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

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
³ ASTRON, The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
e-mail: bassa@astron.nl
⁴ Astro Space Centre, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia
⁵ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
⁶ McGill Space Institute, McGill University, 3550 rue University, Montréal, QC H3A 2A7, Canada
⁷ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Chile
⁸ Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁹ LPC2E – Université d’Orléans / CNRS, 45071 Orléans Cedex 2, France
¹⁰ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, OSUC, 18330 Nançay, France
¹¹ Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹² Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
¹³ Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia

Received: 3 April 2019
Accepted: 7 May 2019

Abstract

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 1–5 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.