Abstract
We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations, with the goal of understanding variability in the low mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on timescales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index γ about 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 Γ = 1.4 − 2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50% of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission can not be 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 disk, that we identified as a hot spot 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 the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.
Highlights
The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disk, that we identified as a hot spot on the companion star
We found the spectra well fitted by a model made by the sum of a thermal component in the form of hydrogen NS atmosphere plus a non-thermal power law tail, both multiplied by the interstellar absorption
We showed that the X-ray and UV light-curve of Cen X-4 during quiescence are both highly variable, with the fractional root mean square variability Fvar = 73.0 ± 1.5% and 50.0 ± 1.4% respectively, while the optical light curve is changing less (Fvar = 10.0 ± 1.6%)
Summary
On the contrary the mechanism powering the optical, UV and X-ray emission in quiescence is still debated. A unifying scenario which can make a clear prediction, systematically matching the spectral energy distribution (SED), from optical up to the X-ray emission, of both NS and BH quiescent LMXB, is still missing. Despite much early observational and theoretical work on quiescent LMXBs, there are several crucial questions still remaining which only simultaneous multiwavelength campaigns can address. Is it accretion at low rates really happening onto NSs, and how exactly does it work? Otherwise, are mass accretion rate fluctuations propagating inwards from the outer disc (UV), up to the NS surface (X-ray), implying that, on the contrary, UV variability is leading the X-ray variability? We present a brief summary of the main results of our fully comprehensive paper [1]
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