The Orbit of NGC 5907 ULX-1

Andrea Belfiore, Ruben Salvaterra, Lara Sidoli, Gian Luca Israel, Luigi Stella, Andrea De Luca, Sandro Mereghetti, Paolo Esposito, Fabio Pintore, Antonino D’Aì, Guillermo Rodrìguez Castillo, Dominic J. Walton, Felix Fürst, Danilo Magistrali, Anna Wolter, Matteo Imbrogno

Research output: Contribution to journalArticlepeer-review


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 − 0.6 + 0.1 days and a projected semi-axis A 1 = 3.1 − 0.9 + 0.8 lt - s . The most likely ephemeris is P orb = 5.6585(6) days, A 1 = 3.1(4) lt-s, and the epoch of ascending nodes passage is T asc = 57751.37(5) MJD. However, there are six similar solutions acceptable within 3σ. 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°.

Original languageEnglish
Article number78
Pages (from-to)1-8
Number of pages8
JournalThe Astrophysical Journal
Issue number1
Early online date9 Apr 2024
Publication statusPublished - 9 Apr 2024


  • High time resolution astrophysics
  • Ultraluminous x-ray sources
  • High mass x-ray binary stars
  • Orbit determination


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