The Unusual Broadband X-ray Spectral Variability of NGC 1313 X-1 seen
  with XMM$-$Newton, Chandra and NuSTAR
  We present results from the major coordinated X-ray observing program on the
ULX NGC 1313 X-1 performed in 2017, combining $XMM$-$Newton$, $Chandra$ and
$NuSTAR$, focusing on the evolution of the broadband ($\sim$0.3-30.0 keV)
continuum emission. Clear and unusual spectral variability is observed, but
this is markedly suppressed above $\sim$10-15 keV, qualitatively similar to the
ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion
disc models designed to approximate super-Eddington accretion, allowing for
both a black hole and a neutron star accretor. With regards to the hotter disc
component, the data trace out two distinct tracks in the luminosity-temperature
plane, with larger emitting radii and lower temperatures seen at higher
observed fluxes. Despite this apparent anti-correlation, each of these tracks
individually shows a positive luminosity-temperature relation. Both are broadly
consistent with $L\propto{T}^{4}$, as expected for blackbody emission with a
constant area, and also with $L\propto{T}^{2}$, as may be expected for an
advection-dominated disc around a black hole. We consider a variety of
possibilities for this unusual behaviour. Scenarios in which the innermost flow
is suddenly blocked from view by outer regions of the super-Eddington disc/wind
can explain the luminosity-temperature behaviour, but are difficult to
reconcile with the lack of strong variability at higher energies, assuming this
emission arises from the most compact regions. Instead, we may be seeing
evidence for further radial stratification of the accretion flow than is
included in the simple models considered, with a combination of winds and
advection resulting in the suppressed high-energy variability.
