Martinez Rubi, V Veligatla, SJ Wijnholds, WN Brouw, G Bernardi, B Ciardi, S Daiboo, G Harker, G Mellema, J Schaye, R Thomas, H Vedantham, E Chapman, FB Abdalla, A Alexov, J Anderson, IM Avruch, F Batejat, ME Bell, MR Bell, M Bentum, P Best, A Bonafede, J Bregman, F Breitling, RH van de Brink, JW Broderick, M Bruggen, J Conway, F de Gasperin, E de Geus, S Duscha, H Falcke, RA Fallows, C Ferrari, W Frieswijk, MA Garrett, JM Griessmeier, AW Gunst, TE Hassall, JWT Hessels, M Hoeft, M Iacobelli, E Juette, A Karastergiou, VI Kondratiev, M Kramer, M Kuniyoshi, G Kuper, J van Leeuwen, P Maat, G Mann, JP McKean, M Mevius, JD Mol, H Munk, R Nijboer, JE Noordam, MJ Norden, E Orru, H Paas, M Pandey-Pommier, R Pizzo, AG Polatidis, W Reich, HJA Rottgering, J Sluman, O Smirnov, B Stappers, M Steinmetz, M Tagger, Y Tang, C Tasse, S ter Veen, R Vermeulen, RJ van Weeren, M Wise, O Wucknitz, P Zarka

Abstract:

The aim of the LOFAR Epoch of Reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. One of the prospective observing windows for the LOFAR EoR project will be centered at the North Celestial Pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. With about 3 nights, of 6 hours each, effective integration we have achieved a noise level of about 100 microJy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 microJy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP.

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