# The global and small-scale magnetic fields of fully convective, rapidly spinning M dwarf pair GJ65A and B

## TL;DR

This study directly compares the large-scale and local surface magnetic fields of the binary M dwarf GJ65AB, revealing significant differences in their global magnetic topologies despite similar stellar properties, which impacts their magnetic activity signatures.

## Contribution

It provides the first direct analysis of both large-scale and local magnetic fields in GJ65AB, highlighting the contrast in their magnetic topologies and fluxes.

## Key findings

- GJ65B has an axisymmetric, dipolar global field of 1.3 kG.
- GJ65A has a weaker, complex 0.3 kG field.
- Both stars have similar total magnetic fluxes around 5-7 kG.

## Abstract

The nearby M dwarf binary GJ65AB, also known as BL Cet and UV Cet, is a unique benchmark for investigation of dynamo-driven activity of low-mass stars. Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as studies of flares, photometric variability, X-ray and radio emission. Here we present a direct analysis of large-scale and local surface magnetic fields in both components. Interpreting high-resolution circular polarisation spectra (sensitive to a large-scale field geometry) we uncovered a remarkable difference of the global stellar field topologies. Despite nearly identical masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like global field with an average strength of 1.3 kG while the primary has a much weaker, more complex and non-axisymmetric 0.3 kG field. On the other hand, an analysis of the differential Zeeman intensification (sensitive to the total magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and 6.7 kG for GJ65A and B, respectively, although there is an evidence that the field strength distribution in GJ65B is shifted towards a higher field strength compared to GJ65A. Based on these complementary magnetic field diagnostic results we suggest that the dissimilar radio and X-ray variability of GJ65A and B is linked to their different global magnetic field topologies. However, this difference appears to be restricted to the upper atmospheric layers but does not encompass the bulk of the stars and has no influence on the fundamental stellar properties.