The Orbit of NGC 5907 ULX-1

TL;DR

This study determines the orbital parameters of the ultraluminous X-ray pulsar NGC 5907 ULX-1, revealing a 5.7-day orbit and suggesting it is a high-mass X-ray binary viewed face-on.

Contribution

First detailed orbital characterization of NGC 5907 ULX-1, providing key parameters and evidence for its high-mass X-ray binary nature.

Findings

Orbital period of approximately 5.7 days.

Projected semi-axis of about 3.1 light-seconds.

Likely face-on high-mass X-ray binary configuration.

Abstract

We report on the orbit of the binary system powering the most extreme ultraluminous X-ray pulsar known to date: NGC 5907 ULX-1 (hereafter ULX1). ULX1 has been the target of a substantial multi-instrument campaign, mainly in the X-ray band, but no clear counterparts are known in other bands. Although ULX1 is highly variable and pulsations can be transient (regardless of the source flux), the timing data collected so far allow us to investigate the orbit of this system. We find an orbital period $P_{orb}=5.7^{+0.1}_{-0.6}\text{ d}$ and a projected semi-axis $A_1 =3.1^{+0.8}_{-0.9}\text{ lts}$. The most likely ephemeris is: $P_{orb}=5.6585(6)\text{ d}$, $A_1 = 3.1(4)\text{ lts}$, and the epoch of ascending nodes passage is: $T_{asc} = 57751.37(5)\text{ MJD}$. However, there are 6 similar solutions, acceptable within $3\,\sigma$. We find further indications that ULX1 is a high-mass X-ray binary. This implies that we are observing its orbit face-on, with an inclination $<5\text{ deg}$.