Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1
Felix Fürst, D. J. Walton, G. L. Israel, M. Bachetti, D. Barret, M. Brightman, H. P. Earnshaw, A. Fabian, M. Heida, M. Imbrogno, Michael Middleton, C. Pinto, R. Salvaterra, T. P. Roberts, G. A. Rodríguez Castillo, N. Webb
Abstract
NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 10 41 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 10 12 G or 10 13 G, however, the strong spin-down during the off-state seems only to be consistent with a very high magnetic field, namely, > 10 13 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.