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

TL;DR

This study monitors the ultra-luminous X-ray pulsar NGC 5907 ULX1 over nearly two decades, revealing a low-state spin-down likely caused by the propeller effect, which constrains its magnetic field to be very high, over 10^13 G.

Contribution

It provides the first evidence of a strong spin-down during a low flux state in a ULXP, offering new constraints on the neutron star's magnetic field using long-term observational data.

Findings

Detected a low-state between 2017-2020 with significant spin-down.

Spin-down during the low-state suggests a magnetic field >1e13 G.

Constraints on magnetic field strength based on accretion and torque models.

Abstract

NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 1e41 erg/s. 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 2003-2022. 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 1e12G or 1e13G, however, the strong spin-down during the off-state seems only to be consistent with a very high magnetic field, namely, >1e13G. 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.