Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1

¹ Quasar Science Resources SL for ESA, European Space Astronomy Centre (ESAC), Science Operations Departement, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: felix.fuerst@sciops.esa.int
² Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁵ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
⁶ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁰ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
¹¹ INAF – IASF Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
¹² INAF – IASF Milano, Via Corti 12, 20133 Milano, Italy
¹³ Centre for Extragalactic Astronomy & Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

Received: 23 September 2022
Accepted: 23 January 2023

Abstract

NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 10⁴¹ erg s⁻¹. The pulsar is known for its fast spin-up during the on-state. Here, we present a long-term monitoring of the X-ray flux and the pulse period between 2003 and 2022. We find that the source was in an off- or low-state between mid-2017 to mid-2020. During this state, our pulse period monitoring shows that the source had spun down considerably. We interpret this spin-down as likely being due to the propeller effect, whereby accretion onto the neutron star surface is inhibited. Using state-of-the-art accretion and torque models, we use the spin-up and spin-down episodes to constrain the magnetic field. For the spin-up episode, we find solutions for magnetic field strengths of either around 10¹² G or 10¹³ G, however, the strong spin-down during the off-state seems only to be consistent with a very high magnetic field, namely, > 10¹³ G. This is the first time a strong spin-down is seen during a low flux state in a ULXP. Based on the assumption that the source entered the propeller regime, this gives us the best estimate so far for the magnetic field of NGC 5907 ULX1.