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Swift monitoring of NGC 5548: X-ray reprocessing and short-term UV/optical variability

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2014
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McHardy, I. M.
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Swift monitoring of NGC 5548: X-ray reprocessing and short-term UV/optical variability
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Author
  • McHardy, I. M.;
  • Cameron, D. T.;
  • Dwelly, T.;
  • Connolly, S.;
  • Lira Teillery, Paulina;
  • Emmanoulopoulos, D.;
  • Gelbord, J.;
  • Breedt, E.;
  • Arevalo, P.;
  • Uttley, P.;
Abstract
Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750 d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short time-scales (tens of days). The UV/optical bands lag the X-ray band with lags which are proportional to wavelength raised to the power 1.23 ± 0.31. This power is very close to the power (4/3) expected if short time-scale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura–Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long-term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.
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Artículo de Publicación ISI
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URI: https://repositorio.uchile.cl/handle/2250/126478
DOI: doi:10.1093/mnras/stu1636
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MNRAS 444, 1469–1474 (2014)
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