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arXiv · 1809.03237

Swift UVOT observations of the 2015 outburst of V404 Cygni

S. R. Oates·S. Motta·A. P. Beardmore·D. M. Russell·P. Gandhi·N. P. M. Kuin·M. De Pasquale·D. Altamirano·A. A. Breeveld·A. J. Castro-Tirado·C. Knigge·M. J. Page·D. Steeghs
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Abstract

The black-hole binary, V404 Cygni, went into outburst in June 2015, after 26 years of X-ray quiescence. We observed the outburst with the Neil Gehrels Swift observatory. We present optical/UV observations taken with the Swift Ultra-violet Optical Telescope, and compare them with the X-ray observations obtained with the Swift X-ray Telescope. We find that dust extinction affecting the optical/UV, does not correlate with absorption due to neutral hydrogen that affects the X-ray emission. We suggest there is a small inhomogeneous high density absorber containing a negligible amount of dust, close to the black hole. Overall, temporal variations in the optical/UV appear to trace those in the X-rays. During some epochs we observe an optical time-lag of (15 - 35)s. For both the optical/UV and X-rays, the amplitude of the variations correlates with flux, but this correlation is less significant in the optical/UV. The variability in the light curves may be produced by a complex combination of processes. Some of the X-ray variability may be due to the presence of a local, inhomogeneous and dust-free absorber, while variability visible in both the X-ray and optical/UV may instead be driven by the accretion flow: the X-rays are produced in the inner accretion disc, some of which are reprocessed to the optical/UV; and/or the X-ray and optical/UV emission is produced within the jet.

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S. R. Oates, S. Motta, A. P. Beardmore, D. M. Russell, P. Gandhi, N. P. M. Kuin, M. De Pasquale, D. Altamirano, A. A. Breeveld, A. J. Castro-Tirado, C. Knigge, M. J. Page, D. Steeghs. 2018-09-10. Swift UVOT observations of the 2015 outburst of V404 Cygni. https://doi.org/10.1093/mnras/stz1998

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