F. Bernardini, E. M. Cackett, E. F. Brown, C. D'Angelo N. Degenaar, J. M. Miller, M. Reynolds, R. Wijnands
The physics of accretion during quiescence in low mass X-ray binaries (LMXBs) is poorly understood, yet there are signs that accretion must be happening. Several LMXBs show variability during quiescence, particularly striking is the case of the nearby neutron star Cen X-4. With the goal of unveiling the real nature of its quiescent variability we conducted the first long-term, multiwavelength simultaneous monitoring of Cen X-4, thanks to 60 observations performed by Swift on a daily basis. During those observations, Cen X-4 is highly variable in all energy bands on timescales from days to months, with the strongest quiescent short-term variability detected in the X-ray band, a factor of 22 drop in only 4 days. The X-ray and the UV and optical emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (spectral index 1.4-2.0). The spectral shape remains constant as the flux varies. Both components are varying in tandem with the X-ray flux, with each responsible for about 50% of the total flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission cannot be due to thermal emission from the stream impact point, and neither to thermal emission from a standard optically thick and geometrically thin disc. The UV emission is produced by reprocessing most likely from the companion star. We found the accretion disc in quiescence to be UV faint. The 110 s correlation timescale suggests that the accreting matter could be in an advection dominated accretion flow state (ADAF), which is likely UV faint, at a distance less to 6150 Schwarzschild Radii.
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http://arxiv.org/abs/1307.2492
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