Magnetar counterparts, kinematics and birth sites with HST and JWST

Abstract

Magnetars are highly magnetised, isolated neutron stars with uncertain formation channels. They comprise a potentially significant fraction of the young neutron star population in the Milky Way, and are implicated in the explosion mechanisms of some of the most powerful explosions in nature. We aim to identify magnetars in the near-infrared with Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) imaging, in order to measure their proper motions and search for their birth sites. Candidate infrared counterparts are selected based on variability, colours and proper motions which are outliers with respect to other sources in the field. Precise proper motions are obtained by tying HST/WCF3 and JWST/NIRcam images to the Gaia reference frame. We newly identify counterpart candidates for PSRJ1622-4950, 1RXSJ 170849.0-400910 and CXOUJ164710.2-455216. The past trajectory of the 1RXSJ 170849.0-400910-associated source coincides with the supernova remnant G346.6-0.2. The transverse velocity distribution of magnetars is found to be marginally inconsistent with young pulsars, due primarily to a dearth of high velocity magnetars. A candidate birth site is identified inside the cone of possible past trajectories in nearly every case. We show, based on the inferred kinematic ages, that characteristic ages may frequently be lower than the true age, but caution that this depends on the reliability of the birth site associations. We conclude that magnetars are similar in terms of their kinematics and birth sites to the wider Galactic neutron star population, consistent with magnetar formation being a common outcome of massive star core-collapse. However, tentative evidence for a dearth of high-velocity magnetars is emerging. If real, this may arise from physical differences in the progenitor population giving rise to magnetars, or from differences in their post-formation velocity evolution.

Figures (12)

• Figure 1: HST cutouts with magnetar 3 $\sigma$ X-ray localisations indicated by blue dashed circles 2014ApJS..212....6O. If a counterpart candidate is identified, its position is indicated by a red circle. Only the latest (2025 epoch) F160W images are shown, with the exception of CXOUJ1647 (which is a 2023 JWST/NIRcam F150W image). Each cutout measures 4$\times$4 arcsec, and they are oriented North up, East left.
• Figure 2: For the cases where a good candidate was identified in the 3$\sigma$ error circle based on distinct kinematics, colour and/or variability, and four epochs were available. Counterparts are labelled with diamonds/triangles/pentagons, other objects in the X-ray localisation regions are labelled as circles with bold outlines. Left panel: comparing images taken minutes apart, here we are seeing the astrometric noise. The points correspond to objects detected in both images and are coloured according to their F125W-F160W colour. Middle: the movement of sources between the first and second epochs of F125W imaging. The bulk motion of sources in the field is clear, while the magnetar counterparts tend to stand apart from this systemic drift due to their large natal kicks. The points are coloured by the difference in F125W between the epochs. Right: as for the middle panel, but the difference between the two epochs in F160W. The derived proper motions and their uncertainties are provided in Table \ref{['tab:mu']}.
• Figure 3: As in Figure \ref{['fig:cp_ident_yes']}, but for the cases where six images were available. The $\mu$-only panels for SGR 1935 compare different filters, F140W versus F125W (the panel with SGR 1935 marked with a hexagon) and F160W (SGR 1935 marked with a square).
• Figure 4: As in Figures \ref{['fig:cp_ident_yes']} and \ref{['fig:cp_ident_yes2']}, but for the cases where no good candidate was identified in the 3$\sigma$ error circle based on distinct kinematics, colour and/or variability. The sources marked with diamonds/triangles/pentagons are the candidates selected solely on P$_{\rm chance}$ grounds by 2022MNRAS.512.6093C, the P$_{\rm chance}$ values are (top to bottom) 0.15, 0.05, 0.03 and 0.005.
• Figure 5: Figure \ref{['fig:cp_ident_no']} continued. More cases where no good candidate was identified in the 3$\sigma$ error circle based on distinct kinematics, colour and/or variability. The sources marked with diamonds/triangles etc. are simply the candidates identified on P$_{\rm chance}$ grounds by 2022MNRAS.512.6093C, the P$_{\rm chance}$ values are (top to bottom) 0.03, 0.18, 0.03 and 0.45. The last two panels for CXOUJ 1647 compare the first epoch in 2017 (F140W) with the F125W and F160W images taken in 2026. We use the F150W JWST/NIRcam image for the 2023 epoch.