# Indication of a massive circumbinary planet orbiting the Low Mass X-ray Binary MXB 1658-298

## TL;DR

This study analyzes X-ray timing data of MXB 1658-298, revealing long-term orbital decay and periodic variations suggestive of a massive circumbinary planet, potentially the most massive known around such a binary.

## Contribution

First detection of a massive circumbinary planet around a low mass X-ray binary using eclipse timing variations and long-term orbital analysis.

## Key findings

- Orbital period decay with a timescale of ~65 million years.
- Periodic residuals indicating a third body with ~20.5-26.9 Jupiter masses.
- Potential identification of the most massive circumbinary planet.

## Abstract

We present an X-ray timing analysis of the transient X-ray binary MXB 1658-298, using data obtained from the RXTE and XMM-Newton observatories. We have made 27 new mid eclipse time measurements from observations made during the two outbursts of the source. These new measurements have been combined with the previously known values to study long term changes in orbital period of the binary system. We have found that the mid-eclipse timing record of MXB 1658-298 is quite unusual. The long term evolution of mid-eclipse times indicates an overall orbital period decay with a time scale of -- 6.5(7) x 10^7 year. Over and above this orbital period decay, the O-C residual curve also shows a periodic residual on shorter timescales. This sinusoidal variation has an amplitude of ~9 lt-sec and a period of ~760 d. This is indicative of presence of a third body around the compact X-ray binary. The mass and orbital radius of the third body are estimated to lie in the range, 20.5-26.9 Jupiter mass and 750-860 lt-sec, respectively. If true, then it will be the most massive circumbinary planet and also the smallest period binary known to host a planet.