The cooling phase of Type I X-ray bursts observed with RXTE in 4U 1820-30 does not follow the canonical F - T^4 relation

TL;DR

This study analyzes X-ray bursts from 4U 1820-30, revealing that their cooling phase does not follow the expected F - T^4 relation, suggesting complex physical processes affecting the neutron star's surface emission.

Contribution

It demonstrates that the flux-temperature relation during burst cooling deviates from the canonical model, indicating variable emitting areas or atmospheric effects.

Findings

Flux-temperature relation fits a broken power law

Departure from F - T^4 suggests changing emitting area

Possible influence of atmospheric composition or residual heat

Abstract

We analysed the complete set of bursts from the neutron-star low-mass X-ray binary 4U 1820-30 detected with the Rossi X-ray Timing Explorer (RXTE). We found that all are photospheric radius expansion bursts, and have similar duration, peak flux and fluence. From the analysis of time-resolved spectra during the cooling phase of the bursts, we found that the relation between the bolometric flux and the temperature is very different from the canonical F - T^4 relation that is expected if the apparent emitting area on the surface of the neutron star remains constant. The flux-temperature relation can be fitted using a broken power law, with indices 2.0$\pm$0.3 and 5.72$\pm$0.06. The departure from the F - T^4 relation during the cooling phase of the X-ray bursts in 4U 1820-30 could be due to changes in the emitting area of the neutron star while the atmosphere cools-down, variations in the colour-correction factor due to chemical evolution, or the presence of a source of heat, e.g. residual hydrogen nuclear burning, playing an important role when the burst emission ceases.