English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

We conducted the first long-term (60 days), multiwavelength (optical,ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swiftobservations from June to August 2012, with the goal of understandingvariability in the low mass X-ray binary Cen X-4 during quiescence. We foundCen X-4 to be highly variable in all energy bands on timescales from days tomonths, with the strongest quiescent variability a factor of 22 drop in theX-ray count rate in only 4 days. The X-ray, UV and optical (V band) emissionare correlated on timescales down to less than 110 s. The shape of thecorrelation is a power law with index gamma about 0.2-0.6. The X-ray spectrumis well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (withspectral index Gamma=1.4-2.0), with the spectral shape remaining constant asthe flux varies. Both components vary in tandem, with each responsible forabout 50% of the total X-ray flux, implying that they are physically linked. Weconclude that the X-rays are likely generated by matter accreting down to theNS surface. Moreover, based on the short timescale of the correlation, we alsounambiguously demonstrate that the UV emission can not be due to either thermalemission from the stream impact point, or a standard optically thick,geometrically thin disc. The spectral energy distribution shows a small UVemitting region, too hot to arise from the accretion disk, that we identifiedas a hot spot on the companion star. Therefore, the UV emission is most likelyproduced by reprocessing from the companion star, indeed the vertical size ofthe disc is small and can only reprocess a marginal fraction of the X-rayemission. We also found the accretion disc in quiescence to likely be UV faint,with a minimal contribution to the whole UV flux.

Daily, multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence