Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1
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.
Item Type | Article |
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Uncontrolled Keywords | astro-ph.HE; X-rays: individuals: NGC 5907 ULX1; X-rays: binaries; Stars: neutron; Magnetic fields; Accretion, accretion disks |
Subjects |
Physics and Astronomy(all) > Astronomy and Astrophysics Earth and Planetary Sciences(all) > Space and Planetary Science |
Date Deposited | 14 Nov 2024 11:11 |
Last Modified | 14 Nov 2024 11:11 |