An Optical Counterpart Candidate for the Isolated Neutron Star RBS1774

TL;DR

This study reports the first optical detection of the neutron star RBS1774 using VLT, revealing an optical flux much higher than expected from X-ray data, challenging existing emission models and suggesting unique surface or geometric properties.

Contribution

First optical counterpart detection of RBS1774 with deep VLT imaging, providing new data on its optical flux and challenging current emission models.

Findings

Optical flux exceeds X-ray blackbody extrapolation by a factor of ~35.

Candidate optical counterpart identified with high positional accuracy.

Optical emission characteristics challenge existing thermal and magnetospheric emission models.

Abstract

Multiwavelength studies of the seven identified X-ray dim isolated neutron stars (XDINSs) offer a unique opportunity to investigate their surface thermal and magnetic structure and the matter-radiation interaction in presence of strong gravitational and magnetic fields. As a part of an ongoing campaign aimed at a complete identification and spectral characterization of XDINSs in the optical band, we performed deep imaging with the ESO Very Large Telescope (VLT) of the field of the XDINS RBS1774 (1RXS J214303.7 +065419). The recently upgraded FORS1 instrument mounted on the VLT provided the very first detection of a candidate optical counterpart in the B band. The identification is based on a very good positional coincidence with the X-ray source (chance probability ~2E-3). The source has B=27.4 +/- 0.2 (1 sigma confidence level), and the optical flux exceeds the extrapolation of the X-ray blackbody at optical wavelengths by a factor ~35 (+/- 20 at 3sigma confidence level). This is barely compatible with thermal emission from the neutron star surface, unless the source distance is d~200-300 pc, and the star is an almost aligned rotator or its spin axis is nearly aligned with the line of sight. At the same time, such a large optical excess appears difficult to reconcile with rotation-powered magnetospheric emission, unless the source has an extremely large optical emission efficiency. The implications and possible similarities with the optical spectra of other isolated NSs are discussed.