An X-ray-UV correlation in Cen X-4 during quiescence

TL;DR

This study reveals a positive correlation between X-ray and UV emissions in Cen X-4 during quiescence, indicating reprocessing in the accretion disk, while optical emission likely originates from the donor star.

Contribution

It provides new observational evidence of X-ray and UV flux correlation in Cen X-4, suggesting reprocessing in the accretion disk as the UV emission source during quiescence.

Findings

X-ray flux varies by a factor of 6.5

Positive correlation between X-ray and UV fluxes

Optical flux shows no significant correlation with X-ray flux

Abstract

Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is seen to be variable on timescales from hundreds of seconds to years, suggesting that at least in this object, low-level accretion is important during quiescence. Here we present results from recent XMM-Newton and Swift observations of Cen X-4, where the X-ray flux (0.5 - 10 keV) varies by a factor of 6.5 between the brightest and faintest states. We find a positive correlation between the X-ray flux and the simultaneous near-UV flux, where as there is no significant correlation between the X-ray and simultaneous optical (V, B) fluxes. This suggests that while the X-ray and UV emitting regions are somehow linked, the optical region originates elsewhere. Comparing the luminosities, it is plausible that the UV emission originates due to reprocessing of the X-ray flux by the accretion disk, with the hot inner region of the disk being a possible location for the UV emitting region. The optical emission, however, could be dominated by the donor star. The X-ray/UV correlation does not favour the accretion stream-impact point as the source of the UV emission.