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

## TL;DR

This study determined the orbital period, inclination, and companion characteristics of the neutron star transient MAXI J1807+132 through photometry, extending known correlations to neutron star systems and estimating its distance and galactic position.

## Contribution

First to measure the orbital parameters of MAXI J1807+132 using optical photometry and extend the magnitude-period correlation to neutron star X-ray transients.

## Key findings

- Orbital period of 4.258 hours determined.
- Binary inclination of 72 degrees estimated.
- Distance to the system is approximately 6.3 kpc.

## 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 $P_{\rm orb} = 4.258 \pm 0.008$ hr. By modelling the light curve, we obtained a binary inclination of $ i = 72\pm5 \, \deg $ and a mass ratio $q = 0.24^{+0.19}_{-0.14}$ ($68$ 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.