Cobalt: A GPU-based correlator and beamformer for LOFAR

Broekema, P. Chris; Mol, J. J. D.; Nijboer, R.; Amesfoort, A. S. van; Brentjens, M. A.; Loose, G. M.; Klijn, Wouter; Romein, John W.

doi:10.1016/j.ascom.2018.04.006

Cited by 42 publications

(33 citation statements)

References 25 publications

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“…These are pointed towards known pulsars that happen to coincide with the SAP field-of-view, or at predetermined positions in the absence of known sources. Finally, the signals from the 12 HBA sub-stations per SAP are summed incoherently (Stappers et al 2011;Broekema et al 2018) to form an incoherent beam with a field-of-view equivalent to that of the station beam. Hence, a total of 222 beams are formed for each LOTAAS pointing.…”

Section: Beam Setup and Tessellationmentioning

confidence: 99%

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

Sanidas

Cooper

Bassa

et al. 2019

A&A

110

132

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We present an overview of the LOFAR Tied-Array All-Sky Survey (LOTAAS) for radio pulsars and fast transients. The survey uses the high-band antennas of the LOFAR Superterp, the dense inner part of the LOFAR core, to survey the northern sky (δ > 0 • ) at a central observing frequency of 135 MHz. A total of 219 tied-array beams (coherent summation of station signals, covering 12 square degrees), as well as three incoherent beams (covering 67 square degrees) are formed in each survey pointing. For each of the 222 beams, total intensity is recorded at 491.52 µs time resolution. Each observation integrates for 1 hr and covers 2592 channels from 119 to 151 MHz. This instrumental setup allows LOTAAS to reach a detection threshold of 1 to 5 mJy for periodic emission. Thus far, the LOTAAS survey has resulted in the discovery of 73 radio pulsars. Among these are two mildly recycled binary millisecond pulsars (P = 13 and 33 ms), as well as the slowest-spinning radio pulsar currently known (P = 23.5 s). The survey has thus far detected 311 known pulsars, with spin periods ranging from 4 ms to 5.0 s and dispersion measures from 3.0 to 217 pc cm −3 . Known pulsars are detected at flux densities consistent with literature values. We find that the LOTAAS pulsar discoveries have, on average, longer spin periods than the known pulsar population. This may reflect different selection biases between LOTAAS and previous surveys, though it is also possible that slower-spinning pulsars preferentially have steeper radio spectra. LOTAAS is the deepest all-sky pulsar survey using a digital aperture array; we discuss some of the lessons learned that can inform the approach for similar surveys using future radio telescopes such as the Square Kilometre Array.

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Section: Beam Setup and Tessellationmentioning

confidence: 99%

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

Sanidas

Cooper

Bassa

et al. 2019

A&A

110

132

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“…3C 196, which was used as the primary fluxdensity calibrator, was observed for 10 minutes at the beginning of the observing run. Data were recorded in 8-bit mode in 231 subbands with a bandwidth of 192 kHz each and averaged into 14 frequency bands, each with 3.12 MHz bandwidth, and correlated with the COBALT (COrrelator and Beamforming Application platform for the Lofar Telescope) correlator (Broekema et al 2018). Given the known highly compact structure of S5 0836+710, we used only the international LOFAR stations 1 and analyzed the data using standard methods of Very-Long-Baseline Interferometry (e.g., Moran & Dhawan 1995) using the AIPS (Astronomical Image Processing System; Greisen 2003) package.…”

Section: Observation and Data Reductionmentioning

confidence: 99%

LOFAR measures the hotspot advance speed of the high-redshift blazar S5 0836+710

Kappes

Perucho

Kadler

et al. 2019

A&A

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Context. The emission and proper motion of the terminal hotspots of AGN jets can be used as a powerful probe of the intergalactic medium. However, measurements of hotspot-advance speeds in active galaxies are difficult, especially in the young universe, due to the low angular velocities and the low brightness of distant radio galaxies. Aims. Our goal is to study the termination of an AGN jet in the young universe and to deduce physical parameters of the jet and the intergalactic medium. Methods. We use the LOw Frequency ARray (LOFAR) to image the long-wavelength radio emission of the high-redshift blazar S5 0836+710 on arcsecond scales between 120 MHz and 160 MHz.Results. The LOFAR image shows a compact unresolved core and a resolved emission region about 1.5 arcsec to the southwest of the radio core. This structure is in general agreement with previous higher-frequency radio observations with MERLIN and the VLA. The southern component shows a moderately steep spectrum with a spectral index of about −1 while the spectral index of the core is flat to slightly inverted. In addition, we detect for the first time a resolved steep-spectrum halo with a spectral index of about −1 surrounding the core.Conclusions. The arcsecond-scale radio structure of S5 0836+710 can be understood as an FR II-like radio galaxy observed at a small viewing angle. The southern component can be interpreted as the region of the approaching jet's terminal hotspot and the halo-like diffuse component near the core can be interpreted as the counter-hotspot region. From the differential Doppler boosting of both features, we can derive the hotspot advance speed to (0.01 − 0.036) c. At a constant advance speed, the derived age of the source would exceed the total lifetime of such a powerful FR II-like radio galaxy substantially. Thus, the hotspot advance speed must have been higher in the past in agreement with a scenario in which the originally highly relativistic jet has lost collimation due to the growth of instabilities and has transformed into an only mildly relativistic flow. Our data suggest that the density of the intergalactic medium around this distant (z = 2.22) AGN could be substantially higher than the values typically found in less distant FR II radio galaxies.

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“…The complex valued beam maps were measured on a regular hexagonal grid, 1.32 and 0.37 deg across for LBA and HBA, respectively. The map size is limited by the number of TABs, spectral windows and stations that the beamformer, COBALT (Broekema et al 2018), can process simultaneously. Per Fourier relation Equation 2 this implies a spatial resolution in the aperture plane of 270 m (HBA at 174 MHz) and 170 m (LBA at 68 MHz), comparable to the diameter of the Superterp (350 m).…”

Section: Methodsmentioning

confidence: 99%

Tied-array holography with LOFAR

Salas

Brentjens

Bordenave

et al. 2020

A&A

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Context. A radio interferometer uses time delays to maximize its response to radiation coming from a particular direction. These time delays compensate for differences in the time of arrival of the wavefront at the different elements of the interferometer, and for delays in the instrument's signal chain. If the radio interferometer is operated as a phased array (tied array), the time delays cannot be accounted for after an observation, so they must be determined in advance. Aims. Our aim is to characterize the time delays between the stations in the core of the LOw Frequency ARray (LOFAR). Methods. We used radio holography to determine the time delays for the core stations of LOFAR (innermost 3.5 km). Using the multibeaming capability of LOFAR we map the voltage beam faster than with a raster scan, while simultaneously calibrating the observed beam continuously. Results. For short radio holographic observations (60 s and 600 s) of 3C196, 3C147, and 3C48 we are able to derive time delays with errors of less than one nanosecond. After applying the derived time delays to the beamformer, the beam shows residuals of less than 20% with respect to the theoretical beam shape. Conclusions. Tied-array holography could be a way towards semi-real-time beam calibration for the Square Kilometer Array.

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Cobalt: A GPU-based correlator and beamformer for LOFAR

Cited by 42 publications

References 25 publications

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

LOFAR measures the hotspot advance speed of the high-redshift blazar S5 0836+710

Tied-array holography with LOFAR

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