NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray source NGC 5907 ULX1 : a vanishing act

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.

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.

Citation

Walton, D., Harrison, F., Bachetti, M., Barret, D., Boggs, S., Christensen, F., …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), Article 122. https://doi.org/10.1088/0004-637x/799/2/122

• Journal Article Type: Article
• Publication Date: Feb 1, 2015
• Deposit Date: Jan 30, 2015
• Publicly Available Date: Feb 16, 2015
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 799
• Issue: 2
• Article Number: 122
• DOI: https://doi.org/10.1088/0004-637x/799/2/122
• Keywords: Black hole physics, X-rays: binaries, X-rays: individual (NGC 5907 ULX1).
• Public URL: https://durham-repository.worktribe.com/output/1413447
• Related Public URLs: http://arxiv.org/abs/1411.5974