On the origin of LS 5039 and PSR J1825−1446

¹ Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: jmoldon@am.ub.es
² National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
³ Max Planck Institut für Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany
⁴ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Received: 9 March 2012
Accepted: 16 April 2012

Abstract

Context. The gamma-ray binary LS 5039 and the isolated pulsar PSR J1825−1446 were proposed to have been formed in the supernova remnant (SNR) G016.8−01.1.

Aims. We aim to obtain the Galactic trajectory of LS 5039 and PSR J1825−1446 to find their origin in the Galaxy, and in particular to check their association with SNR G016.8−01.1 to restrict their age.

Methods. By means of radio and optical observations we obtained the proper motion and the space velocity of the sources.

Results. The proper motion of PSR J1825−1446 corresponds to a transverse space velocity of 690 km s^-1 at a distance of 5 kpc. Its Galactic velocity at different distances is not compatible with the expected Galactic rotation. The velocity and characteristic age of PSR J1825−1446 make it incompatible with SNR G016.8−01.1. There are no clear OB associations or SNRs crossing the past trajectory of PSR J1825−1446. We estimate the age of the pulsar to be 80–245 kyr, which is compatible with its characteristic age. The proper motion of LS 5039 is μ_αcosδ = 7.09 and μ_δ =  −8.82 mas yr^-1. The association of LS 5039 with SNR G016.8−01.1 is unlikely, although we cannot discard it. The system would have had to be formed in the association Ser OB2 (at 2.0 kpc) if the age of the system is 1.0–1.2 Myr, or in the association Sct OB3 (distance 1.5–2 kpc) for an age of 0.1–0.2 Myr. If the system were not formed close to Ser OB2, the pseudo-synchronization of the orbit would be unlikely.

Conclusions. PSR J1825−1446 is a high-velocity isolated pulsar ejected from the Galaxy. The distance to LS 5039, which needs to be constrained by future astrometric missions such as Gaia, is a key parameter for restricting its origin and age.