Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513-46546

Aims: We aimed to monitor the optical linear polarimetric signal of the magnetized, rapidly rotating M8.5 dwarf TVLM 513$-$46546. Methods: $R$- and $I$-band linear polarimetry images were collected with the ALFOSC instrument of the 2.56-m Nordic Optical Telescope on two consecutive nights covering about 0.5 and 4 rotation cycles in the $R$ and $I$ filters, respectively. We also obtained simultaneous intensity curves by means of differential photometry. The typical precision of the data is $\pm$0.46\% ($R$), $\pm$0.35\% ($I$) in the linear polarization degree and $\pm$9 mmag ($R$), $\pm$1.6 mmag ($I$) in the differential intensity curves. Results: Strong and variable linear polarization is detected in the $R$ and $I$ filters, with values of maximum polarization ($p^{*}$ = 1.30$\pm$0.35 \%) similar for both bands. The intensity and the polarimetric curves present a sinusoid-like pattern with a periodicity of $\sim$1.98 h, which we ascribe to structures in TVLM 513$-$46's surface synchronized with rotation. We found that the peaks of the intensity and polarimetric curves occur with a phase difference of 0.18$\pm$0.01, and that the maximum of the linear polarization happens nearly half a period (0.59$\pm$0.03) after the radio pulse. We discussed different scenarios to account for the observed properties of the light curves.