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Title: NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray source NGC 5907 ULX1: A Vanishing Act
Authors: Walton, D. J.
Harrison, F. A.
Rana, V
+15 Co-Authors
Keywords: black hole physics
X-rays: binaries
X-rays: individual: NGC 5907 ULX1
Issue Date: 1-Feb-2015
Publisher: IOP Sciences for The American Astronomical Society
Citation: The Astrophysical Journal, 2015, Vol.799, p7
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.
Description: Open Access.
ISSN: 0004-637X
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Copyright: 2015, The American Astronomical Society
Appears in Collections:Research Papers (A&A)

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