The optical ephemeris and X-ray variability of 4U 1735-44 (V926 Sco)

TL;DR

This study refines the orbital ephemeris of the low mass X-ray binary 4U 1735-44 using optical data from 1984-2007, confirming the system's phase zero and supporting the standard LMXB model with correlated optical and X-ray fluxes.

Contribution

The paper provides a refined optical ephemeris for 4U 1735-44, reconciling previous discrepancies and confirming the system's phase zero with spectral data, supporting the classic LMXB model.

Findings

Optical maximum occurs at spectral phase 0.47 ± 0.05.

Optical flux correlates positively with X-ray flux, with non-X-ray optical flux < 14%.

No orbital modulation, eclipses, or dips observed in X-ray data.

Abstract

Optical observations of the low mass X-ray binary 4U 1735-44 were obtained during 1997-2007 and combined with earlier published observations from 1984-1993 to refine the ephemeris for the system. The linear fit for the time of maximum optical light has the ephemeris HJD = 2445904.0494(90) + [ N x 0.19383222(29)] with a value of chi^2 = 253.5$ for 16 dof and a scatter about phase zero of sigma = 0.061. The new data reconciles the discrepancy between the previous ephemeris and the more recent spectral ephemeris based on emission from the companion star which defined the systems true dynamical phase zero. The optical maximum for 4U 1735-44 now occurs at spectral phase 0.47 \pm 0.05 and thus supports the classic model for an LMXB system. Our data further supports the standard model in several ways. The mean optical flux shows a positive correlation with the RXTE ASM X-ray flux, the relative increases suggesting that the non-X-ray induced optical flux from the companion is < 14 percent of the total optical light from the system. There is no apparent trend in the optical modulation percentage with increasing X-ray flux. The X-ray flux for various ASM energy bands shows no evidence of orbital modulation, eclipses or dips when folded using the new ephemeris.