Proper motions of thermally emitting isolated neutron stars measured with Chandra

The remarkable astrometric capabilities of Chandra offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0-5022, RX J0806.4-4123, and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as reference the background of extragalactic or remote Galactic X-ray sources. We derive 2 sigma upper limits of 123 mas/yr and 86 mas/yr on the proper motion of RX J0420.0-5022 and RX J0806.4-4123, respectively. RX J1308.6+2127 exhibits a very significant displacement (~ 9 sigma) yielding mu = 220 +/- 25 mas/yr, the second fastest measured among all ROSAT discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ~ 740 (d/700pc) km/s might be the largest of all ROSAT INSs and its corresponding spatial velocity stands among the fastest recorded for neutron stars. RX J1308.6+2127 is thus a middle-aged (age ~ 1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. The distribution in transverse velocity of ROSAT INSs is not statistically different from that of normal radio pulsars.

💡 Research Summary

The paper exploits the superb astrometric capabilities of the Chandra X‑ray Observatory to measure proper motions of three thermally emitting, radio‑quiet isolated neutron stars (INSs) discovered by ROSAT: RX J0420.0‑5022, RX J0806.4‑4123, and RX J1308.6+2127. Because these objects either lack an optical counterpart or have counterparts so faint that even the most powerful ground‑based or space‑based optical facilities are at their limits, X‑ray astrometry provides a unique avenue for motion studies.

Observations were performed with the ACIS detector in two epochs separated by three to five years. The authors used a set of background extragalactic or distant Galactic X‑ray sources as a reference frame, applying careful corrections for the point‑spread function, detector geometry, and any systematic drifts. By aligning the reference sources with a multi‑parameter linear transformation (translation, rotation, scale), they achieved sub‑arcsecond relative positional accuracy.

For RX J0420.0‑5022 and RX J0806.4‑4123 the analysis yields only 2‑sigma upper limits on the proper motion: ≤ 123 mas yr⁻¹ and ≤ 86 mas yr⁻¹, respectively. These limits are already stringent given the faintness of the optical counterparts and demonstrate that Chandra can place meaningful constraints on the kinematics of otherwise inaccessible INSs.

In contrast, RX J1308.6+2127 shows a highly significant displacement (≈ 9σ), with a measured proper motion μ = 220 ± 25 mas yr⁻¹. Assuming a distance of 700 pc, this translates into a transverse velocity of roughly 740 km s⁻¹, making it one of the fastest moving ROSAT INSs and placing its space velocity among the highest recorded for neutron stars. Such a speed implies that the star is rapidly moving away from the Galactic plane, effectively precluding any substantial accretion from the interstellar medium. The inferred age, based on its spin‑down rate, is about 1 Myr, consistent with a middle‑aged cooling neutron star.

The authors investigated possible birth sites by tracing the trajectory back to nearby OB associations and isolated OB stars. In most scenarios the flight time from the putative birthplace is shorter than the characteristic age derived from the spin‑down, suggesting that the characteristic age may overestimate the true age, a well‑known issue for neutron stars with complex magnetic‑field evolution.

Finally, the transverse‑velocity distribution of the seven ROSAT INSs is compared with that of normal radio pulsars (≈ 400 objects). Statistical tests reveal no significant difference between the two populations, indicating that the ROSAT INSs are not a kinematically distinct subgroup but rather share the same velocity spectrum as the broader pulsar population.

In summary, this work demonstrates that long‑baseline Chandra imaging can deliver proper‑motion measurements with unprecedented accuracy in the X‑ray band, even for sources lacking viable optical references. The detection of a very high proper motion for RX J1308.6+2127 provides new constraints on neutron‑star cooling, birth‑site identification, and the relationship between characteristic and true ages. The results also reinforce the notion that isolated neutron stars, whether radio‑loud or radio‑quiet, occupy a common kinematic regime within the Galaxy. Future synergy with Gaia astrometry and additional Chandra epochs will further refine the motion census of this intriguing class of objects.