On the origin of LS 5039 and PSR J1825-1446

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.

💡 Research Summary

The paper investigates whether the gamma‑ray binary LS 5039 and the isolated pulsar PSR J1825‑1446 share a common origin in the supernova remnant (SNR) G016.8‑01.1. To address this, the authors reconstruct the three‑dimensional Galactic trajectories of both objects using newly obtained proper motions from radio interferometry (VLA, VLBI) and optical astrometry.

For PSR J1825‑1446, the measured proper motion translates into a transverse velocity of roughly 690 km s⁻¹ if the pulsar lies at a distance of about 5 kpc. This speed is far above the typical circular rotation of the Milky Way disk (~220 km s⁻¹) and indicates that the neutron star received a very strong natal kick at birth. The pulsar’s characteristic age (τ_c ≈ 200 kyr) and the backward integration of its orbit suggest a true age in the range 80–245 kyr, consistent with τ_c. Such an age and velocity are incompatible with the estimated age of SNR G016.8‑01.1 (≈30–50 kyr). Moreover, the past trajectory of the pulsar does not intersect any known OB association or other supernova remnants, reinforcing the conclusion that PSR J1825‑1446 is an isolated high‑velocity pulsar ejected from the Galactic plane and unrelated to the SNR.

LS 5039’s proper motion is measured as (μ_α, μ_δ) = (7.09, ‑8.82) mas yr⁻¹. Because its distance is uncertain (estimates range from 1.5 to 3 kpc), its space velocity is modest, between 30 and 80 km s⁻¹, and roughly follows Galactic rotation. The authors explore two plausible birth environments: the Ser OB2 association at ~2 kpc and the Scutum‑Centaurus OB3 association at 1.5–2 kpc. If LS 5039 originated in Ser OB2, the system’s age would be 1.0–1.2 Myr, which comfortably allows the observed orbital pseudo‑synchronization of the massive companion star. An origin in Sct OB3 would imply a much younger age (0.1–0.2 Myr), making the pseudo‑synchronization scenario unlikely. The authors therefore favour a formation in Ser OB2, but they cannot completely rule out an association with SNR G016.8‑01.1 because the distance uncertainty remains large.

The paper concludes that PSR J1825‑1446 is a high‑velocity, isolated pulsar whose trajectory and age exclude any link to SNR G016.8‑01.1. LS 5039’s origin remains ambiguous; its distance is the critical parameter that determines whether it was born in a nearby OB association or possibly in the SNR. Future astrometric missions, especially Gaia’s forthcoming data releases, will provide the precise parallax and proper motion needed to settle LS 5039’s birthplace and refine its age estimate.