Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1
Salvaterra, R.; Pinto, C.; Israel, G. L.; Bachetti, M.; Earnshaw, H. P.; Roberts, T. P.; Imbrogno, M.; Rodríguez Castillo, G. A.; Barret, D.; Brightman, M.; Webb, N.; Walton, D. J.; Fürst, F.; Fabian, A.; Middleton, M. J.; Heida, M.
Spain, United Kingdom, Italy, France, United States, Germany
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.