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

TL;DR

This work uses time-resolved $R$-band photometry of the neutron star X-ray transient MAXI J1807+132 in quiescence to measure its orbital period and constrain binary parameters. A robust ellipsoidal modulation is detected, yielding an orbital period of $P_{ m orb}=4.258 \pm 0.008$ h and an inclination of $i=72 \pm 5$ degrees, with a mass ratio $q=0.24^{+0.19}_{-0.14}$ and a neutron-star mass $M_{ m NS}=1.2^{+1.1}_{-0.8} M_\odot$. XRBinary/EMCEE modelling also indicates a $R$-band donor flux contribution of $42 \pm 9\%$ and a disc luminosity of $\log(L_d)=32.1\pm0.3$ erg s$^{-1}$. By applying the $M_r{-}P_{ m orb}$ relation to NS XRTs, the distance is estimated at $d=6.3\pm0.7$ kpc, with a Galactic height of $z=1.6\pm0.2$ kpc, supporting a substantial elevation above the Galactic plane. The results extend the dynamical study of NS XRTs and set the stage for future spectroscopic mass measurements of the system.

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 \, °$ 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.

Figures (7)

• Figure 1: 600 s $R$-band image of J1807 in quiescence, obtained with the WFC on the night of 24 June 2022. The target (J1807), the reference star (R), and comparison stars are marked with circles. North is at the top, and east is to the left.
• Figure 2: Light curves obtained on 23-24 June 2022 and 27 July 2022. The time is in units of days since HJD 2459754.5.Upper panel: Seeing conditions for each observation. Middle panel: Light curve of J1807. Lower panel: Light curve of the comparison stars marked in Fig. \ref{['fig:ds9']}, demonstrating photometric stability.
• Figure 3: Lomb-Scargle periodogram of the J1807 light curve for the nights of 24–25 June. The Lomb-Scargle power of the signal is in black, and the window function in orange. The red vertical line marks the main peak at 2.129 hours; dotted grey ticks at the top indicate the daily aliases ($\pm1$, $\pm2$ d$^{-1}$). The period has a significance greater than $4\sigma$.
• Figure 4: Phase-folded light curves on the 4.258 hrs of J1807. The reference epoch used was $T_{0.5}(HJD)=2459755.572$. The observation dates are given in the upper right of each panel.
• Figure 5: Light curve modelling. Upper panel: J1807 data (black dots) and the synthetic light curves based on the 68 per cent confidence intervals from the MCMC analysis (red lines). The teal line indicates the best-fit model. Lower panel: Residuals from the best-fit model (black dots) and for each individual synthetic light curve (red lines).