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NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation

We present a study of the X-ray source NGC 2403 XMM4 (4XMM J073702.2+653934) based on 20 years of archival observations with XMM-Newton, Chandra, Swift and NuSTAR. Although it has previously been classified as an ultraluminous X-ray source (ULX), we show that its luminosity rarely, if ever, passes the $10^{39} \rm ~erg~s^{-1}$ threshold luminosity for a ULX. It does, however, behave very similarly to ULXs, with its 0.3-10 keV spectra well described by two thermal components, the softer of which behaves consistently with the expectations for an advection-dominated disc ($L\propto T^{1.49\pm0.85}$), and we find tentative evidence for an extra spectral component above 10 keV. We also find moderately significant evidence for an absorption feature in one spectrum that could originate in an outflowing wind, although a cyclotron resonance scattering feature is also a possibility. Most intriguingly, we find a possible transient pulsation at $\sim 3.32$ Hz in a short segment of one observation using an accelerated pulsation search. This evidence suggests that NGC 2403 XMM4 is displaying many of the hallmarks of super-Eddington accretion at luminosities between $5 - 10 \times 10^{38} \rm ~erg~s^{-1}$ which, when considered alongside the putative pulsation, points to the presence of a neutron star as the accreting object this system.