Analysis of optical spectroscopy and photometry of the type I X-ray bursting system UW CrB

TL;DR

This study presents two years of optical photometry and spectroscopy of UW CrB, refining its orbital parameters, analyzing optical bursts, and clarifying spectral features, contributing to understanding the system's accretion and burst mechanisms.

Contribution

It provides updated orbital ephemeris, reports new optical bursts, and clarifies the nature of spectral features in UW CrB, enhancing knowledge of its accretion dynamics.

Findings

Refined orbital period to 110.97680(1) min.

Detected 17 new optical bursts, 10 temporally resolved.

Optical bursts occur during a phase gap, indicating non-eclipsed reprocessing site.

Abstract

UW Coronae Borealis (UW CrB) is a low mass X-ray binary that shows both Type 1 X-ray and optical bursts, which typically last for 20 s. The system has a binary period of close to 2 hours and is thought to have a relatively high inclination due to the presence of an eclipse in the optical light curve. There is also evidence that an asymmetric disc is present in the system, which precesses every 5.5 days based on changes in the depth of the eclipse. In this paper, we present optical photometry and spectroscopy of UW CrB taken over 2 years. We update the orbital ephemeris using observed optical eclipses and refine the orbital period to 110.97680(1) min. A total of 17 new optical bursts are presented, with 10 of these bursts being resolved temporally. The average $e$-folding time of $19\pm3$s for the bursts is consistent with the previously found value. Optical bursts are observed during a previously identified gap in orbital phase centred on $\phi=0.967$, meaning the reprocessing site is not eclipsed as previously thought. Finally, we find that the apparent P-Cygni profiles present in some of the atomic lines in the optical spectra are due to transient absorption.