Constraints on optical and near-infrared variability in the localisation of the long-period radio transient GLEAM-X J1627-52

TL;DR

This study investigates the optical and near-infrared environment of the long-period radio transient GLEAM-X J1627-52, finding no clear counterparts or variability, thus constraining models of its nature.

Contribution

The paper provides the first detailed spectrophotometric and variability analysis of the localisation region of GLEAM-X J1627-52, setting limits on potential counterparts and informing models of its origin.

Findings

No white dwarf spectral signatures detected.

No variability or periodicity found in optical/near-infrared data.

White dwarf pulsar scenario not supported by current data.

Abstract

GLEAM-X J1627-52 was discovered as a periodic (~18 min) radio signal over a duration of three months in 2018. It is an enigmatic example of a growing population of 'long-period radio transients' consistent with Galactic origins. Their nature is uncertain, and leading models invoke magnetic neutron stars or white dwarfs, potentially in close binary systems, to power them. GLEAM-X J1627-52 resides in the Galactic plane with a comparatively coarse localisation (~2 arcsecond). Here we study the localisation region to search for spectrophotometric signatures of a counterpart using time-domain searches in optical and near-infrared imaging, and MUSE integral field spectroscopy. No sources in the localisation display clear white dwarf spectral signatures, although at the expected distance we can only provide modest limits on their presence directly. We rule out the presence of hot sub-dwarfs in the vicinity. We found no candidate within our search for variability or periodic behaviour in the light curves. Radial velocity curves additionally show only weak evidence of variation, requiring any realistic underlying system to have very low orbital inclination (i < 5 deg). Two Balmer emission line sources are reminiscent of white dwarf pulsar systems, but their characteristics fall within expected M-dwarf chromospheric activity with no signs of being in a close binary. Currently the white dwarf pulsar scenario is not supported, although longer baseline data and data contemporaneous with a radio active epoch are required before stronger statements. Isolated magnetars, or compact binaries remain viable. Our limits highlight the difficulty of these searches in dense environments at the limits of ground-based data.