A misfired outburst in the neutron star X-ray binary Centaurus X-4

TL;DR

This long-term optical monitoring of Centaurus X-4 revealed a pre-outburst flux rise and a stalled heating front in the accretion disc, providing insights into disc instability and outburst mechanisms in neutron star X-ray binaries.

Contribution

The study presents the first detailed long-term optical light curve of Centaurus X-4, identifying a pre-outburst flux increase and modeling the stalled propagation of an inside-out heating front.

Findings

Detected a ~3-year optical flux rise before the 2020 event.

Observed a stalled heating front likely caused by increasing surface density.

Found that irradiation plays a minor role in optical emission compared to other sources.

Abstract

We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 years. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disc; we detect short-duration flares, likely originating also in the disc, superimposed with a small-amplitude (< 0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (~2300 days) downward trend, followed by a shorter (~1000 days) upward one, is observed in the disc light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disc instability model. For Cen X-4, the rise of the optical flux proceeded for ~3 years, and culminated in a flux increase at all wavelengths (optical-UV-X-rays) at the end of 2020. This increase faded after ~2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disc. The propagation might have stalled soon after the ignition due to the increasing surface density in the disc that the front encountered while propagating outwards. The stall was likely eased by the low level irradiation of the outer regions of the large accretion disc, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.