NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray
  source NGC 5907 ULX1: A Vanishing Act
  We present results obtained from two broadband X-ray observations of the
extreme ultraluminous X-ray source (ULX) NGC5907 ULX1, known to have a peak
X-ray luminosity of ~5e40 erg/s. These XMM-Newton and NuSTAR observations,
separated by only ~4 days, revealed an extreme level of short-term flux
variability. In the first epoch, NGC5907 ULX1 was undetected by NuSTAR, and
only weakly detected (if at all) with XMM-Newton, while in the second NGC5907
ULX1 was clearly detected at high luminosity by both missions. This implies an
increase in flux of ~2 orders of magnitude or more during this ~4 day window.
We argue that this is likely due to a rapid rise in the mass accretion rate,
rather than to a transition from an extremely obscured to an unobscured state.
During the second epoch we observed the broadband 0.3-20.0 keV X-ray luminosity
to be (1.55+/-0.06)e40 erg/s, similar to the majority of the archival X-ray
observations. The broadband X-ray spectrum obtained from the second epoch is
inconsistent with the low/hard accretion state observed in Galactic black hole
binaries, but is well modeled with a simple accretion disk model incorporating
the effects of photon advection. This strongly suggests that, when bright,
NGC5907 ULX1 is a high-Eddington accretor.
