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Durham University

Department of Physics

Staff profile

Publication details for Prof Tim Roberts

Walton, D.J., Harrison, F.A., Bachetti, M., Barret, D., Boggs, S.E., Christensen, F.E., Craig, W.W., Fuerst, F., Grefenstette, B.W., Hailey, C.J., Madsen, K.K., Middleton, M.J., Rana, V., Roberts, T.P., Stern, D., Sutton, A.D., Webb, N. & Zhang, W. (2015). NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray source NGC 5907 ULX1 a vanishing act. Astrophysical journal 799(2): 122.

Author(s) from Durham

Abstract

We present results obtained from two broadband X-ray observations of the extreme ultraluminous X-ray source (ULX) NGC 5907 ULX1, known to have a peak X-ray luminosity of ~5 × 1040 erg s–1. These XMM-Newton and NuSTAR observations, separated by only ~4 days, revealed an extreme level of short-term flux variability. In the first epoch, NGC 5907 ULX1 was undetected by NuSTAR, and only weakly detected (if at all) with XMM-Newton, while in the second NGC 5907 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) × 1040 erg s–1, 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, NGC 5907 ULX1 is a high-Eddington accretor.