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We conducted the first long-term (60 days), multiwavelength (optical,ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swiftobservations, with the goal of understanding variability in the low mass X-raybinary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in allenergy bands on timescales from days to months, with the strongest quiescentvariability a factor of 22 drop in the X-ray count rate in only 4 days. TheX-ray, UV and optical (V band) emission are correlated on timescales down toless than 110 s. The shape of the correlation is a power law with index gammaabout 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere(kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with thespectral shape remaining constant as the flux varies. Both components vary intandem, with each responsible for about 50% of the total X-ray flux, implyingthat they are physically linked. We conclude that the X-rays are likelygenerated by matter accreting down to the NS surface. Moreover, based on theshort timescale of the correlation, we also unambiguously demonstrate that theUV emission can not be due to either thermal emission from the stream impactpoint, or a standard optically thick, geometrically thin disc. The spectralenergy distribution shows a small UV emitting region, too hot to arise from theaccretion disk, that we identified as a hot spot on the companion star.Therefore, the UV emission is most likely produced by reprocessing from thecompanion star, indeed the vertical size of the disc is small and can onlyreprocess a marginal fraction of the X-ray emission. We also found theaccretion disc in quiescence to likely be UV faint, with a minimal contributionto the whole UV flux.

X-ray and UV correlation in the quiescent emission of Cen X-4, evidence of accretion and reprocessing