Long-term timing of four millisecond pulsars

Astronomy & Astrophysics manuscript no. janssen11728 c⃝ESO 2022 May 27, 2022 Long-term timing of four millisecond pulsars G. H. Janssen1,2, B. W. Stappers1,2,3, C. G. Bassa1,4,5, I. Cognard6, M. Kramer7,1, and G. Theureau6,8 1 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK email: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, F-45071 Orl´eans, Cedex 2, France 7 Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany 8 Galaxies, ´Etoiles, Physique, Instrumentation, CNRS URA 1757, 92195 Meudon Principal Cedex, France Received/Accepted 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 Nanc¸ay 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−1respectively. 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−1respectively. 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. Key words. stars: neutron – pulsars: general – pulsars: individual: J1721−2457, J1745−0952, J1810−2005, J1918−0642 1. Introduction In this paper we present improved timing solutions for four millisecond pulsars (MSPs): PSRs J1721−2457, J1745−0952, and J1918−0642, discovered by Edwards & Bailes (2001), and J1810−2005, that was discovered by Camilo et al. (2001). Long- term timing of MSPs is an important tool to determine masses of the individual stars in binary systems, and by constraining secu- lar effects like proper motion, it can be used to improve statistics on the evolution of these systems. In general, masses of pulsars are not easy to determine. In some cases, when the pulsar is in a binary with another compact object, high precision pulsar timing observations on extended timescales can allow for detecting post-Keplerian parameters of the systems, which can be used to separately measure the indi- vidual masses of the stars. Space velocities derived from proper motion measurements of radio pulsars give clues about the evo- lution of these systems and their birth supernovae (e.g. Toscano et al. (1999); Hobbs et al. (2005); Lorimer (2008)). It is believed that recycled pulsars have lower space velocities than normal, slowly rotating pulsars. However as MSPs are generally the most stable rotators, the effects of their space velocities on their rota- tional and orbital parameters will be easier to derive from timing measurements of those systems and could be used to deduce the intrinsic properties of the stars. Another interesting and important use of timing MSPs to high precision, is the formation of a pulsar timing array (PTA, e.g. Hobbs et al. 2008; Janssen et al. 2008b). An instrument like this will use an array of MSPs as the endpoints of a Galaxy-scale gravitational wave (GW) detector. Current estimates predict that to detect a GW background, long-term high precision timing of about 20 MSPs is required (Jenet et al. 2005; Van Haasteren et al. 2008). Increasing the number of stable MSPs in the array will improve the detection significance. In order to better understand these systems in general, and to determine their suitability for in- clusion in a timing a

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