The orbital period and inclination of the neutron star X-ray transient MAXI J1807+132

Abstract

The neutron star X-ray transient MAXI J1807+132 has undergone outbursts in 2017, 2019, and 2023. We conducted an $R$-band time series photometry campaign using the Isaac Newton Telescope during the 2022 quiescent state. We detected a periodic variation in the light curve, consistent with ellipsoidal modulation, which allowed us to determine an orbital period of $P_{\mathrm{orb}}=4.258\pm 0.008$ hr. By modelling the light curve, we obtained a binary inclination of $i=72\pm 5\hat{A}°$ and a mass ratio $q=0.24_{-0.14}^{+0.19}$ ($68$ per cent confidence level). Furthermore, our analysis supports an early M-dwarf companion that contributes between 30 and 50 per cent to the total flux in the $R$-band. We extend the previously established absolute magnitude versus orbital period correlation for black hole X-ray transients to neutron star systems. We applied the correlation to MAXI J1807+132, estimating its distance as $6.3\pm 0.7$ kpc and its height above the Galactic plane to be $1.6\pm 0.2$ kpc.

Published in Astronomy & Astrophysics