We conducted the first long-term (60 d), multiwavelength (optical, ultraviolet, UV, and X-ray)simultaneous monitoring of Cen X-4 with daily Swift observations from 2012 June to August,with the goal... Show moreWe conducted the first long-term (60 d), multiwavelength (optical, ultraviolet, UV, and X-ray)simultaneous monitoring of Cen X-4 with daily Swift observations from 2012 June to August,with the goal of understanding variability in the low-mass X-ray binary Cen X-4 duringquiescence. We found Cen X-4 to be highly variable in all energy bands on time-scales fromdays to months, with the strongest quiescent variability a factor of 22 drop in the X-ray countrate in only 4 d. The X-ray, UV and optical (V band) emission are correlated on time-scalesdown to less than 110 s. The shape of the correlation is a power law with index γ about 0.2–0.6.TheX-ray spectrum iswell fitted by a hydrogen neutron star (NS) atmosphere (kT=59–80 eV)and a power law (with spectral index = 1.4–2.0), with the spectral shape remaining constantas the flux varies. Both components vary in tandem, with each responsible for about 50 per centof the total X-ray flux, implying that they are physically linked. We conclude that the X-raysare likely generated by matter accreting down to the NS surface. Moreover, based on the shorttime-scale of the correlation, we also unambiguously demonstrate that the UV emission cannotbe due to either thermal emission from the stream impact point, or a standard optically thick,geometrically thin disc. The spectral energy distribution shows a small UV emitting region,too hot to arise from the accretion disc, that we identified as a hotspot on the companion star.Therefore, the UV emission is most likely produced by reprocessing from the companion star,indeed the vertical size of the disc is small and can only reprocess a marginal fraction of theX-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with aminimal contribution to the whole UV flux. Show less
