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

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) × 107 yr. Over and above this orbital period decay, the O−C residual curve also shows a periodic residual on shorter time-scales. This sinusoidal variation has an amplitude of ∼9 lt-s and a period of ∼760 d. This is indicative of the 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 ranges 20.5–26.9 Jupiter mass and 750–860 lt-s, respectively. If true, then it will be the most massive circumbinary planet and also the smallest period binary known to host a planet.