VLT observations of the magnetar CXO J164710.2-455216 and the detection of a candidate infrared counterpart

TL;DR

This study used VLT near-infrared observations to identify a candidate infrared counterpart to the magnetar CXO J164710.2-455216, but further variability studies are needed for confirmation.

Contribution

First deep near-infrared imaging of the magnetar's field with adaptive optics, identifying a candidate counterpart and analyzing its properties.

Findings

Candidate counterpart detected with specific magnitudes in J, H, K_S bands.

No significant variability observed between two epochs.

Candidate's colors place it within the main stellar sequence, not conclusively linked to the magnetar.

Abstract

We present deep observations of the field of the magnetar CXOJ164710.2-455216 in the star cluster Westerlund 1, obtained in the near-infrared with the adaptive optics camera NACO@VLT. We detected a possible candidate counterpart at the {\em Chandra} position of the magnetar, of magnitudes $\mathrm{J} = 23.5 \pm 0.2$, $\mathrm{H} = 21.0 \pm 0.1$, and $\mathrm{K}_\mathrm{S} = 20.4 \pm 0.1$. The K$_{\rm S}$-band measurements available for two epochs (2006 and 2013) do not show significant signs of variability but only a marginal indication that the flux varied (at the 2 $\sigma$ level), consistent with the fact that the observations were taken when CXOJ164710.2-455216 was in quiescence. At the same time, we also present colour--magnitude and colour--colour diagrams in the J, H, and K$_{\rm S}$ bands from the 2006 epoch only, the only one with observations in all three bands, showing that the candidate counterpart lies in the main bulk of objects describing a relatively well--defined sequence. Therefore, based on its colours and lack of variability, we cannot yet associate the candidate counterpart to CXOJ164710.2-455216. Future near-infrared observations of the field, following-up a source outburst, would be crucial to confirm the association from the detection of near-infrared variability and colour evolution.