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We present low-radio-frequency follow-up observations of AT 2017gfo, theelectromagnetic counterpart of GW170817, which was the first binary neutronstar merger to be detected by Advanced LIGO-Virgo. These data, with a centralfrequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. Themaximum elevation of the target is just 13.7 degrees when observed with LOFAR,making our observations particularly challenging to calibrate and significantlylimiting the achievable sensitivity. On time-scales of 130-138 and 371-374 daysafter the merger event, we obtain 3$\sigma$ upper limits for the afterglowcomponent of 6.6 and 19.5 mJy beam$^{-1}$, respectively. Using our best upperlimit and previously published, contemporaneous higher-frequency radio data, weplace a limit on any potential steepening of the radio spectrum between 610 and144 MHz: the two-point spectral index $\alpha^{610}_{144} \gtrsim -2.5$. Wealso show that LOFAR can detect the afterglows of future binary neutron starmerger events occurring at more favourable elevations.

LOFAR 144-MHz follow-up observations of GW170817