On the origin of LS 5039 and PSR J1825-1446

TL;DR

This study traces the origins of LS 5039 and PSR J1825-1446 through their galactic trajectories, revealing PSR J1825-1446 as a high-velocity pulsar likely ejected from the Galaxy, and assessing the association of LS 5039 with nearby supernova remnants.

Contribution

The paper provides new proper motion measurements and trajectory analysis for LS 5039 and PSR J1825-1446, challenging previous associations with SNR G016.8-01.1 and estimating their ages and origins.

Findings

PSR J1825-1446 has a transverse velocity of 690 km/s at 5 kpc.

PSR J1825-1446's velocity is incompatible with Galactic rotation and SNR G016.8-01.1.

LS 5039's origin is uncertain; association with SNR G016.8-01.1 is unlikely.

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 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 7.09 and -8.82 mas/yr in right ascension and declination, respectively. The association of LS 5039 with SNR G016.8-01.1 is unlikely, although we cannot to 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.