Long-term timing of four millisecond pulsars

¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK e-mail: gemma.janssen@manchester.ac.uk
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
³ Stichting ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁵ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁶ Laboratoire de Physique et Chimie de l'Environnement, CNRS, 3A Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Galaxies, Étoiles, Physique, Instrumentation, CNRS URA 1757, 92195 Meudon Principal Cedex, France

Received: 26 January 2009
Accepted: 15 February 2010

Abstract

We have timed four millisecond pulses, PSRs J1721-2457, J1745–0952, J1810–2005, and J1918–0642, for up to a total of 10.5 years each using multiple telescopes in the European Pulsar Timing Array network: the Westerbork Synthesis Radio Telescope in The Netherlands, the Nançay Radio Telescope in France and the Lovell telescope at Jodrell Bank in the UK. The long time span has enabled us to measure the proper motions of J1745–0952 and J1918–0642, indicating that they have transverse velocities of 200(50) and 54(7) km s^-1 respectively. We have obtained upper limits on the proper motion of J1721–2457 and J1810–2005, which imply that they have transverse velocities less than 140 and 400 km s^-1 respectively. In all cases, the velocities lie in the range typical of millisecond pulsars. We present pulse profiles for each pulsar taken from observations at multiple frequencies in the range of 350 to 2600 MHz, and show that J1810–2005 shows significant profile evolution in this range. Using our multi-frequency observations, we measured the spectral indices for all four pulsars, and for J1810–2005 it appears to be very flat. The flux density of J1918–0642 shows extensive modulation which we attribute to the combined effects of refractive and diffractive scintillation. We discuss the possible use of including J1721–2457 or J1918–0642 in a pulsar timing array, and find that J1918–0642 will be useful to include when the timing precision of this pulsar is improved over the next few years. We have searched archival optical observations to detect companions of the binary pulsars, but none were detected. However, we provide lower limits on the masses of the white dwarf companions of PSRs J1745–0952 and J1918–0642.