The Orbit of NGC 5907 ULX-1

The Orbit of NGC 5907 ULX-1
Andrea Belfiore (INAF IASF-Mi), Ruben Salvaterra (INAF IASF-Mi), Lara Sidoli (INAF IASF-Mi), Gian Luca Israel (INAF OAR), Luigi Stella (INAF OAR), Andrea De Luca (INAF IASF-Mi), Sandro Mereghetti (INAF IASF-Mi), Paolo Esposito (IUSS Pavia), Fabio Pintore (INAF IASF-Pa), Antonino D’A`i (INAF IASF-Pa), Guillermo Rodr`iguez Castillo (INAF IASF-Pa), Dominic J. Walton (Centre for Astrophysics Research University of Hertfordshire), Felix F"urst (ESA ESAC), Danilo Magistrali (Universidad Pontificia Comillas), Anna Wolter (INAF OAB), Matteo Imbrogno (Dipartimento di Fisica Universit`a Tor Vergata)
arXiv:2405.04574v1 Announce Type: new
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 $arXiv:2405.04574v1 Announce Type: new
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 $