Probing the Low State of the Most Extreme ULX Pulsar

The 3 Ultraluminous X-ray Sources (ULXs) driven by accreting neutron stars well beyond their Eddington limit (L(Edd)) deserve more investigation as they constrain accretion in a regime that eludes simple explanations. They all display superorbital flux modulations and rarely enter a low state not yet understood, that could shed light on their physics. The sources could go sub-Eddington, or the centrifugal push could inhibit accretion (propeller effect), or the pulsar could be obscured by optically thick material (in a disk or winds). The most extreme ULX pulsar is NGC 5907 ULX (Israel et al 2017, I17), at 500 times L(Edd). Current observational campaigns with Swift, XMM-Newton, and NuSTAR have recently seen it entering a low state (<20 L(Edd)) and no recovery after one superorbital period. Now only Chandra can probe its low flux, resolving it from other sources in its galaxy (I17, fig S1; Walton et al 2015), and distinguish a propeller state ( 2 L(Edd)) from other scenarios.