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We present our Swift monitoring campaign of the slowly rotating neutron starBe/X-ray transient GX 304-1 (spin period of ~275 s) when the source was not inoutburst. We found that between its type-I outbursts the source recurrentlyexhibits a slowly decaying low-luminosity state (with luminosities of10^(34-35) erg/s). This behaviour is very similar to what has been observed foranother slowly rotating system, GRO J1008-57. For that source, thislow-luminosity state has been explained in terms of accretion from anon-ionised ('cold') accretion disk. Due to the many similarities between bothsystems, we suggest that GX 304-1 enters a similar accretion regime between itsoutbursts. The outburst activity of GX 304-1 ceased in 2016. Our continuedmonitoring campaign shows that the source is in a quasi-stable low-luminositystate (with luminosities a few factors lower than previously seen) for at leastone year now. Using our NuSTAR observation in this state, we found pulsationsat the spin period, demonstrating that the X-ray emission is due to accretionof matter onto the neutron star surface. If the accretion geometry during thisquasi-stable state is the same as during the cold-disk state, then matterindeed reaches the surface (as predicted) during this latter state. We discussour results in the context of the cold-disk accretion model.

Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1