Orbital evolution and search for eccentricity and apsidal motion in the eclipsing HMXB 4U 1700-37

TL;DR

This study analyzes the orbital evolution of the eclipsing HMXB 4U 1700-37 using long-term X-ray data, measuring orbital decay, constraining eccentricity, and exploring apsidal motion, with implications for understanding its orbital dynamics.

Contribution

It provides new measurements of orbital decay and constraints on eccentricity and apsidal motion using extensive X-ray observations, improving understanding of the system's orbital evolution.

Findings

Orbital period decay rate estimated at -(4.7 ± 1.9) × 10^{-7} yr^{-1}

Eccentricity constrained to less than 0.008 for high apsidal motion rates

Discrepancy discussed between current eccentricity limits and previous radial velocity results

Abstract

In the absence of detectable pulsations in the eclipsing High Mass X-ray binary 4U 1700-37, the orbital period decay is necessarily determined from the eclipse timing measurements. We have used the earlier reported mid-eclipse time measurements of 4U 1700-37 together with the new measurements from long term light curves obtained with the all sky monitors RXTE-ASM, Swift-BAT and MAXI-GSC, as well as observations with RXTE-PCA, to measure the long term orbital evolution of the binary. The orbital period decay rate of the system is estimated to be ${\dot{P}}/P = -(4.7 \pm 1.9) \times 10^{-7}$ yr$^{-1}$, smaller compared to its previous estimates. We have also used the mid-eclipse times and the eclipse duration measurements obtained from 10 years long X-ray light-curve with Swift-BAT to separately put constraints on the eccentricity of the binary system and attempted to measure any apsidal motion. For an apsidal motion rate greater than 5 degrees per year, the eccentricity is found to be less than 0.008, which limits our ability to determine the apsidal motion rate from the current data. We discuss the discrepancy of the current limit of eccentricity with the earlier reported values from radial velocity measurements of the companion star.