We present a detailed , broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13 , a known super-Eddington source , utilizing data from the XMM-Newton , NuSTAR and Chandra observatories .
The broadband XMM-Newton + NuSTAR spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage , suggesting that additional ULXs in the known population may host neutron star accretors .
Through time-averaged , phase-resolved and multi-epoch studies , we find that two non-pulsed thermal blackbody components with temperatures \sim 0.5 and 1.5 keV are required to fit the data below 10 keV , in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column .
The characteristic radii of the thermal components appear to be comparable , and are too large to be associated with the neutron star itself , so the need for two components likely indicates the accretion flow outside the magnetosphere is complex .
We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form , but is terminated by the neutron star ’ s magnetic field soon after its onset , implying a limit of B \lesssim 6 \times 10 ^ { 12 } G for the dipolar component of the central neutron star ’ s magnetic field .
Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs .
Finally , we examine the spectrum exhibited by P13 during one of its unusual ‘ off ’ states .
These data require both a hard powerlaw component , suggesting residual accretion onto the neutron star , and emission from a thermal plasma , which we argue is likely associated with the P13 system .